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20 October 2024, Volume 46 Issue 5

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SEISMOGENIC FAULT OF THE TANGSHAN M_S5.1 EARTHQUAKE ON JULY 12, 2020 AND ITS IMPLICATIONS FOR REGIONAL TECTONICS

CAO Jun, ZHOU Yi, GAO Chen, LIU Shu-feng, CHEN An, ZHANG Su-xin, FENG Xiang-dong, WU Peng, CHEN Zhao-dong

2024, 46(5):  993-1011.  DOI: 10.3969/j.issn.0253-4967.2024.05.001

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On July 12, 2020, a M5.1 earthquake occurred in the Guye District of Tangshan City. This earthquake is notable as the only moderate seismic event exceeding magnitude 5 in the Tangshan area over the past two decades. However, the exact seismogenic fault responsible for this earthquake remains undetermined, complicating efforts to assess future seismic risks in the region. Post-earthquake damage assessments revealed that the macroseismic damage was distributed along two primary fault zones: a long northwest(NW)trending band and a short northeast(NE)trending band. The most significant damage occurred at the intersection of these two bands. Based on the regional geological structure and stratigraphy, field surveys identified the NE-trending Tangshan-Guye fault as a Holocene-active fault, while the NW-trending Mozhouyu fault was classified as a Quaternary fault within the area of greatest damage. Analysis of Sentinel-1A InSAR time-series data revealed differential deformation along the Mozhouyu fault. Relocation results of earthquakes greater than magnitude 1.0 over the past decade in the Tangshan region showed seismic activity distributed in two primary bands. One band aligns with the NE-trending Tangshan-Guye fault, with concentrated activity at its intersection with the Mozhouyu fault. Following the M5.1 earthquake, multiple authorities determined that the focal mechanism indicated a strike-slip earthquake, with two conjugate planes oriented in the NE and NW directions. This finding is consistent with the alignments of the Tangshan-Guye and Mozhouyu faults. Through comprehensive analysis, including post-earthquake field surveys, regional deformation data, and the relocation of smaller seismic events, it was concluded that the surface damage from the Tangshan Guye earthquake followed both NE and NW orientations. Of the two intersecting faults in the damaged area, the Mozhouyu fault is a middle Pleistocene fault, while the Tangshan-Guye fault is the most significant Holocene-active fault in the region. The characteristics of these conjugate faults align with both the source parameters and relocated seismic sequences of the Tangshan Guye earthquake. The right-lateral strike-slip motion along the Tangshan fault zone, combined with regional NE—NEE-directed compressive stress, likely caused the Tangshan-Guye fault to be blocked by the Qinglongshan complex anticline during its eastward expansion. Subsurface data further indicate that the Qinglongshan complex anticline marks a boundary of regional physical property differences. Therefore, it is concluded that the Tangshan-Guye fault and the Mozhouyu fault were the conjugate seismogenic faults responsible for the M5.1 earthquake on July 12, 2020.

The Tangshan Guye earthquake is a typical moderate-intensity strike-slip event in the North China Plain. An analysis of 705 focal mechanism solutions from 2002 to 2020 indicates that most earthquakes in the region are predominantly strike-slip in nature. Historical strong earthquakes in the North China Plain also exhibit high-angle strike-slip faults as their primary seismogenic structures, a conclusion supported by extensive seismological research. A substantial body of seismic studies suggests that the failure of the North China Craton during the early Cenozoic was driven by crustal extension, resulting in the formation of listric(shovel-shaped)normal faults. However, these faults are no longer the main seismogenic structures for present-day earthquakes. Since the late Pleistocene, tectonic activity in the North China Plain has been characterized by the development of new, steeply dipping strike-slip faults, which cut through the older listric normal faults. These steep dip strike-slip faults have become the primary seismogenic structures responsible for regional seismicity. Future seismic hazard assessments in the North China Plain should focus on the activity of these steep dip faults, as they are more likely to generate significant earthquakes. This shift in tectonic stress is attributed to a combination of factors, including the eastward expansion of the Tibetan Plateau, the rigid deformation of the Ordos Block, and the westward subduction of the Pacific and Philippine plates. Since the late Pleistocene, these forces have redefined the tectonic landscape of the region, increasing the likelihood of strike-slip faulting.

STUDY ON PS POINT SELECTION METHOD IN COMPLEX SURFACE ENVIRONMENT

CHEN Kai, XU Xiao-bo, QU Chun-yan, ZHANG Gui-fang, LIAN Da-jun, QIN You-sen

2024, 46(5):  1012-1026.  DOI: 10.3969/j.issn.0253-4967.2024.05.002

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The key challenge of PSInSAR(Permanent Scatter Interferometric Synthetic Aperture Radar)lies in the quality of Permanent Scatter(PS)points, which are difficult to extract accurately in fracture zones due to the complex natural ground cover and unique geomorphological environments. In such areas, the inability to reliably extract high-quality PS points limits the application of PSInSAR for monitoring interseismic deformation. To address the problem of high-quality PS point selection in fault zones and improve the effectiveness of PSInSAR technology for monitoring interseismic deformation, this paper presents a comparative study of coherence coefficient and amplitude deviation double thresholds. The study further integrates the Kolmogorov-Smirnov(KS)test and CR homogeneous pixel method, in addition to conventional coherence coefficient point selection techniques. Sentinel-1A SAR images from March 14, 2015, to February 16, 2020, are used as the data source, with the Wushan-Gangu section of the fault zone on the northern edge of the Western Qinling Mountains, near the northeastern edge of the Qinghai-Tibet Plateau, serving as the test area for PSInSAR processing. The quality and reliability of PS point selection using various methods are compared and analyzed.

