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20 August 2024, Volume 46 Issue 4

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Research paper

THE DISASTER MECHANISM OF THE M_S6.9 EARTHQUAKE IN MENYUAN, QINGHAI PROVINCE, 2022

NIU Peng-fei, HAN Zhu-jun, GUO Peng, LI Ke-chang, LÜ Li-xing

2024, 46(4):  761-782.  DOI: 10.3969/j.issn.0253-4967.2024.04.001

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Earthquake disasters are one of the most significant natural disasters faced by human society. Understanding and mitigating earthquake disasters have always been a key focus of research for seismologists. Conducting investigations on post-earthquake seismic disasters is of great significance for the recovery and reconstruction of disaster-stricken areas, as well as for earthquake prevention and mitigation. Earthquake disasters can be classified into two types based on their mechanisms: one is the destruction caused directly by the seismic vibrations on buildings, lifelines, and other structures; the other is the damage related to geological hazards triggered by earthquakes. The former is mainly related to the density of regional economic layout; the latter seismic geological disasters typically include collapses, landslides, debris flows, ground fissures, ground subsidence, and soil liquefaction. These geological disasters often exacerbate the impact of seismic disasters, posing a more significant threat to human life and property safety. Therefore, it is of great significance to investigate the mechanisms of significant engineering disasters caused by earthquakes, as it can provide important insights for engineering recovery, reconstruction, and site selection. The Qilian-Haiyuan fault zone is an important boundary fault on the northeastern margin of the Qingzang Plateau. It plays a crucial role in absorbing and accommodating the convergence of the Indian Plate towards the Eurasian Plate in a NNE direction. With a total length of approximately 1 000km, it is primarily composed of the Tolaishan fault, the Lenglongling fault, the Jinqianghe fault, the Maomaoshan fault, the Laohushan fault, and the Haiyuan fault, from west to east. On January 8, 2022, a magnitude 6.9 earthquake occurred near the stepover of the Longling and Tuolaishan faults of the Qilian-Haiyuan fault zone. Although the earthquake occurred in uninhabited, sparsely wooded alpine grasslands and did not cause any casualties, it completely destroyed the Liuhuanggou bridge and the south-side Daliang tunnel on the Lanzhou-Xinjiang high-speed railway, a major artery of the Silk Road transportation network in China. This marks the first time that a mainline of the high-speed railway network, which is a showcase of China's economic achievements, has been entirely disrupted by earthquake damage. Based on the high-resolution orthophoto images and digital elevation models(DEMs)obtained through post-earthquake emergency scientific investigations using the unmanned aerial vehicles, this article conducted another field investigation on earthquake disasters in vehicles; this article conducted another field investigation on earthquake disasters in the isoseismal area. First, by investigating geological disasters such as collapses, landslides, and soil liquefaction in the meizoseismal area, as well as the damage to buildings and structures. Then, based on field surveys, a detailed mapping of the reverse-type surface ruptures formed by the Mengyuan earthquake was conducted, identifying the distribution patterns and geometric and kinematic characteristics of the surface ruptures and determining the distribution of coseismic vertical displacements. Additionally, the development of geological disasters caused by this earthquake was analyzed, and the disaster-causing mechanism of the Liuhuang Bridge was discussed. The research indicates that the Liuhuanggou River, located in the isoseismal area, does not exhibit large-scale earthquake landslides and collapses. Instead, only smaller-scale rockfalls and accumulations of rolling stones, as well as localized occurrences of sand liquefaction in certain riverbeds, are observed, which is clearly inconsistent with expectations. In addition to the formation of two strike-slip surface rupture zones, the earthquake also generated a reverse-type surface rupture zone approximately 7.9km long within the Liuhuanggou river on the northern side of the western section of the Lenglongling fault. The rupture zone exhibits an unstable southward trend and is primarily composed of discontinuous arc-shaped compressional ruptures, mole tracks, tensile ruptures, and seismic scarps. Along the surface rupture zone, a total of 35 vertical displacement measurements were obtained, with the minimum displacement of (8±1)cm and the maximum displacement of (49±3)cm. The average vertical displacement is approximately 24cm, and the displacement distribution along the strike is uneven. The surface rupture zone, which cuts nearly vertically across the Lanzhou-Xinjiang high-speed railway Liuhuanggou bridge, has caused extensive surface deformation and displacement. This is the direct cause of the destruction of the Liuhuanggou bridge. This finding suggests that when implementing seismic engineering design measures for major linear projects crossing fault zones, it is important to consider the extensive shear effects of reverse-type surface rupture zones.

