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20 October 2022, Volume 44 Issue 5

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Research paper

DISCOVERY AND SIGNIFICANCE OF CONTINENTAL PILLOW BASALT IN CHAHAR RIGHT BACK BANNER, CENTRAL INNER MONGOLIA

SHI Zhi-wei, BAI Zhi-da, DONG Guo-chen, WANG Xu

2022, 44(5):  1087-1106.  DOI: 10.3969/j.issn.0253-4967.2022.05.001

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Marine pillow basalts are widely developed, while large-scale continental pillow basalts are especially rare in China. The continental pillow basalt newly discovered in Chahar Right Back Banner, Inner Mongolia, is mainly tholeiite and a part of the Hannuoba basalt in Pliocene. In the accumulation sequence, from the bottom to the top, there are grayish-white calcareous mudstone of deep lacustrine, pillow basalt, stomatal basalt, and massive basalt. The pillow basalt has the thickness of about 10~12m and is mainly composed of black pillow body and yellow quenched clastics. The pillow bodies are preserved well and rare in China with complete structure. In detail, most of them are cylindrical, long ellipsoidal, of different sizes, about 0.8~1.5m long, and the largest pillow is about 2m long. Most of the cross sections are nearly circular, with a diameter of about 0.6m, up to 1m. The pillow bodies have obvious concentric layered structure, which can be divided into crust, middle layer and core. The degree of crystallization gradually becomes better from outside to inside. The crust is glassy, the middle layer is mesocrypt structure, and the core has relatively good crystallization, which is of intergranular-intersertal structure. Radial and discontinuous concentric ring fractures often occur in the pillow bodies, of which radial fractures are the most developed. The number of fractures varies from 10 to 20, with a width of 3~5mm. Most of them are filled with calcium and silica. In terms of composition, the pillow body is mainly olivine tholeiite, with porphyritic texture, stomatal-almond and massive structures. Phenocrysts are mainly plagioclase, clinopyroxene and olivine. Plagioclase is in the shape of self-shaped and plated strip, with the size of 1~3mm, the length-width ratio of 3︰1 to 5︰1, and the content of polysynthetic twin is 10%~15%; Clinopyroxene is short columnar, with a size of 0.6~1mm and a content of 5%~8%; Olivine is granular, with the size of 1~2mm and the content of 3%~5%. The matrix is composed of glass-based interlaced structure and intergranular-intersertal structure. It is mainly composed of microcrystalline plagioclase, pyroxene and glassy, accounting for 70%~85%. The basalt has SiO₂ of 52.84% and(Na₂O+K₂O)of 5.46%, belonging to calc alkaline rock(Rittman index σ=3.0<3.3), with obvious fractionation of light and heavy rare earth elements(LREE/HREE=17.52, La_N/Yb_N=24)and weakly Eu negative anomaly(δEu=0.89), enriching large ion lithophile elements(Rb, Sr, Ba, etc.). The pillow bodies are mainly filled with calcareous cemented basaltic quenched clastics, including agglomerate, breccia, and tuff grades, mainly orange basaltic glassy quenched breccia. The determination of tuffaceous quenched clastics enriches the genetic types of volcanic ash. The cements are mainly calcareous and siliceous precipitated by hydrochemistry, a small amount of clay minerals and gypsum can be seen locally. The quenched breccia also contains some calcareous mudstone fragments. It shows that both marine and continental facies can form pillow basalt, and water is a necessary condition, but its formation is not related to water depth, and it is mainly controlled by the temperature and velocity of lava. When the temperature of underwater basaltic magma is between 1150℃ and 1000℃, pillow structure is easy to form, but it is difficult to form pillow lava below 1000℃, and relatively slow velocity is conducive to the formation of pillow body. Continental pillow basalts are usually distributed around craters, which belong to near-crater deposits. They are of definite significance of facies, which is of great practical significances for remodeling the morphology of continental volcanic edifice and studying the volcanic eruption process.

CONE MORPHOLOGY AND ERUPTION MECHANISMS OF THE LATE QUATERNARY VOLCANO IN NORTHERN HAINAN ISLAND

FENG Jing-jing, ZHAO Yong-wei, LI Ni, CHEN Zheng-quan, WANG Li-zhu, LIU Yong-shun, NIE Bao-feng, ZHANG Xue-bin

2022, 44(5):  1107-1127.  DOI: 10.3969/j.issn.0253-4967.2022.05.002

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More than one hundred volcanoes erupted in the northern Hainan Island(NHI)during the Pleistocene and Holocene. With varied volcanic eruption types, and well-preserved volcanic edifice, these volcanoes are ideal for studying the Quaternary volcanic geology and cone morphology. We selected a total of 38 volcanoes in NHI with eruption age less than 200000 years as samples. Based on the unmanned aerial vehicle(UAV)tilt photography technology, we got the high-precision and high-resolution digital orthophoto map(DOM)and digital elevation model(DEM)data for these volcanoes. Combined with Google Earth high-resolution remote sensing satellite image and ALSO(12.5m)digital elevation model(DEM)data, these data provide the way to quantitatively measure and study the high-precision cone morphology. Previously, the method of Hco(height of cones)/Wco(cone base diameter)is commonly used to analyze the shape of cone, which is difficult to extract the information of irregular cones. When the volcanic cone is non-circular, or the elevation of crater rims is not uniform, the morphology parameters from artificial extraction have poor reliability. So, this paper proposes a new method, the ‘Sco^h(horizontal sectional area of volcanic cones)vs H^h(vertical distance from the top of the cone to the cross section)' to accurately analyze the morphology change of cones in northern Hainan Island. This method has two advantages. Firstly, it can deal with cones of any shape to avoid artificial errors. Secondly, it can conveniently filter to the greatest extent the interference factors such as uneven weathering and cone undulation caused by surface vegetation, so that the original morphological characteristics of the cone can be quantitatively displayed. Applying the above method, we accurately analyzed the cone morphology in the NHI. It is indicated that volcanic cones could be divided into three groups in the 'Sco^h vs H^h' projection diagram. The first group with the average(H^h/Sco^h)^h>3m=0~0.6, and about 22% of the cones belong to this type. The second group with the average(H^h/Sco^h)^h>3m=1.0~6.0, and about 65% of the cones in the area belong to this group. The third group with the average(H^h/Sco^h)^h>3m=4.0~12.0, and about 13% of cones belong to this group. Field geological survey confirmed that the first group volcanic cone is composed of volcanic tuff with parallel bedding, as revealed by the Luojingpan volcano, which is a tuff ring cone. Phreatomagmatic eruption is the major volcanism in this group. The second group of volcanic cone is composed of loose scoria or agglomerate, as represented by Maanling volcano. The Strombolian-Hawaii volcanic eruption produced this type of cones. The third group of cones is similar to lava dome and made of block lavas with diameter usually greater than 30cm, as exemplified by Bijialing volcano; therefore it is inferred to be the magma extrusion origin. All lines of evidence indicate that volcanic cones formed by different eruption types exhibit different morphological characteristics, which are accurately reflected in the ‘Sco^h vs H^h’ diagram. Therefore, it is possible to use high-precision DEM data of volcanic cone to constrain volcanic eruption type through this method.