Two sets of coherence coefficient T_γ and amplitude deviation D_γ double thresholds were tested. The coherence coefficient was set at T_γ=0.5, while the amplitude deviation D_γ was set to 0.5 and 0.3, resulting in 19806 and 2485 PS points, respectively. When the amplitude deviation threshold was lowered, the number of PS points on ridgelines decreased significantly, while there was little change in Gangu County, suggesting poor pixel amplitude stability in mountainous areas. Lowering the threshold eliminated many PS points, retaining only those with high amplitude and stable time series, typically found in hard targets like urban buildings. A KS double sample test was then applied in combination with the double threshold method, with all thresholds coefficient T_γ, amplitude deviation D_γ, and KS test P_γ set at 0.5. This approach yielded 1 313 PS points, showing a significant reduction in PS points on ridgelines and in Gangu County, while urban points became more concentrated on hard targets like buildings. Although the KS test reduced PS points in vegetated areas, it did not fully eliminate noise points. Finally, based on the double threshold results of coherence coefficient T_γ=0.5 and amplitude deviation D_γ=0.3, both the KS test and CR homogeneous pixel selection methods were applied. The CR homogeneous pixel method used a phase difference threshold P_γ=0.5 and a temporal phase stability threshold N_γ=50. This yielded 2 485 PS points for the double threshold method, 133 PS points for the double threshold plus KS test, and 414 PS points for the double threshold plus CR homogeneous pixel method. The latter two methods significantly reduced PS points, with a higher concentration of points in Gangu County, consistent with the expectation that PS points predominantly correspond to hard targets like buildings.

Statistical analysis of the results demonstrated that the combination of the coherence coefficient, amplitude deviation, and CR homogeneous pixel method provided the highest quality PS points, effectively excluding noise points in vegetated areas. The combination of coherence coefficient, amplitude deviation, and KS test ranked second, improving accuracy in urban areas but failing to eliminate noise in vegetated areas. Using Sentinel-1A SAR images and the Wushan-Gangu fault as the test area for time series PSInSAR processing, the accuracy of PS point selection was further verified. A comparative analysis of deformation monitoring results from the three methods revealed that both the KS test and CR homogeneous pixel method improved the accuracy of fault deformation monitoring, with the CR homogeneous pixel method yielding superior results. Monitoring data from 2015 to 2020 showed that the deformation rate of the northern block of the Wushan-Gangu fault ranged from -2 to -0.2mm/a, with an average deformation of approximately -1.7mm/a. In contrast, the southern block exhibited a deformation rate between 0.3 and 0.5mm/a, with an average deformation of about 1.8mm/a The relative average deformation rate between the northern and southern blocks was 0.7mm/a, indicating left-lateral strike-slip movement. Among the three methods, the double threshold plus CR homogeneous pixel method produced PS points with the smallest deformation rate standard deviation, indicating more stable and reliable deformation results.

A NEW REFERENCE SCHEME FOR THE DELINEATION OF ACTIVE BLOCK BOUNDARIES IN THE SICHUAN-YUNNAN EXPERIMENTAL SITE

SUN Xiao, LU Ren-qi, ZHANG Jin-yu, WANG Wei, SU Peng

2024, 46(5):  1027-1047.  DOI: 10.3969/j.issn.0253-4967.2024.05.003

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Active block boundaries represent areas where significant crustal stress accumulates, leading to concentrated tectonic deformation and frequent seismic activity. These boundaries are crucial for understanding the patterns of strong earthquakes within mainland China. The China Seismic Experimental Site, located in the Sichuan-Yunnan region, is a key area of tectonic deformation caused by the collision and convergence of the Indian and Eurasian plates. This region plays a vital role in transferring tectonic stress between western China and adjacent plates.

This comprehensive study analyzes the integrity, three-dimensional characteristics, hierarchy, and tectonic activity of blocks within the Sichuan-Yunnan region, following established schemes and criteria for defining active block boundaries. After detailed research, the major active fault zones in the region have been divided into three primary active block boundary zones and sixteen secondary boundary zones.

A new reference scheme was developed by considering several factors, including the historical distribution of strong earthquakes, the hierarchical patterns of earthquake frequency and magnitude, spatial variations in present-day deformation as revealed by GNSS data, and deep crustal differences indicated by gravity data and velocity structures. The Jinshajiang-Honghe Fault, Ganzi-Yushu-Xianshuihe-Anninghe-Zemuhe-Xiaojiang Fault, and Longmenshan Fault are identified as the primary active block boundary zones, while faults such as the Lijiang-Xiaojinhe, Nantinghe, and Longriba faults are classified as secondary boundary zones.

Through an integrated analysis of seismic activity, current deformation patterns, fault sizes, deep crustal structures, and paleoseismic data, the study estimates that the primary boundary zones have the potential to generate earthquakes of magnitude 7.5 or greater, while the secondary boundary zones could produce earthquakes of magnitude 6.5 or greater.

The expansion of geophysical exploration, including shallow and deep earth data, has allowed for a transition in the study of active tectonics from surface-focused to depth-focused, from qualitative to quantitative, and from two-dimensional to three-dimensional analysis. By integrating multiple data sources, i.e. regional geology, geophysics, seismicity, and large-scale deformation measurements, this study presents a more refined delineation of active blocks in the Sichuan-Yunnan region.

The new delineation scheme provides a scientific basis for future mechanical simulations of interactions between active blocks in the Sichuan-Yunnan Experimental Site. It also offers a framework for assessing the probability of strong earthquakes and evaluating seismic hazards. The purpose of this study is to re-analyze and refine the delineation of active block boundaries using high-resolution, coordinated data while building on previous research.

In summary, the Sichuan-Yunnan region’s primary fault zones are divided into three primary and sixteen secondary active block boundary zones. The study concludes that primary boundary zones are capable of generating magnitude 7.5 or greater earthquakes, while secondary zones can produce magnitude 6.5 or greater earthquakes. While the current block delineation scheme offers a valuable foundation, further discussion and refinement of certain secondary boundary zones are needed as detection and observational data improve. This study provides an essential framework for analyzing the dynamic interactions between active blocks, identifying seismogenic environments, and assessing seismic risks in the Sichuan-Yunnan region.