STUDY ON THE EFFECT OF EXCESS TOPOGRAPHY ON LANDSLIDES INDUCED BY LUDING M_S6.8 EARTHQUAKE IN 2022

QIU Heng-zhi, MA Si-yuan, CHEN Xiao-li

2024, 46(4):  783-801.  DOI: 10.3969/j.issn.0253-4967.2024.04.002

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Strong earthquakes in mountainous regions are prone to triggering severe geological disasters, such as landslides, collapses, and debris flows. These disasters are characterized by their wide distribution, large scale, and high frequency, making them among the most destructive secondary effects of earthquakes. In recent years, the central and eastern parts of the Qinghai-Tibet Plateau have experienced frequent strong earthquakes, leading to varying degrees of earthquake-induced landslide disasters.
Landscape evolution is significantly influenced by tectonic activities and river incising, which alter the materials of hillslopes and their topographic characteristics. Earthquakes can significantly affect the spatial distribution of co-seismic landslides, particularly in areas with excess topography. In tectonically active zones, rock uplift and river erosion gradually increase slope angles and decrease slope stability. Weathering, freeze-thaw cycles, and seismic vibrations can reduce rock strength, leading to slope erosion and landslides. Once these processes occur, the slope tends to reach a critical state of stability, which is characterized by the presence of excess topography. Landslides can rapidly reduce hillside elevations, limiting terrain relief and impacting landform evolution. Excess topography, defined as rock mass inclined at angles greater than a specified threshold, is closely related to unstable slope masses. The essence of a landslide is the disruption of slope equilibrium, often reflected in the presence of excess topography. However, the influence of excess topography on the distribution of co-seismic landslides is not well understood.
Earthquake-induced landslides occur when slopes become unstable and slide due to seismic forces. The instability arises when ground motion alters the internal friction angle and cohesion forces along rock mass defects, making them unable to resist the gravitational forces that cause sliding. The weight of the slope material plays a crucial role in this process. As a key component of landscape evolution, landslides significantly shape geomorphic forms, as indicated by the presence of excess topography. The undulating terrain of a region is the result of long-term structural and surface erosion interactions, as well as material migration and distribution. Landslide development is closely related to the local environment, particularly geomorphic conditions. Seismic landslides also play a vital role in shaping and reorganizing active orogenic belts, influencing subsequent landscape evolution.
On September 5, 2022, a M_S6.8 earthquake struck Luding county, Sichuan province, China, with the epicenter in Hailuogou Glacier Forest Park(29.59°N, 102.08°E), at a focal depth of 16km and a maximum intensity of Ⅸ degrees. The earthquake, lasting approximately 20 seconds, was strongly felt across many parts of Sichuan Province and induced numerous landslides, causing significant damage. The affected area, located at the transition between the Qinghai-Tibet Plateau and the Sichuan Basin, features a typical alpine and canyon landscape with steep terrain and river incision, providing favorable conditions for landslides. The long-term and intense tectonic activity in the eastern Qinghai-Tibet Plateau has resulted in complex topography and geomorphology, providing the material basis and external conditions for earthquake and landslide disasters.
With advancements in science and technology, the Digital Elevation Model(DEM)has become widely used in geoscience research. As DEM accuracy improves, its applications have evolved from qualitative descriptions of geomorphic morphology to semi-quantitative and quantitative analyses of various geomorphic parameters. Geomorphic parameters reveal the structural geomorphic information within topography, essential for understanding regional characteristics and evolution mechanisms. The Luding earthquake serves as a case study for analyzing the influence of topography on the distribution of co-seismic landslides.
In this study, through post-earthquake remote sensing image analysis we identified 1 485 landslides(covering approximately 14.83km²)and analyzed their spatial distribution. Field surveys revealed that most co-seismic landslides are shallow, with relatively small thicknesses, primarily located along the Dadu River. Excess topography calculations based on the ALOS 12.5m DEM and subsequent quantitative analysis of its correlation with co-seismic landslides indicate a strong relationship: with a 30° threshold, excess topography peaks are found along the Dadu River and its tributaries, coinciding with the majority of landslide occurrences. A total of 91.7% of co-seismic landslides are within areas of varying excess topography heights. However, the average height of excess topography in landslide areas(~80m)significantly exceeds landslide thicknesses, suggesting that the Luding earthquake only mobilized a small fraction of the total excess topography. The remaining excess topography may represent potential unstable slopes for future landslides. Furthermore, the spatial distribution of landslides induced by previous earthquakes, such as the Wenchuan earthquake in 2008, the Lushan earthquake in 2013, and the Ludian earthquake in 2014, shows a high degree of consistency. This underscores the importance of understanding the relationship between excess terrain and landslide distribution to enhance the accuracy of earthquake-induced landslide predictions.