PROGRESS IN MONITORING AND RESEARCH OF ACTIVITY OF VOLCANOES IN NORTHERN HAINAN ISLAND

GUO Ming-rui, YU Hong-mei, HU Jiu-chang, WANG Xi-jiao, ZHENG Zai-zhuang, WANG Gui-dan

2022, 44(5):  1128-1141.  DOI: 10.3969/j.issn.0253-4967.2022.05.003

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Volcanic eruption is a manifestation of crustal movement. The upper mantle inside the earth is filled with hot magma. Under great pressure, the magma will break out of the earth’s crust from the weak place along the volcanic channel and the volcanic throat. Volcanic eruption is the most intense display of the earth’s internal heat energy on the surface. Some activity characteristics and signals will be generated before volcanic eruption. Some countries have successfully predicted volcanic eruptions, and avoided a large number of casualties and economic losses.
Volcano monitoring is the basis for accurate volcano prediction and an effective method to reduce volcanic disasters. Volcano monitoring is to monitor the activity state of underground magma through various observation methods, and to capture the precursors of volcanic eruption.
The northern Hainan Island(also called Qiongbei)is one of the areas with frequent volcanic activity since the Quaternary in China. A total of 10 phases and 59 eruptions have occurred in Qiongbei area, forming more than 100 volcanoes of different sizes and shapes. Lavas with an area of about 4000km² are distributed in seven cities and counties of Haikou, Wenchang, Qionghai, Ding’an, Chengmai, Lingao and Danzhou, and Yangpu Economic Development Zone. More than 30 volcanoes with different sizes in the Shishan and Yongxing areas of the Quaternary volcanic area are arranged in a north-west direction, forming a typical central volcanic group, and the distribution of this volcanic group is controlled by the Changliu-Xiangou fault zone. The volcanic activity of the Ma’anling-Leihuling volcanic group(the largest)continued until the Holocene. This volcano is only 15km away from Haikou City, and is a typical urban volcano with potential eruption risk. Once it erupts, it will inevitably cause widespread public concern.
In order to acquaint us with the activity of Qiongbei volcano and prevent sudden volcanic disasters, a volcano monitoring network was set up in 2007 by the Hainan Earthquake Agency, which has been running well and has accumulated a large amount of observation data. We collected and sorted out the seismic, deformation, fluid and geomagnetic observation data in the Qiongbei volcanic area in the past 23 years(including the regional seismic network data before the establishment of the volcano monitoring stations), and used various methods to analyze the data. The relationship between regional volcanism and tectonics is obtained, the activity background of Qiongbei volcano is known, and the future development direction of the volcano monitoring network is proposed. In this article, we summarized and reviewed the Qiongbei volcano systematically. The results can provide a scientific basis for the planning and construction and disaster prediction of the Qiongbei volcanic area, and offer technical support for the development of follow-up volcano monitoring.
The diurnal variation of magnetic field and load/unload response ratio are used to study the electromagnetic induction of the changing magnetic field in the earth’s interior and analyze the changes in the electrical conductivity of the fluid in the deep crust of the earth. The load/unload response ratio is the function of station location and electrical conductivity of the medium underground, and thermal motion of underground masses will increase the electrical conductivity underground. The purpose of monitoring the temperature of water is to monitor the changes in geothermy at different depths, and compare and analyze the changes in water temperature at different depths and levels between the two wells. The result shows that there is no significant increase of energy in the Qiongbei volcanic area. Statistical analysis on the recorded earthquakes in the past 23 years is carried out to obtain the information about the normal background seismic activity. In recent years, the earthquake level in the volcanic area is slightly lower than the annual average(6 events), which all belong to volcanic tectonic earthquakes. The 7 GNSS observation networks can monitor the crustal deformation in Qiongbei area, and the data reflect the regional stress changes. The analyes on the results of multi-period mobile GNSS observation show that the observation stations mainly move in the southeast direction, without deflection, and the speed and direction are consistent. Each observation station has 6 baselines relative to the Ma’anling base station in the crater, and the baseline length is in a range of 72.3~42.8km. The baseline changes are mainly in a tortuous pattern, within a magnitude of ±15mm, and there are no obvious compression or extension.

The above analysis on the existing data of geomagnetism, fluids, seismometry and GNSS measurements shows that the volcanic activity of northern Hainan Province is currently in a stable state.

APPLICATION OF RECRYSTALLIZED CARBONATES TO THE DATING OF BEDROCK FAULTS IN CARBONATE ROCK AREA--A CASE STUDY ON THE R-SHEAR FAULT OF THE NORTHERN SECTION OF THE NAPO FAULT SYSTEM

JI Hao, LIU Chun-ru, ZHANG Pei-quan, LI Bing-su, NIE Guan-jun, WEI Chuan-yi, YIN Gong-ming

2022, 44(5):  1142-1155.  DOI: 10.3969/j.issn.0253-4967.2022.05.004

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The history of fault activity is the basis for understanding earthquake activity, evaluating earthquake risk and reducing earthquake hazards. At present, the determination of fault activity age can be divided into two categories according to different dating objects: One is the sediments related to fault activity to define the activity age, mainly through the dating of dislocated and non-displaced strata and landforms. For sediment dating, there are currently many dating techniques available with high reliability, such as ¹⁴C method, luminescence dating, etc. Therefore, those dating methods are currently the main ones for determining the chronology of fault movement. The other is the dating of materials and geomorphic surfaces directly related to fault activity. Such as fault gouges, bedrock slip surfaces(including associated or secondary substances on fault surfaces or slip surfaces), veins filled in fault zones, etc. However, the Quaternary activity of the faults in the carbonate rock area is difficult to determine due to the lack of Quaternary sediment cover. The latest research shows that recrystallized carbonate formed on the fault surface in carbonate rock area provides an ideal material for the study of such faults using ESR method. In the carbonate rock area, during the fault activity, the heat produced by friction leads to melting of the frictional surface of the rocks and produces a thin layer(~2mm)of recrystallized carbonates. The recrystallized carbonate with obvious fault scratches is the product of dynamic metamorphism of the frictional surface due to the local high temperature and pressure environment generated by the rapid friction of the two walls of fault during the fault activity, and its age coincides with the fault movement. The recrystallization experienced by recrystallized carbonate can clear the accumulated ESR signals before the fault occurs, and the new ESR signals accumulation begins after the fault movement. Therefore, the chronology of fault activity can be obtained by determining the age of recrystallized carbonate on the fault surface using the ESR method. The Napo fault zone controls the earthquake breeding and occurrence of the Youjiang seismic zone, so it is of great significance to study the tectonic activity in Guangxi region. However, due to the limitation of dating materials, the chronology of Napo fault zone is very scarce. In order to solve this problem, in this paper, three recrystallized carbonate samples from an R-shear fault in the north section of Napo fault system were collected for ESR analysis. The results show the chronology of this fault activity is approximately 200kaBP. Based on the Riedel shear model, it can be speculated that the main fault of the Napo fault system may have undergone tectonic activity after 200kaBP. Consequently, recrystallized carbonates produced by fault activity in the carbonate bedrock region have broad application prospects as dating materials that directly record fault activity information.