Research paper

THE STUDY OF CRUSTAL THICKNESS AND POISSON'S RATIO IN TENGCHONG VOLCANO AREA BY H-к-c METHOD

ZHANG Tian-ji, LI Qiu-feng, LI Feng-ying, ZHONG Yu-sheng, DUAN Hong-jie

2024, 46(5):  1048-1065.  DOI: 10.3969/j.issn.0253-4967.2024.05.004

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Tengchong volcanoes are not extinct but a group of dormant volcanoes with magma underground. The Tengchong volcanic area is a unique geological condition integrating magma activity, earthquakes, and hot springs. Crustal thickness and Poisson’s ratio are two important parameters that characterize crustal structure and material composition and are crucial for accurately detecting the location and scale of magma chambers in the Tengchong volcanic area. However, previous studies on obtaining crustal thickness and Poisson’s ratio in the Tengchong volcanic area only used nine volcanic network stations, which had insufficient resolution and could not effectively constrain the position of magma chambers. The traditional H-κ method is adopted to an isotropic crust with a flat Moho. The crust maybe anisotropic and the Moho is dipping. In the presence of a complex crustal structure with azimuthal anisotropy or dipping Moho, the H-κ results may be biased. So, we extracted 4 268 P receiver functions from teleseismic wave data recorded at 23 digital seismic stations. A H-κ-c method with harmonic corrections is used to obtain crustal thickness and Poisson’s ratio in the Tengchong volcano area. Before the harmonic corrections to the P receiver functions, we perform the incident moveout corrections and back azimuthal binning of 5°. The H-κ-c method can correct the influence of crustal anisotropy and dipping interfaces on receiver functions by harmonic transformation, can acquire more stable and reliable crustal thickness and wave velocity ratios, and can obtain information on the inclination of the Moho and crustal azimuthal anisotropy. Based on previous research, we discussed the crustal deformation mechanism of the Tengchong block and revealed the corresponding relationship between crustal structure, heat flow, earthquakes, and magmatism.

Results show the fast-wave polarization directions with a dominant NW-SE orientation in the north and change to a dominant NE-SW orientation in the south, and delay times varying between 0.06 and 0.80s, with a mean of 0.40s. It is consistent with the Tengchong block undergoing clockwise rotation around the EHS. There is an inclined Moho surface and strong azimuthal anisotropy in the intersection of the Tengchong Fault, Yingjiang Fault, and Longchuan Fault. The strong azimuthal anisotropy in the Tengchong block maybe related to the strong influence of the upwelling of the deep thermal material from the upper mantle. The fast wave polarization direction parallel to the Longling-Ruili fault indicates that the observed anisotropy may be related to the fracture of the fault. The crustal thickness ranges from 32 to 39km, and the Poisson’s ratio ranges from 0.235 to 0.326. There exist three Moho-uplifting centers, one in Gudong-Qvshi-Mazhan-Tengchong, the other in Qingshui-Xinhua, another in Zhen’an-Longxin-Xiangda. The very high Poisson’s ratio(σ>0.3) is consistently located within these three locations. We speculated that the Moho-uplifting and higher Poisson’s ratio at the three sites denote the existence of three magma chambers. The horizontal scale of the three magma chambers is respectively 20km×35km, 20km×20km, 25km×25km, and separately controlled by the Tengchong Fault and Longchuanjiang Fault, Tengchong Fault and Longchuan Fault, Nujiang Fault and Longling-Ruili Fault. The locations of the magma chambers are different from that obtained by the same receiver function method, which the different seismic stations and stacking methods may cause. The locations of the magma chambers are not exactly the same as those of the geothermal anomaly areas and the mantle-derived volatile release anomaly areas measured by the surface hot springs. The reason for this difference may be the ground temperature and the mantle-derived volatile component, which are the measurement results of the surface hot spring. The Poisson’s ratio value we calculated is the average value of the entire crust. Under the four Holocene volcanic craters of HeikongShan, Dayingshan, Laoguipo, and Ma’anshan, there is an interconnected magma chamber, the most vigorous volcanic activity since the Holocene. There are almost no earthquakes occurring in the crust at the center of the Moho uplift; most earthquakes are distributed in the crust around the Moho-uplifting centers. This may be because the hot magma heated the crust, resulted in the rocks in the crust being plastic, and it is difficult to accumulate large strains. Our results have important reference value and guiding significance for earthquake and volcanic activity monitoring, earthquake prevention and disaster reduction in the Tengchong volcanic area.