3D MODELING AND MAXIMUM POTENTIAL SEISMIC ASSESS-MENT OF THE EASTERN MARGIN FAULT OF DAXING UPLIFT

ZHANG Ya-jing, LI Zheng-fang, ZHOU Ben-gang, XIAO Hai-bo

2024, 46(4):  802-820.  DOI: 10.3969/j.issn.0253-4967.2024.04.003

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The eastern margin fault of Daxing uplift is an important boundary fault in the southeast of Beijing Plain. The fault is located in the southeast of properthe Xiadian Fault and is distributed in the correct order en echelon with the Xiadian Fault, which controls the development of Langgu secondary depression under the extensional tectonic background. Recent shallow seismic reflection profiles and borehole data have found evidence of Holocene activity in the eastern margin of the Daxing Uplift, which has changed the conclusion in recent decades that it has not been active since the late Quaternary. Because the fault is a right order echelon with the Xiadian Fault, and it is similar to the Xiadian Fault in structure, and the Xiadian Fault had the Sanhe-Pinggu M8 earthquake in 1679, it is inferred that the fault has the risk of a large earthquake. It has essential crucial application value to the seismic hazard survey in Beijing. Also, it poses a new challenge to the upper limit of the maximum potential earthquake magnitude of the fault on the eastern margin of the Daxing Uplift.
Quaternary sediments cover the fault on the east margin of Daxing Uplift and are in a hidden state, which results in its geometric features and deep and shallow coupling relationships that cannot be visually demonstrated by two-dimensional data two-dimensional data cannot visually demonstrate. It is of great significance to establish a three-dimensional model of hidden active faults for the hazard assessment of seismic active faults. In this paper, by collecting the fine location data of small earthquakes in this area and collating several shallow seismic geophysical profiles and deep seismic reflection profiles, SKUA-GOCAD 3D geological modeling software is used to build 3D models of the eastern margin fault of Daxing Uplift and the Xiadian Fault based on the section modeling method, and the distribution of the two faults in 3D space is simulated. The geometric features and the relationship between the depth and shallow structure of the two faults are revealed, including 1)a three-dimensional fault model and stratigraphic information map; 2)a three-dimensional model diagram of fault distribution according to dip Angle; 3)Three-dimensional model diagram of fault distribution according to depth and a three-dimensional map of small earthquake distribution. The 3D map shows that there are strong structural similarities between the faults on the eastern margin of the Daxing uplift and the Xiadian faults. The contrast map shown by depth shows that both faults are deep and shallow faults, the shallow faults disappear at about 15km underground, and the deep faults extend downward to cut the lower crust and the Moho surface. The contrast diagram displayed by apparentthe dip Angle clearly reflects that the two faults have obviously different dip angles in-depth and shallow. The deep fault is almost steep, and the shallow fault shows obvious differences in different sections. The distribution range of small earthquakes is 0-25km, of which the dominant distribution range is 10-20km. Therefore, it is speculated that the east margin fault of Daxing Uplift may have the seismogenic capacity similar to the Sanhe-Pinggu M8 earthquake in 1679. However, as existing studies have shown that the activity of the Xiadan fault and its southern extension section-eastern margin of the Daxing Uplift in this region gradually weakens from north to south, the maximum potential earthquake magnitude of the east margin fault of the Daxing Uplift is inferred in this paper to be less than Sanhe-Pinggu M8 earthquake in 1679.
Finally, by using the structural analogy of the Xiadian Fault on the eastern margin of the Daxing Uplift, and based on the structural similarity of the two faults, this paper evaluates the maximum potential earthquake magnitude that may be induced by the Daxing Fault using different experiential relations of magnitude-fault rupture scale fitted by predecessors in North China. The conclusion is as follows: the distribution range of the magnitude of the earthquake is 7.3-7.4.
Based on the structural analogy with the Xiadian Fault and the empirical relationship between magnitude and rupture scale, the maximum potential earthquake magnitude induced by the eastern margin fault of Daxing uplift is estimated to be magnitude 7.5. This conclusion has important scientific guiding significance for earthquake disaster prevention and control in the capital area, and should be paid attention to and actively take prevention and avoidance measures.

EVIDENCE OF HOLOCENE ACTIVITY OF NALATI FAULT ZONE WITHIN THE TIANSHAN

WANG Lei, REN Zhi-kun, HE Zhong-tai, JI Hao-min, LIU Jin-rui, GUO Long, LI Xing-ao

2024, 46(4):  821-836.  DOI: 10.3969/j.issn.0253-4967.2024.04.004

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The Tianshan orogenic belt, extending across the Euro-Asian plates, is one of the most significant intracontinental orogenic belts globally. Spanning over 2 500km, it traverses China, Kazakhstan, Kyrgyzstan, and Uzbekistan from east to west. The belt has been continuously uplifted due to the collision of the India-Eurasia plate during the Cenozoic era. The Tianshan is divided into three segments: North Tianshan, Middle Tianshan, and South Tianshan. The crust in this tectonic region is being shortened in the north-south direction, and a series of NEE-trending or NWW-trending strike-slip faults have developed to accommodate the deformation. The Nalati fault zone serves as the collision suture between the Central Tianshan block and the Tarim block and marks the boundary between Central and South Tianshan. This fault zone trends NEE and extends southwest into Kyrgyzstan, connecting to the Nikolayev line. Its eastern segment is located north of the Dayouludusi basin. The north-south shortening rate is approximately 2.0mm/a, and the horizontal strike-slip rate is about 2.9mm/a. Reports indicate a north-south shortening rate of 0.8-1.1mm/a since the late Quaternary, suggesting it is a significant Holocene active fault zone. However, research on this fault zone's activity is limited, with most studies focused on its eastern segment. Research on other sections remains scarce.
This study focuses on the middle segment of the Nalati fault zone in Tekes county, Ili Prefecture. The Tekes section trends ENE, starting from Qiongkushitai village in the east, passing through Kalatuori, Ayakeaqia, and Kalawenkeer, and reaching Burili in the west, spanning approximately 55km. Methods employed include remote sensing image interpretation, field geological investigation, UAV aerial surveys, trench excavation, Radiocarbon-14 dating, and semi-automatic horizontal dislocation measurement. The main findings are as follows: 1)The linear geomorphological features of the Tekes segment are prominent, with typical fault geomorphological signs such as fault cliffs, triangles, scarps, bulges, gate ridges, passes, guanmen mountains, and left-lateral dislocation ridges and gullies widely observed.; 2)Small Unmanned Aerial Vehicle Mapping and LaDiCaoz semi-automatic dislocation measurement and analysis indicate a minimum horizontal displacement of approximately 3.4m; 3)Faults are developed in the Proterozoic and Paleozoic strata. A trench 4m long and 1.6m wide excavated at a series of fault reverse scarps revealed a sedimentary event of the sag pond at the hillside, indicating at least four paleo-earthquake events; 4)To date the paleo-earthquake events, we collected 11 sediment samples for Radiocarbon-14 dating at the BETA Analytic laboratory. Results show that the sample at a depth of 2m is about(7.06±0.03)ka BP, and the latest colluvial wedge is about(1.67±0.03)ka BP; 5)Using OxCal age correction, the ages of the four paleo-earthquake events were determined at a 95.4%confidence level: event E1 occurred between 2757BC and 413AD, event E2 between 3581BC and 429BC, event E3 between 4702BC and 3932BC, and event E4 between 5742BC and 5230BC. In summary, we propose that the middle segment of the Nalati fault zone has been active since the Holocene.