QUATERNARY TECTONIC FEATURES OF THE FUMAYING HIDDEN AREA OF THE ANQIU-JUXIAN FAULT

WANG Lei, XU Hong-tai, WANG Zhi-cai, YANG Chuan-cheng, ZHANG Jian-min, WANG Dong-lei, XIA Nuan, CAI Ming-gang, LU Ren-qi, REN Zhi-kun

2022, 44(5):  1156-1171.  DOI: 10.3969/j.issn.0253-4967.2022.05.005

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The Anqiu-Juxian Fault is an important seismogenic fault in the eastern China, along which many strong earthquakes occurred in history. From north to south, the fault can be divided into Anqiu section, Juxian-Tancheng section and Xinyi-Sihong section. According to the spatial distribution, occurrence and activity characteristics of the fault, the Anqiu section also can be divided into five sub-sections, which are the north of Changyi sub-section, the Changyi-Nanliu sub-section, the Anqiu-Mengtuan sub-section, the Qingfengling sub-section and the Mengyan sub-section. Since the late Quaternary, the activity of the Anqiu-Juxian Fault can be divided into two branches, namely, the west branch F_5-1 and the east branch F_5-2. There is a hidden area of the fault around Fumaying village, Weifang, Shandong Province. In order to find out the fault features of the hidden area on the Anqiu-Changyi segment of the fault, the geological-geomorphological investigation, shallow artificial seismic prospecting, combined drilling, trenching and OSL dating were carried out. Through the above work, we obtained the following understandings: 1)The results of geological and geomorphological survey and shallow seismic profiles show that the Meicun-Shuangguan segment of Anqiu-Juxian Fault can also be divided into F_5-1 and F_5-2. The branch F_5-2 of the fault is hidden in the Quaternary layers, and the west branch F_5-1 exposes at the east slope of the hills. The area between the two branches of the fault is the Fumaying hidden area in this paper. 2)The combined drilling section in the Fumaying hidden area shows that the east branch F_5-2 passes through between the drillings Z₄ and Z₅, and the upper breakpoint can be inferred to extend to the interior of the layer w₂ of Heituhu formation of Holocene series, buried at the depth of 4.2~6.9m. The shell was sampled as ¹⁴C dating sample at the bottom of Heituhu formation, and the result from the sample No. ¹⁴C-1 is(9.79±0.03)kaBP, indicating that the latest active age of the east branch is the Holocene. 3)Between the two branches, a long strip-shaped Quaternary basin is formed along the east branch fault. The Quaternary at the west side of the fault developed well, and the layers of lower Pleistocene, middle Pleistocene, upper Pleistocene and Holocene can be seen in the drilling cores; Only the upper Pleistocene and Holocene deposited at the east side. This phenomenon indicates that the Fumaying Basin deposited in the early-middle Quaternary and has the characteristics of the faulted basin. 4)There are different activities in different periods between the two branches of the Anqiu-Juxian Fault. The Fumaying hidden segment of the east branch F_5-2 was active obviously in the early-middle Quaternary, and still active since the late Quaternary. According to the geological and geomorphological survey, there is a set of the late Pleistocene yellow silt strata offset by the west branch F_5-1, indicating that the latest active age of the west branch is the late Pleistocene.

RESEARCH ON INTEGRATING INTERSEISMIC DEFORMATION RATE FIELDS OF MULTI-TRACK INSAR

HUA Jun, GONG Wen-yu, SHAN Xin-jian, WANG Zhen-jie, JI Ling-yun, LIU Chuan-jin, LI Yong-sheng

2022, 44(5):  1172-1189.  DOI: 10.3969/j.issn.0253-4967.2022.05.006

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With the rapid development of interferometric synthetic aperture radar(InSAR)technique, massive and high-quality interferograms make the surface displacement monitoring over a larger area become available. And it has become a daily and professional work to acquire large-scale surface deformation in many countries or regions. However, the standard frame width of spaceborne SAR imagery is limited. Generally, the width of single SAR image is tens of kilometers or even hundreds of kilometers. As a result, large research area may not be covered by single track, such as interseismic displacements in Tibetan plateau and coseismic displacements in Wenchuan earthquake. It is necessary and crucial to integrate multi-track InSAR displacement on the purpose of acquiring large-scale surface deformation observations. This study focuses on the application requirements of large-scale interseismic deformation rate field reconstruction. We first generated a simulated dataset and quantitatively analyzed the error sources in integrating multi-track InSAR displacement rate products into a regional displacement map, including the inconsistency of reference point, side looking incident angle and other non-deformation signals(residual atmospheric errors, time difference of acquiring SAR images, etc.). Especially the influence of incident angle of different tracks was quantitatively explained and discussed. The deformation rate obtained by InSAR technology is relative to a specific reference point, which is a specific coherent target selected in the processing of InSAR data, and its active characteristics are often unknown. This makes the deformation rate field of different tracks have a systematic deviation, which needs to be compensated for integration. GPS displacement rate is usually applied to uniform reference datum. The imaging time of multi-track SAR images is different, and the length of time is also different, which makes the deformation rate in the overlapping area of adjacent tracks different. This is mainly caused by the nonlinear component of the deformation rate. When the time span and imaging time are not different, the images of observation time are generally not considered in interseismic deformation observation. Through the simulated experiment, it is found that the influence of incident angle must be considered to obtain high-precision and large-scale InSAR deformation rate field. And a second-order polynomial could be used to correct incident angle. Afterwards, we proposed a ratio method based on polynomial estimation to correct the incident angle. Additionally, the application scenario and effect of integration after transforming to ALD(Azimuth Look Direction)are discussed and analyzed for large-scale strike-slip faults. This study takes the southeastern Tibetan plateau as an experimental site. The integration of large-scale InSAR deformation rate field is conducted and analyzed. The GPS horizontal velocity field covered by InSAR data is used to calibrate the reference datum. In order to obtain the InSAR deformation corresponding to GPS station, we take the GPS point as the center with a radius of 5km, and use the inverse distance weighting method to obtain the InSAR deformation rate value. After the reference datum is corrected, the difference of InSAR deformation rate in overlapping areas is significantly reduced. The developed ratio method and conversion to ALD direction are used to integrate the InSAR displacement products reconstructed based on Sentinel-1 satellite data. The results show impacts of incident angle differences on the adjacent tracks and also prove the reliability of the strategies applied in this study. Under the condition of ensuring that the time coverage of multi-track SAR data is basically the same, the difference between the reference datum and the incident angle should be taken into consideration firstly to realize the fusion and integration of multi-track InSAR deformation rate field. And the developed ratio method and conversion to ALD direction are able to suppress or avoid the impact of incident angle for acquiring large-scale deformation field with high precision and high resolution. The ratio method based on polynomial estimation could correct the influence of incident angle. But the final result corrected by ratio method is located in the unilateral side view reference coordinate system, which is likely to cause far-field deformation and visual distortion. The strategy of conversion to ALD direction could also be applied for integration of large-scale InSAR deformation rate field, and is suitable for large-scale strike-slip fault areas dominated by horizontal displacement. The two strategies are of great significance to realize large-scale and high-precision InSAR deformation rate field reconstruction and they are successfully applied to integration of multi-track InSAR deformation products in the southeast of Tibetan plateau. And the strategy of conversion to ALD direction is slightly better than the ratio method, which shows that ALD transformation strategy is more suitable for southeastern Tibetan plateau. It shows that the actual displacements of the target area are mainly dominated by horizontal displacements, and the premise assumption of ALD conversion strategy is basically tenable. There is no unknown parameter estimation in the ALD conversion strategy, and it is simple and direct, while the ratio method requires polynomial fitting of the overlapping region, which is affected by the residual error in the deformation rate field.