DOUBLE-DIFFERENCE RELOCATION OF YUNNAN YANGBI M_S6.4 EARTHQUAKE SEQUENCE ON MAY 21, 2021 AND TECTONIC IMPLICATIONS

XU Yong-qiang, LEI Jian-she, HU Xiao-hui

2024, 46(5):  1066-1090.  DOI: 10.3969/j.issn.0253-4967.2024.05.005

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At 21:48 on May 21, 2021(Beijing time), the M_S6.4 earthquake occurred in western Town(25.700°N, 99.880°E), Yangbi County, Dali, Yunnan Province, with a focal depth of 10km(China Earthquake Networks Center). The Yangbi earthquake is a typical type of foreshock-mainshock-aftershock earthquake, which had a significant impact on the local residents and attracted great attention from society. To better understand the seismogenic structure and mechanism of this earthquake, the present study relocates the May 21, 2021 Yangbi M_S6.4 earthquake sequence, collected from the China Earthquake Networks Center from 2021 to June 18, 2022. Finally, 2681 precisely located events are obtained through the double-difference relocation algorithm. Our results show that the Yangbi earthquake sequence extended for about 32km, mainly along the NW-SE direction, and it is an overall echelon structure changing from narrow in the northwest to broad in the southeast. The dominant depth of the earthquake sequence is 5-10km. The foreshocks were mainly active in the northern section of this earthquake sequence, with the mainshock being a unilateral rupture. The aftershocks primarily extended in the southeast direction, but the southeast extension process was not simply a unilateral extension. Multiple secondary oblique activity sequences were derived on the west side of the sequence. With the continuous release of stress in the study area, only the main rupture continued to be active in the southeastern section of the sequence in the later stage of activity. Still, the secondary oblique ruptures that evolved was no longer active. The average location errors of these earthquakes are about 0.47km in the east-west direction, about 0.50km in the north-south direction, and 0.62km in the vertical direction, and the average RMS travel-time residual is 0.22s.

This study collects broadband digital seismic waveform data of earthquakes with M_S≥4.0 on the main fault of the earthquake sequence recorded by regional seismic networks in Yunnan, Sichuan, and other areas from the International Earthquake Science Data Center. The focal mechanism solutions of the major earthquake events are obtained using the gCAP full waveform inversion method. The results show that the focal mechanism solutions of earthquakes with M_S≥4.0 on the main fault all have an NW-SE oriented nodal plane I, consistent with the dominant distribution of the NW-SE oriented sequence. Except for the nodal plane I of the Yangbi M_S5.6 earthquake, which has a northeast dipping angle, all other focal mechanism solutions have a southwest dipping nodal plane I, which was consistent with the sequence orientation as shown in the vertical cross sections. According to the inclination angles of the P, B, and T axes, the inverted focal mechanism solutions all belong to a strike-slip type.

In this study, the parameters of the seismic fault plane are fitted in segments according to the distribution density of small-to-medium-sized earthquakes. The results show that the strike trending of the main fault plane varies between 126°-137° and gradually increases from north to south, dipping towards the southwest. The dip angle varies between 79°-87° gradually decreasing from north to south. There are four secondary oblique faults with variations in striking directions of 157°, 338°, 157° and 313° from north to south, corresponding to dip angles of 86°, 87°, 87°, and 86°, respectively.

Based on the above research results, combined with the background stress field and V_P/V_S tomographic results, it is inferred that the Yangbi earthquake occurred on the high-dip-angle and NW-SW strike-slip faults in the southwest mountainous areas of Yangbi County. These faults consist of a strike-slipping main fault and multiple secondary crisscrossing small faults, which may be jointly affected by regional stress and deep fluid activity.

REGIONAL PROPAGATION MEDIUM PARAMETERS FOR GROUND MOTION ESTIMATION

ZHENG Xing-qun, TAO Zheng-ru, BAI Kai

2024, 46(5):  1091-1105.  DOI: 10.3969/j.issn.0253-4967.2024.05.006

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Estimating the possible ground motion generated by future earthquakes can provide input for seismic design, which is an important research direction in engineering seismology. It is widely used in seismic risk analysis, earthquake zoning, post-earthquake emergency, earthquake disaster mitigation planning. Ground motion is estimated in two ways: first, the attenuation relation of ground motion is simplified to express the relationship between ground motion and earthquake size, focal characteristics, propagation medium and site influence; Second, the near-fault ground motion of large earthquakes is synthesized by using finite fault source model and Green’s function. Both of the two ways describe the influence of source, propagation medium and site conditions. Among them, the propagation of seismic waves in the medium can be expressed in terms of geometric spreading term and inelastic attenuation term. In this study, the characteristics of these two regional propagation media are studied from two perspectives: the geometrical spreading term and the inelastic attenuation term. Firstly, the northeastern region of Japan, with abundant seismic records, is taken as the research area, and the effects of magnitude, focal depth, and Moho depth are discussed by using a genetic algorithm. The results show that the Moho depth significantly influences the geometrical spreading parameters. The results were applied to the regions of Sichuan, Xinjiang and Yunnan in western China, and the geometric spreading terms were determined according to the depth of the Moho plane and the least square method was used to fit the geometric spreading term. On this basis, from the geological point of view, according to the geological time, earth heat flow value, surface elevation, Moho buried depth, upper crust shear wave velocity and seismic activity, the regions are re-divided, and the inelastic attenuation parameters Q₀ and η are obtained by using genetic algorithm. According to the above six factors, it can be found that the stress drop is distributed in 30-130bar, κ₀ in 0.025-0.06s, Q₀ in 220-350, η in 0.56-0.96. Among them, the discussion of the influence of various influencing factors on the Q₀ in this paper can be described as follows: When grouped according to geological time, the value of Q₀ decreases from Paleozoic to Cenozoic. When divided into two groups according to geothermal conditions, the Q₀ value of the low heat area is greater than that of the high heat area. When it is divided into three groups according to the elevation, which are less than 1 000m, between 1 000 and 3 000m and above 3 000m, the Q₀ value decreases with the elevation increase. When the buried depth of the Moho surface is divided into deep and shallow buried areas, the Q₀ value of the deep buried area is greater than that of the shallow buried area. When the shear wave velocity of the upper crust is divided into two groups, the Q₀ value of the low-speed region is smaller than that of the high-speed region. When the seismic activity is divided into strong seismic activity and weak seismic activity, the Q₀ value of the weak seismic activity region is greater than that of the strong seismic activity region. By analyzing the inversion results, some rules of quality factor distribution in the study area can be summarized: seismic wave attenuation is faster and Q value is usually lower in the areas with high ground heat flow, shallow Moho surface, low shear wave velocity and strong seismic activity; With the increase of geological time from ancient to present and elevation, the Q value decreases gradually. Combined with the calculated value of the regional crustal medium parameter Q, which is required for ground motion estimation, the division scheme of the inversion area is given, that is, the eastern part of Sichuan and Tianshan region of Xinjiang are the regions with high Q values, Yunnan, central and western parts of Sichuan and Kashi region of Xinjiang are the regions with low Q values, and the northwestern part of Yunnan and west and central part of Sichuan are the regions with relatively lower Q values. During the study of the inelastic attenuation term, the study area was redivided according to the influence of a single factor. In the follow-up research, the study area will be redivided according to the cross-over of different influencing factors to explore the influence of the inelastic attenuation term. At the same time, the next step will continue to improve the zoning standards in western China and draw the zoning map.