COMPREHENSIVE STUDY OF THE CURRENT CONNECTION MODE OF A NORMAL FAULT STEPOVER: AN EXAMPLE OF THE CHANFANG STEPOVER ON THE KOUQUAN FAULT IN THE SHANXI RIFT SYSTEM, CHINA

HUA Chun-yu, SU Peng, SHI Feng, XI Xi, GUO Zhao-wu

2024, 46(4):  837-855.  DOI: 10.3969/j.issn.0253-4967.2024.04.005

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The overlapping area between the ends of adjacent fault segments is known as a fault stepover. The normal fault stepover has two endmember connection modes, i.e., soft-link mode and hard-link mode. The soft-link stepover's border faults are connected through a relay ramp, and the border faults' displacements are transmitted through the bending deformation of the relay ramp. The hard-link stepover's border faults are connected through a breaching fault, and the border faults' displacements are transmitted through the faulting deformation of the breaching fault. Distinguishing the current connection mode of a normal fault stepover can shed light on the evolution stage of the normal fault. It can also indicate the potential earthquake rupture pattern in the stepover, which is important for evaluating the seismic hazard of engineering sites within the stepover. The straightforward technique to distinguish the current connection mode of a normal fault stepover is to determine whether an active breaching fault exists within the stepover. However, in many cases, due to the small amount of accumulated offset and human modification of the breaching fault, it is always hard to observe fault scarps in the field even though the fault stepover is deforming under the hard-link mode.
The Shanxi Rift System is a prominent intracontinental rift zone in East Asia. It comprises a series of left-stepping en échelon grabens bounded by high-angle normal faults. It is distributed in an S-shaped geometry with a narrow, NNE-trending zone in the middle and two broad, NEE-trending extensional zones in the north and south. The Shanxi Rift System is one of the strong earthquake-prone regions in China. Since 780 BC, the Shanxi Rift System has hosted three M8 earthquakes, five M7-7娻 earthquakes, and a series of M6-7 earthquakes. The Kouquan fault is the western border fault of the Datong Basin in the northern part of the Shanxi Rift System. A stepover is developed near the Chanfang village on the Kouquan fault, which we named the Chanfang stepover.
In this study, we use a combination of the tectonic geomorphological investigation in the field, high-resolution topographic data analysis, and Ground Penetrating Radar(GPR)surveying to study the current connection mode of the Chanfang stepover. Three fault outcrops on the border faults of the Chanfang stepover are investigated. The outcrop D1 is in an alluvial fan covered by loess on the southwestern boundary fault of the Chanfang stepover. Two branch faults are present at this outcrop. One offsets a bedrock surface and the alluvial fan's gravel layer. The other is the boundary between a gravel layer and the loess, where imbricated gravel can be observed. The fault outcrop D2 is also on the southwestern boundary fault of the Chanfang stepover. The fault at the outcrop D2 offsets a gravel layer and the vertical offset of the top of the gravel layer is approximately 2m. The fault outcrop D3 is located on the northeastern boundary fault of the Chanfang stepover. At the outcrop D3, the fault separates the gneiss of the Archean Jining Group from the loess. Based on the Chinese GF-7 satellite stereo imagery, we obtain the high-resolution digital elevation model(DEM)covering the Chanfang stepover and identify two levels of geomorphic surfaces, i.e., T1 and T2. The surface T1 is an alluvial fan, mainly developed in the piedmont areas. The surface T2 is an erosion surface distributed in the bedrock mountain. To quantify the deformation pattern within the Chanfang stepover, we construct a series of topographic cross-sections on the surface T1 and find a gentle geomorphic scarp within the stepover. We conduct two GPR surveying lines across the Chanfang stepover. On the GPR images, we identify two known faults, F₁ and F₃, that previous researchers have mapped and a buried fault, F₂, that has not been constrained previously.
The Fault F₂ observed by the GPR is consistent with the geomorphic scarp constrained on the DEM, suggesting that a breaching fault exists in the Chanfang stepover. The existence of the Chanfang breaching fault indicates that the connection mode of the Chanfang stepover is the hard-link mode. We thus infer that the future earthquakes on the Chanfang stepover may cause concentrated surface ruptures on the breaching fault. This study shows that the combination of the tectonic geomorphologic investigation in the field, high-resolution topographic data analysis, and the GPR survey can effectively locate near-surface, slow, active normal faults. This comprehensive technique can be used for the connection mode of a normal fault stepover.