COSEISMIC SLIP DISTRIBUTION OF THE 2021 M_W5.8 BIRU(TIBET, CHINA)EARTHQUAKE AND THE COULOMB STRESS VARIATION

YU Shu-yuan, ZHANG Guo-hong, ZHANG Ying-feng, DING Juan, ZHANG Jian-long, FAN Xiao-ran, WANG Shao-jun

2022, 44(5):  1190-1202.  DOI: 10.3969/j.issn.0253-4967.2022.05.007

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On March 19, 2021, an earthquake of M_W5.8 occurred in Ruju County, Tibet. The epicenter of the earthquake is located to the west of Xiaqu town in the Qiangtang Basin in the north of the Qinghai Tibet Plateau. The regional structure around the earthquake is complex and there are many faults developed there. To the south of the epicenter is Peak Shaqiongya and to its west is Peak Ceduo, both are high-altitude peaks. Since 1970(up to July 22, 2021), 27 earthquakes with M_W≥5 have occurred within 200km of the area, most of which are of normal faulting, indicating that tensile stress plays a dominant role in the area. The largest earthquake is the Naqu earthquake with M_W5.9 that occurred near the Bengco Fault in 1972. Therefore, this earthquake is a rare medium strong earthquake in this area. Studying the seismogenic structure of this earthquake has reference significance for understanding the geological structure, fault movement characteristics and seismic rupture attributes of normal faulting earthquakes in this area. In this paper, the coseismic deformation field of this earthquake is obtained by using D-InSAR technology. It shows that the maximum uplift and subsidence are 5cm and 6cm respectively, the long axis direction is NE, and the LOS deformation signs of ascending and descending orbit are the same. It is preliminarily determined that the seismogenic fault is a normal fault, striking NE. On this basis, based on the SDM software and Okada method, the initial seismogenic fault is constructed with a length of 27.56km(92.78°~93.00°E), a width of 20km(set in the SDM program), a dip angle of 50°~63°, and a rake angle of -150°~0°; the smoothing factor is set to 0.08, and the maximum number of iterations is 10000. Secondly, the fault dip angle of the selected range is tested with a step of 1°, and the optimal fault dip angle is determined to be 55°. On this basis, a single fault model is used to divide the fault plane into 1km×1km fine slip distribution model of 1km sub fault structure. Finally, the distribution characteristics of coseismic slip on fault plane are inversed by SDM. For T143 of ascending orbit, the residual RMS is 1.0cm, and for T77 of descending orbit, the residual RMS is 0.4cm. The combined ascending/descending residual range is -0.016~0.015m, and the residual RMS is 0.3cm. Through comprehensive analysis, the model fitting of ascending orbit data is good, which indicates that the inversion result of slip distribution is reliable. According to the determined optimal parameters of the fault, the fine slip distribution model of the fault is drawn. The sliding surface presents an elliptical centralized distribution as a whole, and the maximum sliding amount reaches 0.2m; The average rake obtained by inversion is about -55.56°, the moment magnitude is M_W5.8, the macro epicenter is located at(31.94°N, 92.85°E)and the seismogenic fault is a normal fault with left-lateral strike-slip component.
By setting the friction coefficient to 0.4, taking the Young’s modulus as 3×10¹⁰ and Poisson’s coefficient of 0.25, the coseismic Coulomb stress changes triggered at different depths are calculated with column algorithm at intervals of 5km. The results show that the coseismic Coulomb stress with the seismogenic fault as the receiving fault has the coseismic effect mainly in two directions: One is the NE-SW direction, which is mainly manifested as the Coulomb stress increase in the North Nierong Fault at the southern edge of the Qiangtang block and some sections of the Bangonghu-Nujiang fault zone, and the influence range decreases with the increase of the depth. The other direction is NW-SE, and the Coulomb stress decreases and changes with the increase of depth. The Coulomb stress image of 5km underground shows the alternating distribution of positive and negative stress changes in strike-slip earthquakes. The area with the largest increase in stress is located in the northeast of the Biru earthquake and the stress is partially released. The stress increase at 5km northeast of the epicenter of Biru earthquake turns to stress decrease at 10km northeast of the earthquake, the stress increase area decreases in the northeast and expands in the southwest of the main shock. At the depth of 15km, the stress in the north of the epicenter further decreases, and the stress in the southwest of the epicenter further increases. At the same time, the stress decrease area is relatively large, and the stress decreases in most areas near the epicenter of the earthquake. The slip amount of the fault in the deep is very small, so the magnitude of stress increase at the depth of 20km is relatively small, and the corresponding stress decrease area is relatively large. On the whole, most of the subsequent earthquake events occur at the depth of 5~15km, which is consistent with the stress increase area at the corresponding depth. At the same time, the stress increase at the depth of 5~20km is located at the south and north end of the rupture zone, and its ΔCFS≥0.01MPa, so the seismic risk is worthy of attention. Based on the comprehensive analysis, this paper calculates and analyzes the variation and distribution characteristics of Coulomb stress by taking the North Nierong Fault and the Bangonghu Nujiang fault zone as the receiving faults. The Coulomb stress generated by the earthquake in the depth of 5~20km is calculated by using the above two receiving faults, which is mainly negative. However, the earthquake has obvious Coulomb stress loading effect on the west section of Bangonghu-Nujiang Fault and the east section of North Nierong fault zone. Through comprehensive analysis, the Biru earthquake has a certain static stress loading effect on the above two receiving faults, and there has been no strong earthquake on the two faults for a long time, which needs attention. The Qinghai-Tibet Plateau is composed of several sub blocks. The plate movement is dominated by horizontal and vertical uplift in the east-west direction, and many large active fault zones are developed along the block boundaries. This earthquake is located in the Qiangtang block in the central part of the Qinghai-Tibet Plateau.
There are many conjugate shear faults developed in the block. Among them, the Bangonghu-Nujiang fault zone is a large-scale near EW-strike fault zone at the southern boundary of Qiangtang Basin. According to the interpretation of remote sensing images and historical geological data, medium strong earthquakes of M_S5~6 occurring in this area in the history are all located at the intersection of the NE trending fault. The epicenter of the M_W5.8 Biru earthquake is located at the intersection of the NE-trending Bangong Lake-Nujiang fault zone and the NW-trending North Nierong fault zone. This paper considers that the occurrence of this earthquake is related to the tectonic environment where the Qiangtang block locates, and the slip type of the seismogenic fault has the typical characteristic of seismic rupture inside the Qiangtang block. Judging from the continuous interference fringes of the deformation field of the ascending track, the coseismic rupture did not reach the surface. At the same time, the coseismic surface deformation of this earthquake is concentrated around the North Nierong Fault and Bangong Lake-Nujiang fault zone, which are 9km and 10km away from the earthquake respectively, and the seismogenic fault dips to the northwest. According to the historical research data of the study area, this paper preliminarily believes that the seismogenic fault of this earthquake is a NE-direction hidden secondary fault located in the north of the west section of the Bangong Lake-Nujiang fault zone. The fault activity is mainly of normal faulting with a small amount of strike slip. The relationship between this fault and the main fault needs to be determined comprehensively based on the on-site seismo-geological survey.