USING SEISMIC AMBIENT NOISE HORIZONTAL-TO-VERTICAL SPECTRAL RATIO(HVSR) METHOD TO DETECT SITE RESPONSE AND SHALLOW SEDIMENTARY STRUCTURE IN XIONG’AN AREA

RUAN Ming-ming, LIU Qiao-xia, DUAN Yong-hong, WANG Shuai-jun, ZHENG Cheng-long, WANG Liang

2024, 46(5):  1106-1122.  DOI: 10.3969/j.issn.0253-4967.2024.05.007

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The construction of the Xiong’an New Area is a national strategy and a long-term plan outlined by the Chinese government. To support the urban planning and development of this area, many scholars have conducted a series of geophysical surveys aimed at understanding the detailed subsurface structure. The Horizontal-to-Vertical Spectral Ratio(HVSR)method, first introduced by Nakamura, has recently gained widespread use for investigating shallow subsurface structures, site response, and microzonation.

In this study, we utilized a large seismic array with an interstation distance ranging from 500 to 1000 meters, deployed across the Xiong’an New Area. The array consisted of over 900 short-period seismographs, covering most of the area. Using ambient-noise recordings, we removed nonrandom transient signals from the waveform data with a short-term-average over long-term-average detector automatic picking algorithm, and applied the Konno-Ohmachi algorithm to smooth the HVSR curves. For each site, we analyzed the amplitude of the peak value of the HVSR curve(A)and the corresponding frequency(f₀). Both parameters were further elaborated through the creation of contour maps using the Kriging interpolation method. Additionally, the peak frequencies from the HVSR curves were used to calculate the sedimentary thickness, based on an average shear-wave velocity and the frequency-depth formula.

The frequency map shows that the peak frequencies range between 0.6 and 1.1Hz, with an overall peak frequency of about 0.7 to 1.0Hz. The lowest frequencies were found predominantly in the vast eastern area of the study region, corresponding to geological features such as the Niubei Slope, Niutuozhen High, and Baxian Sag. According to the frequency-depth formula, a lower peak frequency indicates greater sediment depth. The variation in peak frequencies across stations highlights changes in the bedrock interface, which correspond to fault structures depicted on the geological map. Furthermore, high-amplitude areas were mainly located between the Rongxi fault and Rongdong fault, suggesting an impedance contrast between shallow and deeper layers. Stratigraphic profiles reveal that Quaternary and Tertiary sedimentary layers directly overlie the crystalline basement composed of Proterozoic metamorphic rocks. Combined analysis of peak frequency and amplitude aligns well with the available geological data. Our analysis produced 3D depth images of the Quaternary sedimentary layer interface across the study area, clearly imaging a significant seismic impedance interface at depths of 100-220m. This shallow interface corresponds to the contrast between the Tertiary rocks and the overlying Quaternary sedimentary layers. The sediment thickness progressively increases from east to west across the study area. Interfaces derived from the HVSR profiles display similar characteristics to those on the geological map and are consistent with borehole data and results from the high-density resistivity method. Moreover, we established a power-law relationship correlating the fundamental site resonance frequencies with sedimentary cover thickness obtained from borehole data in the Xiong’an New Area. The undulating characteristics of the sedimentary layers correspond closely to fault locations and geological tectonic units, confirming that faults such as the Rongxi, Rongdong, Niuxi, Niudong, and Xushui-Dacheng faults serve as boundaries for secondary geological tectonic units, influencing the structure of the near-surface sedimentary layers.

We developed a 3D shallow subsurface sedimentary model for the Xiong’an New Area and created contour maps of amplitude(A)and peak frequency(f₀). The results both support and extend previous understandings of the region’s structure. This study demonstrates that the HVSR method, in conjunction with a large seismic array, is a rapid and effective technique for investigating shallow subsurface structures and seismic site responses. The exploration of sedimentary structures and seismic site response characteristics, which are closely related to earthquake hazards, provides a critical foundation for seismic fortification and urban planning in the Xiong’an New Area.

STUDY ON THE VARIATION CHARACTERISTICS OF GRAVITY FIELD AND APPARENT DENSITY IN URUMQI AND ITS SURROUNDING AREAS

KONG Xiang-kui, LIU Dai-qin, AILIXIATI·Yushan, LI Jie, CHEN Li, LI Rui, CHEN Rong-liu

2024, 46(5):  1123-1150.  DOI: 10.3969/j.issn.0253-4967.2024.05.008

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Using seasonal gravity observation data from Urumqi and its surrounding areas, collected between April 2019 and April 2022, this paper applies absolute gravity control to perform classical adjustment calculations, identifying the spatial-temporal evolution characteristics of the gravity field in the study area. The relationship between seismic activity and gravity change has long been a topic of interest. The time-varying gravity field is a fundamental physical field that reflects the migration of mass and directly represents the internal tectonic movements of the Earth and surface mass redistribution. The link between earthquakes and gravity changes is primarily related to tectonic movement and variations in mass(density)within the Earth’s interior.