THE SPATIAL AND TEMPORAL CHARACTERISTICS OF PRESENT-DAY SEISMICITY IN NORTHEASTERN LONGMENSHAN FAULT ZONE

HU Nan, LONG Feng, WANG Ying, XU Liang-xin

2024, 46(4):  856-875.  DOI: 10.3969/j.issn.0253-4967.2024.04.006

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The Longmenshan fault zone, situated along the eastern margin of the Tibetan plateau, represents a significant thrust tectonic belt characterized by pronounced segmentation. It is delineated into northern and central-southern segments at Beichuan, and along its depth, it features three sub-parallel fault belts: the Houshan fault, the Central fault, and the Qianshan fault, extending from the northwest to the southeast. Geological research indicates that since the Quaternary, the central-southern segments of the Longmenshan fault zone have exhibited considerable seismic activity, whereas the northern segment has shown minimal signs of movement. However, paleo-earthquake studies have identified substantial historical seismic events in the Qingchuan fault, a component of the northern segment, dating back to the Holocene. The devastating 2008 Wenchuan earthquake(M_S8.0), which occurred in the middle section of the Longmenshan fault zone, resulted in a 240-km-long surface rupture along the Central fault. A multitude of aftershocks radiated northward from the epicenter, with no discernible surface ruptures observed in the northern segment. This study aims to provide a comprehensive analysis of the kinematic features of the northern segment by re-evaluating the Wenchuan earthquake's aftershocks and employing focal mechanisms derived from previous studies.
Seismic activity is intrinsically linked to active tectonics, and the precise localization of minor earthquakes can offer critical insights into the underlying seismogenic processes and mechanisms. In this paper, we have compiled early aftershock relocation data and further refined the relocation of small earthquakes using an integrated seismic location technique. Seismic phase data were obtained from the networks in Sichuan, Gansu, and Shaanxi over the past decade, spanning from 2010 to 2020. To mitigate the impact of crustal velocity variations, an optimal one-dimensional velocity model for the study area was initially inverted using the VELEST program. The Hypo2000 program was then utilized to adjust the initial seismic source positions, followed by the application of the double-difference method for the relocation of minor earthquakes. The reliability of the localization outcomes, determined using the LSQR method, was verified by the SVD method. Consequently, 10 653 minor earthquakes were relocated with an average travel time residual of 0.053s, a horizontal location error of 281m, and a vertical location error of 260m.
In the southern extremity of the study area, the relocated earthquakes are predominantly aligned along the parallel faults flanking the primary rupture zone. In the south-central region, the relocated earthquakes exhibit deviations from the rupture zone, revealing multiple seismic clusters. Towards the northern end, the relocated earthquakes demonstrate a migration from the main rupture towards the Qingchuan fault. The depth profiling of seismic sources reveals that the relocated earthquakes are concentrated between 8-15km deep, all situated above the 500℃ isothermal surface. The depth profile in the southern region continues the characteristics of the main rupture surface of the Wenchuan earthquake, while the dip angle becomes increasingly steep as it progresses northward. The northern end's depth profile suggests an interaction between the rupture surface and the Qingchuan fault. Additionally, the analysis of 32 focal mechanisms exceeding M_L4.0 within the study area corroborates the geometrical structures of the fault zone, as revealed by the spatial distribution of the relocated earthquakes, further validating the reliability of relocation.
A comprehensive analysis suggests that the current seismicity in the northern section of the Longmenshan fault zone is multifaceted, with ongoing activity on the main rupture surface(afterslip)and slip on secondary new rupture surfaces triggered by the mainshock. It is hypothesized that the spatial distribution of the relocated earthquakes retains segmented characteristics. In the southern region of the study area, thrust slip induced by the main rupture continues; in the middle region, new ruptures are concurrently active with the main rupture; and in the northern region, influenced by the high velocity of the upper crust around Ningqiang-Mianxian, the rupture zone vanishes at the surface, with the deep triggering of the Qingchuan fault by stress transfer being evident. In conclusion, the complex spatial characteristics of the current seismic activity in the northern section of the Longmenshan are attributed to the interplay of pre-existing faults, new ruptures, and the main rupture, reflecting the spatially heterogeneous process of stress transfer and adjustment following the Wenchuan earthquake, potentially linked to the complex geological structure of the region.

LATE CENOZOIC FLOOD BASALT ERUPTION IN DALINOR, INNER MONGOLIA

CHANG Li-wen, ZHAO Yong-wei, LI Ni, SUN Jin-heng

2024, 46(4):  876-892.  DOI: 10.3969/j.issn.0253-4967.2024.04.007

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Fissure eruption refers to volcanic activity where magma is expelled to the Earth's surface through cracks in the crust. During such eruptions, large volumes of low-viscosity lava flow rapidly along these fissures, forming extensive flood-like lava fields known as “flood basalts”. Fissure eruptions are characterized by their significant magma scale and devastating impact. Although there is no historical record of volcanic fissure eruptions in China, numerous late Cenozoic lava platforms have been discovered in XilinGol League and Chifeng, Inner Mongolia. These platforms likely represent overflow basalts formed by continental volcanic fissures. In this study, we focus on investigating the lava platform within the Dalinor volcano group located in XilinGol area using tools from volcanic physics and geochemistry. The XilinGol League in Inner Mongolia hosts extensive late Cenozoic lavas, encompassing an area of approximately 10 000km² and representing one of China's largest basaltic provinces. This volcanic field, formed through flood basalt volcanism, has remained active since the late Cenozoic and poses potential risks of future eruptions. To enhance our understanding of its origin and assess eruption hazards associated with intracontinental flood basalt volcanism, this study focuses on the latest lava platforms within the Beilike region of the Dalinor Volcanic Field. In this study, we investigate volcanic activity by examining its eruptive characteristics, including magma temperature, lava viscosity, and lava flow velocity. During the volcanic eruption, a significant portion of the lava engulfed the gently sloping surfaces of fluvial and lacustrine deposits, resulting in the topographic formation of expansive lava plateaus. Throughout this process, surface lava underwent condensation to give rise to pahoehoe lavas. In contrast, the underlying plastic lava continued its effusion along eruptive fissures, causing vertical compression of the solidified crust and subsequent formation of lava ridges. In the vicinity of the eruptive fissure, a series of low ridges measuring 3-8 meters in height and tens to hundreds of meters in length emerged. At a distance over 3km from the eruption fissure, the lava flow exhibits a banded distribution with an elevated central portion and inclined edges, displaying characteristic lava levees. Columnar jointing is commonly observed at the periphery of the plateau profile. The profile exhibits stacked layers of lava, indicating a laminar flow movement of the lava flow. These geological features are consistent with the characteristics typically associated with flood basalt eruptions. The whole rock geochemistry indicates that the lava in Beilike belongs to olivine tholeiitic basalt and alkali olivine basalt. The lava is characterized by phenocrysts primarily composed of olivine and clinopyroxene, particularly augite. The Mg# values of clinopyroxene vary between 58.59 and 80.69. The eruption temperature of Beilike lava is determined by applying clinopyroxene-melts geothermo-barometry inversion, yielding a range of 1 123.2-1 173.4℃. Additionally, the viscosity of the erupting lava is obtained using a previously established calculation model, resulting in values ranging from 30 to 187Pa·S. This paper investigates the flow dynamics of “fissure eruption” through an analysis based on principles of lava fluid mechanics while considering relevant physical properties. We assume that high-temperature lava behaves as a Newtonian fluid and consider zero overflow velocity near the vent during fissure eruptions. The influence on lava flow primarily stems from surface slope, gravity, and inherent fluid properties rather than temperature variations throughout the process. We assume that the surface slope is 0.5 degrees. The lava temperature is 1 120℃, and the thickness of the lava is 1m. By adopting this approach, we can calculate the maximum attainable flow velocity upon reaching a stable state. It has been calculated that during fissure eruptions in the study area, the velocity of lava flows primarily ranged from 0.4m/s to 1m/s, with occasional instances reaching as high as 2.5m/s. The present study unveils the eruptive characteristics of flood basaltic volcanism in the Belike region. It establishes the physical parameters of the lava flows, thereby providing essential data for formulating strategies to mitigate future volcanic eruption disasters. The key parameters presented here not only contribute to the understanding of volcanism in Inner Mongolia but also hold significant reference value for basaltic volcanism in other continental intraplate environments across China. This research will enhance our understanding of this unique form of volcanism while providing a scientific basis for mitigating volcanic disasters. Moreover, the calculation methods and steps employed to derive these parameters may readily apply to other volcanic fields.