INFLUENCE OF GROUNDWATER AND LAND SUBSIDENCE ON THE MOBILE GRAVIMETRY ALONG THE YISHU FAULT ZONE

LI Shu-peng, ZHU Yi-qing, JIA Yuan, CUI Hua-wei, YIN Hai-tao, WU Shuang, WANG Feng-ji, LU Han-peng

2022, 44(5):  1203-1224.  DOI: 10.3969/j.issn.0253-4967.2022.05.008

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Yishu fault zone is the Shandong section of the Tanlu fault zone, and it is the structural boundary between the Luxi fault block and the Ludong fault block. In history, the Tancheng M_S8¹/₂earthquake in 1668 and the Anqiu M_S7.0 earthquake in 70BC occurred with serious casualties and property losses. The Anqiu-Juxian Fault is a Holocene active fault in the Yishu fault zone. Predecessors have conducted a lot of research on the fault by means of field geological survey, magnetotelluric measurement, small earthquake activity analysis, GPS velocity field, etc. and found that the north section of the Yishu fault zone is currently in the locking stage and has the potential for strong earthquakes with M_S≥7.
The mobile gravimetry is an effective means for medium and long-term prediction of strong earthquakes. The gravimetry along the Yishu fault zone was conducted using the LCR-G and CG-5 relative gravimeters every half year from 2010 to 2020. The mobile gravimetry anomalies occurred in the north section of the Yishu fault zone in 2019, and the area was delineated as an earthquake risk area based on the mobile gravimetry anomaly. Then the anomaly was confirmed to be caused by groundwater decrement. Therefore, it is necessary to make clear whether the mobile gravimetry anomalies are caused by tectonic or non-tectonic background field variations.
In order to provide effective mobile gravimetry data for medium and long-term forecasting of earthquakes, we divide the Yishu fault zone area into plain area and mountainous hilly area according to the different geographical landforms to study the tectonic background and characteristics of stratum lithology, geomorphology and hydrogeology of the two types of areas. Based on the 22 periods of high-precision mobile gravity data from 2010 to 2020, the high temporal-spatial resolution GPS images and the distribution variation characteristics of groundwater are analyzed, the influence of groundwater and vertical crustal deformation on gravimetry is calculated quantitatively using the infinite plane layered model and the approximate proportional relationship between gravity and vertical crustal deformation, and the spatial distribution of gravity effects of groundwater and vertical deformation and the corrected gravity variation image are obtained. Then, combined with the leveling and GPS research results in recent years, the possibility of earthquake in the near future in the Yishu fault zone is judged. Finally, the time-sequence curve of gravity and gravity effects of groundwater and vertical deformation of Guangrao, Shouguang and Changyi stations are drawn to summarize the time-varying characteristics and synchronization of groundwater, vertical deformation and gravity.
Through the above correction processing, we find that:
(1)The multi-year gravity anomaly in the north section of the Yishu fault zone since August 2010 has a strong correlation with the decline of groundwater level and land subsidence in the region. The maximum impact of groundwater lowering on gravity is -102.21μGal, and the maximum impact of land subsidence is 190.56μGal. The superposition effect is 195.72μGal. The gravity field after correction is stable as a whole, and the variation in the positive and negative transition regions is not dramatic, indicating that there is no large differential movement of underground materials. Combined with the leveling and GPS research data, it is speculated that there is less possibility of strong earthquakes in the north section of the Yishu fault zone in the near future.
(2)The influence of groundwater and land subsidence on the mobile gravimetry mainly occurs in the north section of the Yishu fault zone, which is distributed in Quaternary strata, and there is little such interference in the bedrock-outcropped middle and south section of the Yishu fault zone. When gravity anomaly occurs in the plain area due to the variation of non-structural background field, it is likely to misidentify it as an earthquake precursor. Therefore, in the study of gravity anomaly, groundwater and vertical crustal deformation correction should be carried out first to eliminate the gravity anomaly caused by the variation of the non-tectonic background field.
(3)Deep confined water funnel area is often accompanied by land subsidence and obvious gravity variations. Phreatic decline generally does not cause land subsidence, but water loss will cause negative gravity change to a certain extent.
(4)In the daily correction of mobile gravimetry data, the mean annual correction constant can be calculated according to the magnitude variation law of groundwater and vertical crustal deformation in recent years, which has certain positive significance for the analysis, application and related research of mobile gravimetry data.

GEOCHEMICAL CHARACTERIZATION OF FAULT GAS IN MACRO-SEISMIC INTENSITY AND AFTERSHOCK DISTRIBUTION OF JINGHE M_S6.6 EARTHQUAKE ON AUGUST 9, 2017

ZHU Cheng-ying, YAN Wei, MA Rong, LI Zhi-hai, WANG Cheng-guo, HUANG Jian-ming, ZHOU Xiao-cheng

2022, 44(5):  1225-1239.  DOI: 10.3969/j.issn.0253-4967.2022.05.009

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An M_S6.6 earthquake struck Jinghe County, Boltala Prefecture, Xinjiang at 7:27:52 am on August 9, 2017, with the epicenter located in the eastern section of the Cusongmuqike piedmont fault. In order to study the activity characteristics of the faults around Jinghe after the M_S6.6 earthquake, the field survey and selection of the Cusongmuqike piedmont fault and the Jinghe section of the Bolokenu-Aqikekuduke Fault were carried out from October 18 to November 3, 2017. Mobile monitoring of soil gas geochemistry was carried out at 191 survey points respectively on 12 survey lines of Cusongmuqike piedmont fault and two survey lines on the Jinghe section of Bolokenu-Aqikekuduke Fault, and the concentrations of Rn, H₂ and CO₂ emitted from soil across the fault were observed. The results show that: 1)The concentrations of soil gases Rn, CO₂ and H₂ were consistent, and the high values appeared near the faults, with obvious single-peak characteristics. 2)The concentration of Rn, CO₂ and H₂ increased gradually from west to east and reached the highest value near the main shock and aftershock areas, and the concentration of soil gas reached the lowest value in the Jinghe section of the Bolokenu-Aqikekuduke Fault. 3)There is a good consistency between Rn and CO₂ concentration anomaly intensity, and the distribution regions of both are basically opposite to H₂ concentration anomaly intensity. The high values of soil gas Rn and CO₂ anomaly intensity mainly concentrated in the west of the epicenter area of Jinghe M_S6.6 earthquake(line 7, 8, 9 and 10 area), which was the aftershock concentration area, and the lowest value of H₂ anomaly intensity appeared in this area. Reason may be that the aftershock concentration area was also the fault activity enhancement area, with the occurrence of aftershocks, stress increased, the permeability of rock decreased, and the gas in fault pores released sustainedly, causing the concentration change of gases emitted from fault. Since Rn cannot participate in the migration activities such as diffusion and convection, it was brought to the surface from deep underground carried by CO₂, this could explain the abnormal synchronization of Rn and CO₂. Hydrogen, as the element with the smallest particle size, lightest mass, fastest migration speed and strongest penetration, mainly comes from the trapped hydrogen in the pores and fissures of deep rock. After the earthquake, the pores in the rock were damaged, and hydrogen rapidly emitted to the shallow part or atmosphere. On the one hand, with the occurrence of aftershocks, the amount of H₂ from shallow gas reservoirs was decreasing. On the other hand, the generation of H₂ produced by chemical reactions in rocks decreased with the decrease of fault activity. The measurement was done more than 70 days after the occurrence of the main shock, during which the activity of strong aftershocks tended to end and the activity of the fault also weakened. 4)Concentrations of soil gas Rn, CO₂ and H₂ have a good consistency with the distribution of aftershocks and the macroseismic intensity, so, carrying out in a timely manner the soil gas geochemical observation is an effective means and method for earthquake trend judgment, soil gas geochemical observation in and around the earthquake area is a kind of effective means and methods for earthquake trend judgement, and the cross-fault soil gas concentrations can also be used as an important supplement to the macroseismic intensity evaluation.