By examining gravity field changes at half-year and one-year scales and analyzing gravity profile images in relation to geological structures, this paper explores the characteristics of gravity field variations in Urumqi and its surrounding areas. To effectively separate gravity anomalies at different depth levels, wavelet multi-resolution analysis is employed to decompose the gravity field anomalies, distinguishing regional from local anomalies in the study area. Specifically, the wavelet multi-scale analysis method is applied to process gravity field dynamic data from 2019-2020, 2020-2021, and 2021-2022. This method helps isolate and interpret abnormal signals in the gravity field, improving the reliability of earthquake precursor gravity anomalies.

The gravity source characteristics provide insight into the physical property changes of the crust. In this study, the “equivalent source” inversion model is used to determine the dynamic characteristics of the crust’s apparent density. The multi-period gravity point values obtained through the adjustment method serve as input data for the equivalent source apparent density change model in the study area.

The results indicate that the gravity field in the study area exhibits clear zonation, with predominant negative changes and alternating positive and negative gravity anomalies. The wavelet gravity details show that the anomaly areas align with geological structures, and the estimated source depth, as determined by the power spectrum, is consistent with the Crust1.0 model. The inversion of the flow gravity data reveals the variation characteristics of the crust’s equivalent apparent density, which correlate well with the time-varying gravity field. Multi-scale decomposition of gravity anomalies at different depth levels further illuminates the physical property changes of the crustal medium, as reflected by the equivalent source density model. These findings, when combined with the regional tectonic background and seismic activity, offer valuable insights. The research presented in this paper provides a foundational understanding of gravity field trends in Urumqi and its surrounding areas, contributing to future predictions of gravity field changes in the region.

THE CHARACTERISTICS AND MECHANISM OF GRAVITY AND MAGNETIC FIELD CHANGES BEFORE AND AFTER THE 2014 HUOSHAN M_S4.3 EARTHQUAKE

LIANG Xiao, CHU Fei, XU Ru-gang, SUN Hong-bo, XIAO Wei-peng, WANG Jun

2024, 46(5):  1151-1171.  DOI: 10.3969/j.issn.0253-4967.2024.05.009

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Before the Huoshan earthquake, significant anomalies were detected in both the gravity and lithospheric magnetic fields. To comprehensively analyze the variations in these fields and their underlying mechanisms before and after the Huoshan earthquake, we used mobile gravity data from 2010 to 2015 and mobile geomagnetic data from 2013 to 2014 in Anhui Province and surrounding regions. Our analysis focused on gravity field changes from two perspectives: (1)the spatial and temporal variations in the gravity field and(2)the time series of gravity point values across the seismogenic fault(Tudiling-Luoerling fault). Additionally, we corrected for diurnal variations and long-term trends in the geomagnetic field, allowing us to track changes in the three components of the lithospheric magnetic field—total intensity, magnetic inclination, and declination—before and after the earthquake. Using wavelet multi-scale decomposition, we calculated and analyzed wavelet details at different decomposition scales for the gravity and magnetic field variations in the first half year before the Huoshan earthquake. Finally, in conjunction with underground fluid data, we examined the seismogenic background and explored the underlying reasons for the precursory anomalies observed in the geophysical fields.

The research yielded the following conclusions: Prior to the Huoshan earthquake, an anomalous high-gradient zone in both the gravity field and the total strength of the lithospheric magnetic field was observed, extending approximately 100km. Notably, the total magnetic field strength in the Huoshan area significantly decreased before the earthquake. The gravity field exhibited a small initial decline, evolving into a high-gradient anomaly zone parallel to the seismogenic fault, which culminated in the earthquake occurring near the zero contour of gravity change. The alignment of the zero lines of gravity and magnetic field changes with the strike of the seismogenic fault suggests that tectonic faults play a critical role in controlling crustal deformation and underground fluid migration. When combined with a comprehensive analysis of the regional stress field, underground fluid dynamics, and variations in the gravity and magnetic fields, this information can be instrumental in identifying and assessing seismic risk zones.

The Huoshan region is highly susceptible to seismic activity due to the influence of the Bayan Har block in the Qinghai-Tibet region, which induces stress field fluctuations in the area. The M_S7.0 Lushan earthquake in Sichuan, on April 20, 2013, significantly impacted the regional stress field, resulting in the opening of the “Huoshan Seismic Window.” In the six months preceding the Huoshan earthquake, there was an increase in crustal movement, as well as a marked rise in minor seismic activity. These factors accelerated the adjustment of the regional stress state and the migration of underground fluids, leading to expanded variations in regional gravity and magnetic fields. The Huoshan M_S4.3 earthquake exhibited distinct precursory anomalies. The wavelet multi-scale decomposition of gravity and magnetic field changes suggests that the source of the Huoshan earthquake likely originated in the middle to lower crust. The deformation and material migration in the upper crust appeared to be influenced by processes in the middle and lower crust, with energy accumulation in the upper crust triggering the opening of the “Huoshan Seismic Window” and the subsequent earthquake. Additionally, the extreme point of the wavelet details in the lithospheric magnetic field change was located near the zero line of the wavelet details in the gravity field and the fault development area.

This study concludes that regional stress fluctuations, crustal deformation, and underground fluid migration are controlled by fracture structures. The migration of underground fluids and other materials results in notable changes in the gravity and magnetic fields, particularly in areas with concentrated fault activity, underscoring the potential for predicting earthquakes using geophysical precursor signals such as gravity and lithospheric magnetic field changes.