STUDY ON SURFACE WAVE TOMOGRAPHY OF THE A'ERSHAN VOLCANOES

HOU Jie, WU Qing-ju, YU Da-xin, YE Qing-dong

2024, 46(4):  893-915.  DOI: 10.3969/j.issn.0253-4967.2024.04.008

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Since the Cenozoic era, a series of intraplate volcanic groups have developed along the east and west sides of the Songliao Basin in the eastern part of the Central Asian orogenic belt. The A'ershan volcanic group is one of the Cenozoic intraplate volcanoes in the eastern section of the Central Asian orogenic belt. Further study of this volcanic group is of great significance for exploring and understanding the genesis of intraplate volcanoes in the eastern section of the Central Asian orogenic belt. In the past, the distribution of mobile seismic stations and some fixed stations used for imaging research on the A'ershan volcanic group was relatively sparse and did not fully cover the A'ershan volcanic group. The resolution of the crust-mantle structure obtained in the past was also slightly insufficient for exploring the genesis mechanism of the A'ershan volcanic group. This article utilizes the vertical teleseismic waveforms of 29 broadband mobile seismic stations near the A'ershan volcanic group and 8 fixed stations around them from May 2019 to December 2021. Through frequency-time analysis technology, 11 775 Rayleigh wave phase velocity dispersion between two stations is extracted. After excluding non-monotonic rise and phase velocity dispersion curves that differ significantly from most dispersion distributions, 11 010 high-quality Rayleigh wave fundamental phase velocity dispersion curves were ultimately obtained. Then, based on classical ray theory, the two-dimensional phase velocity distribution with a period of 10-80s and a grid size of 0.5°×0.5° is inverted by using the traditional dual station method. Except for areas not covered by radiation in the edge zone, the lateral spatial resolution of phase velocity in the study area is basically within 50km. The checkerboard test also showed that dividing the grid size of the study area into 0.5°×0.5° is feasible, and anomalies with a central area scale less than 0.5°×0.5° can also be identified. Afterward, the CRUST1.0 model was used as the initial crustal model, and the PREM model was used as the initial mantle model. The crustal thickness results obtained from the receiver function were used to constrain the thickness of each layer in the initial crustal model, further reconstructing the three-dimensional S-wave velocity structure of the crust and upper mantle in the study area. The results show that: within the range of the middle and lower crust, the S-wave velocity in the A'ershan volcanic area exhibits apparent low-velocity anomalies. Based on the characteristics of the high wave velocity ratio in the area, it is speculated that there may be a crustal magma chamber in the A'ershan volcanic group. There are multiple high-velocity anomaly structures within a depth range of 40-150km in the A'ershan volcanic group. The difference in the depth of high-velocity anomalies indicates the heterogeneity of the lithosphere thickness, and it is speculated that the thickness of the lithosphere in the A'ershan volcanic area does not exceed 100km. The deeper distribution of high-velocity anomalies may represent the dismantled lithosphere, while the shallower distribution of high-velocity anomalies may represent the undeveloped lithosphere or residual lithosphere after dismantling, reflecting the possibility of lithospheric detachment and subsidence in the region. There are S-wave low-velocity anomalies in the upper mantle on the north and south sides of the A'ershan volcanic group, connecting the asthenosphere and the exposed positions of the A'ershan volcanic group on the surface. The low-velocity anomalies on the north and south sides merge at a depth of 150km. Based on the high heat flux value, high V_P/V_S, and crustal thinning characteristics of the surface near the distribution area of the A'ershan volcanic group, as well as the previous conclusion based on remote seismic P-wave and S-wave travel time tomography results that there is a clear connection between the low-velocity anomaly below the A'ershan volcanic group and the southern edge of the Songliao Basin in the deep mantle, it is speculated that this low-velocity anomaly may be caused by the upwelling of asthenosphere material caused by the detachment of the lithosphere in the Songliao Basin.