MODIS LST RECONSTRUCTION ALGORITHM BASED ON DIURNAL CORRELATION OF SURFACE TEMPERATURE OF SIMILAR LAND FEATURES

CUI Jian-yong, ZHANG Man-yu, SONG Dong-mei, LUO Sheng, SHAN Xin-jian, WANG Bin

2022, 44(5):  1240-1256.  DOI: 10.3969/j.issn.0253-4967.2022.05.010

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Land surface temperature(LST) is an indispensable parameter for studying the exchange of matter and energy between the earth surface and the atmosphere, the changes of surface processes, geothermal detection and precursors of seismic thermal anomalies, etc. However, cloud cover phenomenon results in a large number of null values in MODIS LST products, which limits the wide application of LST. In this study, a MODIS LST reconstruction algorithm based on LST diurnal variation correlation of similar land features at different time is proposed. Taking Hotan, Xinjiang as the research area, and using MODIS 8-day LST products from 2003 to 2015 as experimental data, based on the correlation of LST between similar features at different times of a day, with the help of the surface cover type, the regression models of morning, afternoon, evening and early morning surface temperature were established respectively. The LST fitting of the three was regressed to the LST of the early morning, and then the optimal combination of the fitting results of morning, afternoon and evening was taken to achieve the two-step reconstruction of the early morning LST. The experimental results show that the minimum error of this method is 0.57K, the errors are all below 1.2K, and the average error is 0.92K. It is proved that this method has also a good complement effect when applied to the reconstruction of surface temperature of the remaining three time periods. Compared with the existing LST complement methods, the proposed method achieves higher accuracy and rate of complement with a small amount of auxiliary data, which can provide a solid data basis for the research of surface process based on temperature and seismic thermal anomaly detection. The main conclusions of this study are verified as follows:
(1)By establishing regression models for MOD11A2 morning and evening and MYD11A2 afternoon and MYD11A2 early morning surface temperature of different land cover types, it can be seen that second-order polynomial fitting effect is better and the model has better stability.
(2)For the study area, the accuracy of using night data to complement the early morning LST is the highest, but the spatio-temporal continuity of reconstructed LST cannot be completely guaranteed. If morning and afternoon LST data are added, the complement rate can be further improved.
(3)The reconstruction data of land surface temperature based on this method has better regional consistency and no step change. Compared with the LST compensation method proposed in recent five years, it is found that this method uses less auxiliary data and has relatively high accuracy of compensation. The errors after compensation are all below 1.2K, with an average error of 0.92K and a minimum error of 0.57K, which can meet the requirements of surface temperature compensation.
(4)At the same time, the missing data of MODISLST data in the other three time periods of a day are reconstructed respectively in this paper, with high reconstruction accuracy. Based on this method, the accuracy of data reconstruction in the early morning and evening is higher than that in the morning and afternoon. Therefore, in the future anomaly extraction work, the evening or early morning data can be given priority as the main data source, so as to achieve accurate anomaly extraction.
(5)In order to verify the universality of this method in different regions, two sub-regions with different land cover types and locations in the study area were randomly selected to carry out regional usability evaluation, and the results showed that the accuracy of reconstruction in both regions was high, and missing value reconstruction could be achieved.
In conclusion, the proposed method in this paper provides a new idea and method for MODIS land surface temperature reconstruction. Compared with the existing LST complementary method, the proposed method achieves higher complementary accuracy and complementary rate with a small amount of auxiliary data, which can provide a solid data foundation for the study of surface processes based on temperature. In addition, the complete surface temperature data after reconstruction can provide data support for geothermal anomaly detection and extraction, and lay a theoretical foundation for the interpretation of seismic thermal anomaly mechanism.

STUDY ON FINE CRUSTAL STRUCTURE OF THE SANHE-PINGGU EARTHQUAKE REGION BY HIGH-RESOLUTION SPECTRAL DECOMPOSITION METHOD

LI Qian, FENG Shao-ying, QIN Jing-jing, TIAN Yi-ming

2022, 44(5):  1257-1272.  DOI: 10.3969/j.issn.0253-4967.2022.05.011

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Deep seismic reflection detection technology is considered to be one of the effective technical means to detect the fine lithospheric structure and the deep structure in earth’s interior. However, the detection depth of deep seismic reflection technology is large, and the geological tectonics of target area are usually complex(such as there are many fault zones, the terrain elevation changes greatly), which makes the explosion condition of source and the receiving condition of seismic wave very poor in the process of seismic data acquisition. This results in that the deep seismic reflection data has low frequency, narrow frequency band, low signal-to-noise ratio and resolution, and weak reflection energy. In addition, due to the existence of various forms of complex geological bodies in the deep crust and the energy absorption and attenuation caused by the long-distance propagation of seismic wave, etc., the reflection wave groups of the middle and lower crust in the deep seismic reflection profile is often characterized by weak reflection energy, poor continuity, interleaved or zonal distribution, which brings difficulties to the correct interpretation of seismic data and the understanding of geological structure.
With the emergence of high-resolution time-frequency analysis methods, it becomes more and more meaningful to try to use new time-frequency analysis methods to analyze the time-frequency distribution characteristics and phase extraction of deep seismic reflection data. The sparse constraint inverse spectral decomposition(SCISD)is a time-frequency analysis method based on the unsteady convolution model. This method describes the spectral decomposition problem as a linear inversion problem, in which minimizing the l1 norm of the time-frequency spectrum of the seismic signal is adopted as a sparsity constraint term, and it has the ability to generate high-resolution time-frequency spectrum and phase spectrum. Compared with transform-based spectral decomposition methods(such as short-time Fourier transform, continuous-wavelet transform and S-transform), this method has higher time-frequency resolution, more concentrated time-frequency distribution, and good anti-noise performance. It can extract the corresponding time-varying wavelet phase information, and is not affected by the shape and length of time window. Due to these good features, the SCISD method has achieved certain results in low-frequency anomaly detection, reservoir identification and carbonate cave-type reservoir prediction for petroleum geophysical exploration. On the one hand, it can provide high time resolution for thin reservoir prediction, on the other hand, it can provide wavelet phase change characteristics for gas reservoir detection and carbonate cave identification. However, it has not yet been applied to deep seismic reflection data. Therefore, this paper applies the SCISD method to the deep seismic reflection data in the Sanhe-Pinggu M8.0 earthquake area. The test of single-channel data shows that this method can effectively identify the seismic amplitude anomaly and reconstruct the seismic signal more accurately. The time-frequency characteristic analysis and data reconstruction of the deep seismic reflection data in the Sanhe-Pinggu M8.0 earthquake area were carried out. It is found that the reconstructed section and time-frequency section have higher signal-to-noise ratio and resolution than the original section, and improve the continuity of the seismic profile events. It can not only calibrate the frequency components at different times of weak signals in the deep seismic reflection data, but also clearly depict the characteristics of low-frequency details on the stacked section. The high-resolution time-frequency phase spectrum has rich phase information, and the phase characteristics change significantly. It can be used for fault identification, which is helpful to judge the tectonic boundary. The wavelet reconstruction technique and the high-resolution single-frequency phase spectrum are helpful to characterize the internal reflection structure of the Moho transition zone. The SCISD method is suitable as the spectral decomposition method for deep seismic reflection data.
By analyzing the time-frequency characteristics of the original reflection seismic data and purposefully reconstructing the deep seismic reflection data, the effective weak signals in the deep crust can be extracted. The joint comparative analysis of time-frequency profile and reconstructed profile is helpful to deeply study the deep fine structure and establish an intuitive understanding of the deep structure. This method analyzes the deep seismic reflection profile from a new perspective, which is beneficial to discover new geological understandings, has very important research significance, and can be widely used in the interpretation of deep seismic reflection data.