GRAVITY CHANGES BEFORE THE PINGYUAN M_S5.5 EARTHQUAKE OF 2023

LI Shu-peng, HU Min-zhang, ZHU Yi-qing, HAO Hong-tao, YIN Hai-tao, JIA Yuan, CUI Hua-wei, LU Han-peng, ZHANG Gang, WANG Feng-ji, LIU Hong-liang

2024, 46(5):  1172-1191.  DOI: 10.3969/j.issn.0253-4967.2024.05.010

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On August 6, 2023, an earthquake with M_S5.5 occurred in Pingyuan County, Dezhou City, Shandong Province, which is the largest earthquake in the Shandong region in the past 40 years. Before the earthquake, Shandong Earthquake Agency conducted biannual mobile gravity measurements near the epicenter, observed the spatiotemporal gravity field changes for the four years leading up to the earthquake, and made a certain degree of medium-term prediction, predicting that the epicenter location(36.00°N, 116.10°E)would be about 130km from the actual epicenter. This suggests that it is potentially feasible to carry out medium-term prediction of moderate earthquakes based on the temporal and spatial variations of the gravity field in the tectonically weak North China. Therefore, the study of the gravity changes before the 2023 Pingyuan M_S5.5 earthquake can help to deepen the understanding of the relationship between the time-space variations of the gravity field and the moderate earthquakes, enrich the database of “magnitude and gravity anomalies” in North China, and improve the science and accuracy of identifying and determining the medium- and long-term anomalies of earthquakes.

The mobile gravity data utilized in this paper were processed and calculated using the classical adjustment method in LGADJ software. This process involved corrections for earth tide, instrument height, monomial coefficient, air pressure, and zero drift, resulting in absolute gravity values for each measurement point. Eight absolute gravity points, including Jiaxiang, Tai'an, and Zibo, served as the starting reference points. The average accuracy of the observed data point values during each period ranged from 8.5 to 16.0μGal, indicating relatively high precision. Subsequently, the calculation results of the two data sets were subtracted to obtain the relative gravity change. This change was then interpolated on a continuous grid using the Surface module of GMT mapping software and subjected to 50-km low-pass filtering. Finally, the dynamic evolution image of the gravity field was generated.

Based on these results, this study analyzes the characteristics of regional gravity field changes since September 2019. These findings are integrated with information on deformation fields, seismic source mechanisms, and dynamic environments to explore the relationship between gravity changes before the earthquake and the seismic mechanism. The results indicate the following:

(1)Since May 2022, precursory anomalies have been detected in the gravity field changes around the epicenter. Between May 2022 and April 2023, there was a significant increase in positive gravity changes exceeding +50μGal and a spatial extent exceeding 160km in the south of the epicenter, with positive-negative differences exceeding 70μGal on both sides of the epicenter. However, the gravity changes near the epicentre remained stable and in a “locked” state. The magnitude, range, and duration of gravity changes before the earthquakes align with previously summarized indicators.

(2)Between September 2021 and September 2022, distinct four-quadrant distribution characteristics emerged in the regional gravity field changes. And the spatial distribution of regional gravity field changes corresponds to horizontal deformation fields, seismic source mechanisms, and coseismic displacement fields. Precisely, the compression zones of the seismic source mechanism and the inflow and subsidence areas of the coseismic displacement field correspond to regions of surface compression and gravity decrease before the earthquake. Similarly, the expansion zones of the seismic source mechanism and the outflow and uplift areas of the coseismic displacement field correspond to of surface expansion and gravity increase before the earthquake.

(3)The leading cause of the gravity changes anomaly before the Pingyuan M_S5.5 earthquake was the migration of deep-seated fluid materials, with the gravity effects generated by upper crustal deformation being a secondary factor. It is believed that the subduction of the Pacific Plate caused high-speed eastward migration of the relatively weak lower crust flow, dragging the upper crust eastward. The more rigid upper crust accumulated stress and strain during this process, developing numerous micro-fractures, while tectonic heterogeneity led to an east-west compression and north-south extension pattern. The fluid migration from compressed to expanded areas caused positive and negative differential changes in the gravitational field around the epicenter, culminating in the earthquake.

THERMAL INFRARED ANOMALIES OF MODERATELY STRONG EARTHQUAKE IN XINJIANG AND SURROUNDING REGIONS

ZHANG An-he, ZHONG Mei-jiao, AISA Yisimayili, LIU Ping

2024, 46(5):  1192-1206.  DOI: 10.3969/j.issn.0253-4967.2024.05.011

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Xinjiang and its surrounding areas are one of the regions with the most frequent seismic activities and the largest intensity in the Chinese mainland. Therefore, conducting relevant earthquake prediction research is crucial for disaster prevention and mitigation. However, due to the limited natural conditions and other factors in the region, the number of site observation stations is small and the their distribution density is low. It is difficult to carry out earthquake prediction only using site observation. Remote sensing technology has the advantages of being all-weather and large-scale. With the development of remote sensing technology, many scholars have found that there are different degrees of thermal anomalies before strong earthquakes. At present, a variety of thermal anomaly extraction methods have been formed. Among them, the relative power spectrum method can remove the thermal radiation changes affected by non-tectonic activity factors such as topography, ground object types, and meteorology to highlight the thermal radiation anomalies caused during earthquake preparation. This method has been applied in Xinjiang and surrounding areas for many years. However, in the past few years, the technique for studying seismic thermal anomalies in Xinjiang and its surrounding areas has primarily focused on a single seismic event, lacking systematic combing of earthquake cases and analysis of prediction efficiency.