CHARACTERISTICS OF UPPER CRUSTAL SHEAR WAVE SPLITTING IN THE NORTHEASTERN TIBETAN PLATEAU

LI Shu-yu, GAO Yuan, JIN Hong-lin, LIU Tong-zhen

2024, 46(4):  916-933.  DOI: 10.3969/j.issn.0253-4967.2024.04.009

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The northeastern part of the Tibetan plateau, comprising secondary blocks such as Qiangtang, Bayan Har, Qaidam and Qilian, forming a complex tectonic pattern. This region, located at the interface between the South China Block and North China Block, has been at the forefront of the Indo-Eurasian plate collision, experiencing significant tectonic deformation. Consequently, it serves as an ideal natural laboratory for the study of plate tectonics, crustal dynamics, and seismic activity. Shear wave splitting is a method used to study the anisotropy of media, based on the phenomenon where shear waves split into two sets of wave trains, fast and slow, due to the anisotropy of the medium during propagation. In the mid-to-upper crust, this splitting characteristic is often identified through the analysis of local earthquake waveforms. The fast wave direction typically aligns with the oriented arrangement of vertical cracks, governed by the regional principal horizontal compressive stress direction. In contrast, the slow wave is nearly perpendicular to the fast wave, and its time delay is closely related to the crack geometry and internal fluid state, indirectly reflecting the degree of medium anisotropy. In this study, we have collected waveforms of local small earthquakes from January 2010 to September 2021 on the northeastern Tibetan plateau and calculated two anisotropy parameters: fast-wave polarization direction and slow-wave time delay, using shear wave splitting analysis. We subsequently construct a detailed spatial distribution map of the anisotropic parameters of the upper crust. The fast-wave polarization direction is dominated by an ENE direction, roughly parallel to the regional principal compressive stress direction, indicating that the anisotropy of the upper crustal medium is mainly controlled by regional tectonic stress. Several relatively weaker secondary fast-wave polarization directions, including NNW, WNW, and near EW, vary widely across the northern and southeastern parts of the Qilian block and the northern part of the Qiangtang block. These directions are approximately parallel to the widely distributed NW-trending faults, indicating the influence of the fault system. The fast-wave polarization directions on the northeastern edge of the Qaidam block are more discrete, with the northern margin stations showing WNW direction dominance and the north-central part showing NE or weaker NW dominance, affected by the combined effects of stress, faults, rock properties, and other factors. The slow-wave delay time serves as a quantitative indicator of the anisotropy, reflecting variations in stress level within the medium. With the thrust fault system in the northern part of the Qilian block, the slow-wave time delay varies from 1.7ms/km to 6.3ms/km, averaging(3.2±2.1)ms/km. Notably, these time delays are larger in the east than in the west, reflecting differences in the stress environment. The southeastern Qilian block and the northeastern margin of the Qaidam block exhibit a relatively uniform average time delay of(5.1±2.4)ms/km, with an overall range of 2.5ms/km to 5.7ms/km. The similar distribution of time delays may be related to similar rock properties and tectonic environments. At the northern edge of the Qaidam basin, the WNW-oriented fast-wave polarization direction, coupled with a relatively consistent slow-wave time delay ranging from 3.1ms/km to 4.5ms/km, may be a response to the high-pressure metamorphism of fractures in the deep crust. The northern part of the Qiangtang block shows a stable degree of deformation, as evidenced by the slow-wave time delay averaging(4.5±0.8)ms/km with a small standard deviation. Both the northern Qiangtang block and the periphery of the Lajishan faults(encompassing the southeastern Qilian and northeastern Qaidam blocks)host volcanic arcs and reservoir formations. However, the former exhibits shorter time delays compared to the latter, potentially attributed to differences in rock physical properties and the tectonic environment. Due to the heterogeneous distribution of data, further studies are needed to gain a more comprehensive understanding of upper crustal deformation.

EMPIRICAL EXTRAPOLATION MODEL OF SITE SHEAR WAVE VELOCITY AND ITS APPLICABILITY IN SHANDONG PROVINCE

LI Zhi-heng, XIE Jun-ju, LI Ke-wei, WEN Zeng-ping, LI Xiao-jun, WANG Zhi-cai, XU Hong-tai, ZHAO Xiao-fen, ZHANG Na