EXPERIMENTAL STUDY ON THE P-WAVE VELOCITY OF THE CRUST IN THE LAOHUSHAN FAULT ZONE IN GANSU PROVINCE

SONG Gang, YANG Xiao-song

2022, 44(5):  1273-1289.  DOI: 10.3969/j.issn.0253-4967.2022.05.012

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The previous research shows that dense small earthquakes may occur in the Laohushan fault zone in the coming decades. Micro-earthquakes have obvious near-field characteristics due to low earthquake magnitude, high main frequency and fast energy attenuation of signal. Near-field crustal wave velocity structure, especially the shallow velocity structure, has a great impact on the precise positioning of high-frequency micro-earthquakes. To this end, the P-wave velocity (V_P) of representative rocks in this area was systematically measured at room temperature and ambient pressure of 50~500MPa. The experiment result shows that:
(1)V_P shows a logarithmic increase with pressure in the low pressure range and a linear trend increase with the pressure when exceeding the critical pressure P_c. P_c is in the range of 200~450MPa, and is 262.5MPa on average. The average P_c of crystalline rock is 250MPa and the average P_c of sandstones is 271.4MPa. The P_c of sandstones is significantly higher than that of crystalline rocks. This is attributed to the low pressure range, where the cracks and pores are closing with increasing pressure, and the cracks and pores are almost completely closed in the high pressure range.
(2)Under ambient temperature, the velocity pressure derivative in nonlinear domain of velocity-pressure curve is much larger than that in linear domain.
(3)The intrinsic velocity of intrusive rocks is greater than that of sandstone and metamorphic sandstone.
Combined with previous geothermal survey data, seismic tomography and field geological survey results, the following conclusions are obtained:
(1)Under the condition that temperature varies with depth, the influence of temperature on P-wave velocity is less than that of pressure on P-wave velocity in the depth corresponding to the pressure below the critical pressure P_c at the 500MPa pressure interval deduced by simple fitting. Temperature has a strong effect on P-wave velocity at depths where the pressure is higher than the critical pressure P_c. The experimental measurements are higher than the actual formation velocity, the difference is about 0.2142km/s.
(2)The lower limit and upper limit of mean P-wave velocities obtained by experimental measurements are smaller than the lower limit and greater than the upper limit of P-wave velocities obtained by previous inversions, respectively. It can be attributed to the high wave velocity of intrusive rocks and metamorphic rocks outcropped in the shallow part of the stratum, and the low wave velocity of sedimentary rocks in the deep part of the stratum. This is consistent with the geological survey results in this area.
(3)The combination of velocity in nonlinear domain and velocity in linear domain can provide a more real reference of P-wave velocity structure model in this region.

CONSTRAINING MOHO CHARACTERISTICS IN THE NORTH-EASTERN MARGIN OF TIBET PLATEAU WITH FREQUENCY-DEPENDENCE OF RECEIVER FUNCTION

SONG Ting, SHEN Xu-zhang, MEI Xiu-ping, JIAO Yu-yuan, LI Min-juan, SU Xiao-yun, JI Wan-jing

2022, 44(5):  1290-1312.  DOI: 10.3969/j.issn.0253-4967.2022.05.013

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The northeastern margin of Tibet Plateau is the frontier area where the Tibet Plateau expands and grows towards the interior of the continent. This area is located at the junction of the Tibet Plateau, Ordos and South China blocks. It is also located in the north central section of the north-south seismic belt, the east-west boundary of the Chinese mainland. A variety of structural factors cause dramatic changes in the depth of Moho in the region, and the structural relationship is complex. Many large active faults such as the East Kunlun Fault, the northern margin fault of the West Qinling Mountains, and the Haiyuan-Qilian Mountains Fault have developed. Understanding how the plateau expands in the northeastern margin will help to reveal the mystery of the deformation mechanism of Tibet Plateau.
In order to better understand the coupling relationship of fault systems and regional tectonic evolution in the northeastern margin of Tibet Plateau, it’s necessary to explore its crust-mantle transition zone. The material and energy of crust and mantle are constantly differentiated and exchanged in Moho discontinuity, accompanied by a series of geological evolution. Therefore, detecting the thickness and complexity of the crust-mantle transition zone can provide major information for the material and energy exchange, rock phase transformation and component changes of the crust mantle, and help to further reveal the regional geodynamic process.
In this paper, the waveforms of earthquakes from November 1, 2009 to November 30, 2011 recorded by 70 high-density temporary seismic stations in southeastern Gansu Province in the northeastern margin of the Tibet Plateau are used. Teleseismic waveform records with epicenter distance of 30°~90° and surface wave magnitude greater than or equal to 5.5 are selected. Using time-domain iterative deconvolution method, receiver functions of selected data are calculated. Then, by changing the value of Gaussian filter factor in deconvolution, we get receiver functions of different frequency. By analyzing each stations’ frequency-dependence of receiver functions, i.e. the multiple wave crests that occurred in the Moho Ps converted phase and its multiples when the frequency is high, and classifying and summarizing them, we got 5 types of frequency-dependence of receiver functions. Based on our previous numerical test results, we characterized the corresponding S-wave velocity structure in the crust-mantle transition zone beneath the station by fitting the observation structure. In order to explore the characteristics of the lateral continuous change of the crustal structure under the observation array, we stacked the receiver functions along the profile according to the transmission conversion point. At the same time, in order to eliminate the time difference of seismic phase caused by different epicentral distance and focal depth, and highlight the Ps converted wave and its multiples respectively, we set the ray parameter to 0.065s/deg before stacking, and correct the receiver functions of three frequencies of different seismic phases respectively. Then, the frequency characteristics of the receiver function stacked along the profile are analyzed and the corresponding S-wave velocity structure of the Moho is obtained. The results show that the frequency-dependence of the Moho-related receiver functions in the study area is variable, particularly obviously different between the two sides of most fault zones, reflecting the complexity and non-uniformity of the structure of crust-mantle transition zone. The S-wave velocity we constructed is not a first-order discontinuity but gradually increasing with the increase of depth. It may indicate the high viscosity of the crust beneath the research area, which is not conducive to the flow of lower crustal material. Besides, it can be inferred that there may be some mafic material in the crust-mantle boundary. This complex transition zone between the crust and the mantle in this area can be considered as the effect of upwelling and underplating of hot materials in the upper mantle.