To further summarize the characteristics of seismic thermal infrared anomalies in Xinjiang and its surrounding areas, improve its prediction indicators, and improve the scientific and accuracy of earthquake prediction in this area, based on the blackbody brightness temperature data of FY-2 geostationary meteorological satellite, we extract the thermal infrared relative power spectrum anomaly of Xinjiang and surrounding regions from 2008 to 2021 by using the relative power spectrum method and analyze the prediction efficiency of earthquakes with different magnitudes, and summarizes the relationship between thermal infrared anomalies and corresponding earthquakes. The results show that: 1)The band 1 of the thermal infrared relative power spectrum has the highest corresponding rate of 44% for earthquakes above 5 in Xinjiang and its surrounding areas, but only 6.0-6.9 earthquakes have passed the significance test. The R-value is 0.342, which is greater than R₀(0.306). The dominant occurrence time of M5.0-5.9 earthquakes is within 3 months after the beginning of the anomaly and within 0.5 months after the end of the anomaly, while that of M6.0-6.9 earthquakes is three months and 7-12 months after the end of the anomaly. The dominant seismogenic areas of each magnitude range are within 200km from the edge of the anomaly area to the surrounding area. 2)The abnormal area and duration of the 6.0-6.9 earthquakes corresponding to the thermal infrared relative power spectrum anomaly positively correlate with the magnitude, and all pass the significance test. The peak and magnitude did not pass the significance test in the two magnitude ranges. 3)This anomaly occurs most frequently in the Altun area and has a high correspondence rate. The seismic correspondence ratio in the southern Xinjiang region is higher than that in the northern Xinjiang region; The anomaly in the basin has a higher seismic correspondence ratio and higher earthquake magnitude; 4)Most anomalies occurred in spring, and the seismic correspondence ratio of anomalies in autumn was the highest; 5)The proportion of epicenter mechanism solution types of corresponding earthquakes is consistent with that of various kinds in Xinjiang, and most events were shallow earthquakes. Shallow earthquakes may be more likely to cause thermal infrared anomalies.

THE INFLUENCE OF SEISMIC SOURCE CHARACTERISTICS ON VELOCITY PULSE DISTRIBUTION IN SCENARIOS: A TEST IN HUYA FAULT

JI Zhi-wei, LI Zong-chao, ZHANG Yan, JU Chang-hui

2024, 46(5):  1207-1225.  DOI: 10.3969/j.issn.0253-4967.2024.05.012

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In August 1976, within a week, three earthquakes with a magnitude of 6.5 or higher occurred at the border of Songpan County and Pingwu County in Sichuan Province, China. The seismogenic structure of the three earthquakes is the Huya Fault. The Huya Fault is still a strong active fault, and there is still a possibility of major future earthquakes in the Songpan and Pingwu regions. Historical earthquake records only represent earthquakes that have occurred, and there is uncertainty in estimating earthquake motion using existing records, especially in near-fault areas without strong earthquake records. Estimating near-fault ground motion has become a research hotspot in the interdisciplinary field of seismology and engineering seismology in recent years. The research and methods differ from the design of earthquake motion methods for earthquake safety evaluation in engineering. They are developed by integrating earthquake source physics, seismic wave propagation theory, and engineering seismology. Based on the geological and geomorphological characteristics of the Songpan-Pingwu area in Sichuan Province and the process framework for constructing scenario earthquake models, we have developed three scenario earthquake source models with a magnitude of M_W7.0. These models include rupture models and source mechanisms related to the Huya Fault. The seismic source parameters were referenced from existing statistical models. Utilizing the three-dimensional finite difference method, we can simulate the long-term ground motion of scenario earthquakes for its facile discretization and computational efficiency. We set virtual observation stations within the calculated area, enabling the acquisition of velocity wave fields and waveforms across diverse earthquake scenarios. Besides, the velocity pulse identification method is combined to identify the seismic motion of the virtual station to study the distribution characteristics of regional velocity pulses. We use the pulse recognition method to identify velocity pulses of earthquake motion(observed or simulated earthquake motion). It can be summarized as a continuous wavelet transform of two orthogonal components of earthquake motion to determine whether it is a pulse. When the pulse index PI>0, the original record is determined to be a pulse, and the larger the PI, the stronger the pulse characteristics of the original record. When the pulse index PI<0, the original record is deemed nonpulse. This method can obtain the pulse amplitude and pulse period. Finally, the obtained results will be fitted with the probability distribution curve of velocity pulses to explore the impact of rupture mode and source mechanism on the distribution of velocity pulses. The results of this article indicate that: 1)The rupture mode is significant to the distribution of regional velocity pulses. For strike-slip faults, the velocity pulses caused by unilateral rupture mode are mainly in the E-W direction, and the peak value of the pulses does not exceed 50cm/s. The range of pulse distribution and the peak intensity of strong vibrations generated on the surface are smaller than the bilateral rupture mode. Strong velocity pulses not only appear near the projection area of faults on the surface but also trigger velocity pulses at a distance from the epicenter due to the directional effect of rupture. 2)The shape of the velocity pulse probability distribution curve is similar to the simulated velocity pulse distribution characteristics, and there are significant differences in the distribution of seismic motions under different source mechanisms. The current velocity pulse probability distribution model only considers the rupture characteristics and the relative position relationship between stations and faults without considering the influence of source parameters such as rupture velocity. There are deviations in the fitting effect for different components, such as E-W and N-S. The speed pulse period identified by virtual stations varies from 1-7s. By adding structural measures to the building structure, the natural vibration period of the structure can be changed, thereby avoiding the potential hazards of pulse-type seismic motion. More actual observation data is needed to study the distribution of velocity pulse periods in the future. This article’s simulation results are consistent with the existing understanding of earthquake motion. However, our study employs a simplified crustal structure characterized by horizontal layers, temporarily ignoring the site condition in the simulation of long-period ground motion. We do not encapsulate the complexities introduced by the site conditions. Average shear wave velocity at 30m underground (V_S30) is a factor that affects the number of pulse recognition. In addition, this article does not discuss the effects of rupture speed, the number of asperities, and the position of asperities. Therefore, we will conduct more in-depth research on these factors in our subsequent work. The research in this article calculates the scenario earthquake of the Huya Fault under rupture mode and focal mechanism. The research results can be used for seismic analysis of long-period structures, providing a reference for the construction of significant projects and seismic hazard analysis near the Huya Fault. This article referred to previous research when setting parameters for scenario earthquakes. However, due to the limitations of statistical models, the set scenario earthquakes cannot fully represent the Huya Fault situation and the region’s possible earthquake scenarios.