ANALYSIS OF THE SEISMOGENIC STRUCTURE OF THE JUNE 2021 M_S5.8 MANG’AI EARTHQUAKE IN NORTHERN QAIDAM BASIN

ZHANG Bo-xuan, ZHENG Wen-jun, CHEN Jie, HE Xiao-hui, LI Qi-lei, ZHANG Dong-li, DUAN Lei, CHEN Gan

2022, 44(5):  1313-1332.  DOI: 10.3969/j.issn.0253-4967.2022.05.014

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The M_S5.8 Mang’ai earthquake occurred in northwest Qaidam Basin on June 16, 2021. There are the Santai reverse fault-fold belt, Xiaoqulin reverse fault-fold belt, Lenghu reverse fault-fold belt and Eboliang reverse fault-fold belt developed in the seismic area from north to south, which converge along the direction of the Altun Fault in the northwest and spread out in the southeast basin. As the boundary between the southern margin of Qilian Shan and the northern margin of Qaidam Basin, the Mesozoic and Cenozoic tectonic deformation is very complex. The study of seismogenic capacity and mechanism of fold-related faults in this area is beneficial to further understand the strain distribution pattern and tectonic deformation mechanism between the southern Qilian Shan and the northern margin of Qaidam Basin.
This earthquake occurred in the eastern section of the NW-trending Lenghu anticline in the northwestern margin of Qaidam Basin. The focal depth of the earthquake is about 13km determined by using CAP method, and the focal mechanism solution is thrust type. The parameters of the double-couple nodal planes obtained in this paper are similar to those obtained by GFZ and USGS, respectively. Combined with surface geology, satellite images and seismic reflection profile interpretation, it is considered that the earthquake occurred in the southeast of the Lenghu reverse fault-fold belt, and the seismogenic structure may be one of two deep concealed thrust faults dipping north and south, respectively, which control the growth of the anticline under the east of Lenghu No.7 anticline.
According to the position of the growth strata interpreted by the seismic reflection profile, the latest rapid deformation in the eastern section of Lenghu anticline began around the sedimentary period of the Shizigou Formation in the Late Cenozoic, and the activity has continued till now.
This M_S5.8 Mang’ai earthquake only partially ruptured the underlying thrust fault of the east of Lenghu No.7 anticline, but did not rupture to the surface, so it was a typical folding earthquake.
According to the empirical formula of the rupture area-magnitude of reverse fault, we have estimated the upper limit of the maximum magnitude of many Quaternary active anticlines in the seismic region. All of these structures have the structural condition for generating M_W5.9~7.2 earthquakes, and may cause strong earthquakes of magnitude 7 or above due to cascading earthquake rupture. Therefore, the seismic risk of the earthquake area in the future cannot be ignored, and it is urgent to carry out more detailed research on the activity behavior of these structures.

SPATIOTEMPORAL HETEROGENEITY AND APPLICATION OF b VALUES IN HYDRAULIC FRACTURING INDUCED SEISMICITY

JIANG Cong, JIANG Chang-sheng, YIN Xin-xin, WANG Rui-jia, ZHAI Hong-yu, ZHANG Yan-bao, LAI Gui-juan, YIN Feng-ling

2022, 44(5):  1333-1349.  DOI: 10.3969/j.issn.0253-4967.2022.05.015

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Induced earthquakes and the corresponding seismic risk are rising concerns for the smooth implementation of new industrial activities such as the exploitation of unconventional oil and gas resources and has attracted broad attention from both the public and academia. As a result, many associated scientific problems need to be further examined. The magnitude-frequency distribution(FMD)is fundamental to seismicity characterization, where systematic study of b values for induced earthquakes could reveal the regional accumulated stress, subsurface structural characteristics, as well as seismic risks of induced seismicity.
In this study, we systematically reviewed the values, spatial-temporal heterogeneity, physical mechanism, dominating factors, as well as the application status of b values on hydraulic fracturing induced earthquakes in the past ten years. Multiple case analysis shows that the b value varies over a wide range(0.6~2.9)and exhibits large spatiotemporal heterogeneity. Felt earthquakes are often preceded by decreased b values and often occur in the regions with relatively low b backgrounds. In addition, fault activation or felt induced earthquakes are often accompanied by b values less than 1.0, despite that b>1.0 are commonly observed in the process of fracture expansion. Thus, the b value is promising for estimation of the state of faults(i.e., maturity and criticality).
This study further assessed the factors that may affect b values, including objective factors such as in-situ stress field, fault geometry, fault maturity and focal depth, as well as subjective factors associated specific construction conditions, such as injection volume and injection rate. We then summarized multiple possible physical mechanisms, including the pore pressure, in-situ differential stress, maximum shear stress, and the non-uniformity of geological conditions. Although the discussed factors and physical mechanisms imply the multiple complexities associated with the b value that may challenge the effectiveness of its utilization, the b value remains a preliminary but effective evaluation for first-order estimation induced-earthquake hazard. For example, in the cases of deep hypocenter, high differential stress, high fault maturity, developed initial fracture network or bedding, or when the pore pressure, fault geometry and in-situ stress field meet the conditions of high probability fault slip tendency, the b value is often less than 1.0. In fact, due to the limited understanding of the seismic risk induced by hydraulic fracturing, b value still serves as a key parameter for the seismic risk analysis such as earthquake rate prediction and maximum magnitude prediction, as well as risk control technologies such as “Traffic Light System”(TLS), the b value has been widely used in hydraulic fracturing.
Finally, we discussed the misunderstandings and challenges of b-value estimations for hydraulic fracturing induced earthquakes. For instance, b values calculated from different methods are less comparable and the quality of seismic catalogues, especially the reliability of magnitude measurement, also impact the accurate estimation and physical interpretation of b value. In addition, the mutation point of when the fault is about to reach its critical stage cannot be accurately identified through the temporal evolution of the b value alone. Even in cases where the mutation point is identified, the shut-down of current industrial operation does not guarantee the prevention of a subsequent felt event. Such challenges limit the effective utilizations of b values toward mitigating the seismic hazard associated with hydraulic fracturing induced earthquakes.
After clarifying the consensus and controversial scientific issues, we speculate that the b value for induced earthquakes may serve as one preliminary criterion for the evaluation of reservoir reconstruction, the estimation of reservoir stress state and the mitigation of induced seismicity hazard. Our study summarized and evaluated the b-value characteristics for hydraulic fracturing induced earthquakes. The paper could serve as a scientific reference for the industrial, regulating and research communities that are interested in non-conventional energy exploration and/or seismic safety supervision.