2024, 46(4):  934-954.  DOI: 10.3969/j.issn.0253-4967.2024.04.010

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Site shear wave velocity is a pivotal parameter for site classification and for quantitatively assessing the site's impact on ground motion. It has extensive applications in engineering seismic design and rapid post-earthquake damage assessment. China's seismic design standard, GB50011-2010, primarily uses two indicators for site classification: the thickness of the soil layer and the equivalent shear wave velocity of the top 20m of soil. In contrast, the United States and Europe utilize the average shear wave velocity, V_S30, at a 30m depth for site classification. Studies have indicated that considering only the top 20m of soil in classification overlooks the influence of deeper low-velocity layers on long-period structures. Additionally, reliance on the top 20m's shear wave velocity can be problematic due to its sensitivity to the properties of the fill layer and the potential unreliability of measurements in this shallow depth. To address these issues, scholars in China advocate increasing the depth considered in site classification from 20m to 30m. Current standards focus on soil layers not exceeding 20m, resulting in engineering boreholes and shear wave velocity measurements that rarely exceed this depth, especially in harder sites where boreholes often extend less than 10m. The development of site shear wave velocity extrapolation models is crucial for accurate site classification and ground motion parameter determination, particularly in the absence of deep borehole data.
Various extrapolation methods have been proposed, including the constant velocity method, velocity gradient method, and conditional independence method. The constant velocity method assumes a uniform velocity below the measured depth, while the velocity gradient method fits empirical relationships in a linear or logarithmic form. The conditional independence method leverages correlations between instantaneous and average shear wave speeds at various depths. Domestic research has led to the establishment of regional shear wave velocity extrapolation models, though their applicability is often limited to specific regions. The selection of the most suitable extrapolation method for a given region requires further investigation.
This study focuses on Shandong province, a region within China's North China Seismic Zone with a significant risk of strong earthquakes. With nearly 80% of the province requiring seismic fortification of at least Ⅶ-degree intensity, research into shear wave velocity extrapolation models is of practical importance for site categorization and seismic defense. Utilizing extensive shear wave velocity profiles and borehole lithology data, this study applies constant velocity, velocity gradient, and conditional independence methods to establish regional extrapolation models. It evaluates the applicability and accuracy of these methods in Shandong and recommends an empirical model for shear wave velocity extrapolation.
Key findings include: 1)For borehole depths less than 10m, the empirical extrapolation models for V_S20 and V_S30, utilizing the three discussed methods, exhibit considerable inaccuracies. Caution is advised when applying the wave velocity predictions from this study to depths under 10m. Notably, the BCV method tends to significantly underestimate when extrapolating from shallow data. The BCV method's predictions become more reliable and exhibit reduced error only when borehole depths exceed 10m for V_S20 and 15m for V_S30; 2)The empirical extrapolation models for V_S20 and V_S30 in Shandong province, developed using the velocity gradient method, align well with actual measurements. These models' regional applicability is supported by comparative regional analyses. The V_S30 predictions for Shandong are found to be generally lower than those in Japan but closer to those in California and the Beijing plain, with a slight increase in the higher wave speed range; 3)Considering the models' accuracy and regional applicability, the study advocates for the empirical extrapolation models of V_S20 and V_S30 for Shandong Province based on the conditional independence method. These models minimize total prediction errors across various depths. While the BCV model's performance improves at greater depths, the velocity gradient extrapolation model's efficacy diminishes.
Overall, this study contributes to the advancement of seismic design practices in Shandong province by offering empirical extrapolation models for V_S20 and V_S30, enhancing the understanding of ground motion characteristics and supporting more robust seismic resilience strategies.

STUDY ON SNR AND ACCURACY OF GEOELECTRICAL RESISTIVITY OBSERVATION

ZHANG Yu, WANG Lan-wei, ZHANG Shi-zhong, ZHANG Xing-guo, HU Zhe

2024, 46(4):  955-971.  DOI: 10.3969/j.issn.0253-4967.2024.04.011

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The geoelectrical resistivity observation method has played a significant role in earthquake monitoring and prediction research over the past 55 years. In the current geoelectrical resistivity observation system, the geoelectrical resistivity is indirectly measured by measuring the supply current and the artificial potential difference. To meet monitoring forecasting needs, the measurement system is required to have small measurement errors and high accuracy. When the observation instruments are applied to a seismic station, the primary influence is the site noise disturbance, the observation accuracy depends on the signal-to-noise ratio(SNR)of the measurement process. This study combines the measurement instruments and measurement methods used in the current network of geoelectrical resistivity observation stations and studies the relationship between observation accuracy and SNR in geoelectrical resistivity observation. Analyses the changes of SNR under the conditions of different observation configurations on the ground and borehole.
Since SNR is related to the intensity of the interference and the artificial potential difference, also related to the supply current and the observation configuration. To improve the SNR under the interference conditions of a certain observing site, it can be done in 2 ways: to increase the supply current, and to reduce the electrode distance. Every doubling of the artificial potential difference requires a doubling of the power supply current and a 4-fold increase in power supply, with a corresponding increase in the SNR of 6 dB; if the SNR is increased by 20dB, it is equivalent to a 10-fold increase in supply current and a 100-fold increase in power supply, which is more difficult to achieve. When the power supply current is unchanged, and the main interference effect is a uniform field, the electrode distance reduction can effectively improve the SNR. If the ground electrode distance is reduced to 1/3 of the original, or the borehole electrode distance is reduced to 1/4 of the original, the SNR can be increased by about 20dB.
Why could borehole observations improve observation accuracy?Because electrode distance is reduced. Burying the electrode at a certain depth in the ground cannot improve the SNR of observation but will reduce the SNR with the increase of electrode burying depth and decrease. However, when the electrode burial depth reaches a certain depth, i.e., the ratio of electrode burial depth H and measurement electrode distance MN is greater than 4, the SNR reaches a stable level and no longer decreases, with a maximum decrease of about 6dB. Therefore, borehole observation must reduce the observation electrode distance while burying the electrodes deeper to effectively improve the measurement SNR and reduce the influence of electromagnetic interference. Based on the observation results of different depths and electrode distances of Jiangning station, discussed the SNR and accuracy of geoelectrical resistivity observation and analyzed the rationality of the design of downhole devices.
Through the above study, the following conclusions can be obtained:
(1)The SNR and observation accuracy of resistivity observations are related to the number of measurements. As the number of measurements increases, the SNR and the accuracy improve. However, when the number of measurements exceeds 20, the improvement of SNR and accuracy becomes less noticeable.
(2)When the primary interference source is a uniform field, and the power supply current is constant, the SNR increases significantly with the shortening of the pole distance. To meet the requirement of observation accuracy, it can be considered to increase the observation SNR.
(3)Reducing the observation electrode distance can increase the SNR and improve the observation accuracy. Under the same observation electrode distance, the SNR of the borehole configuration is smaller than that of the surface configuration. It decreases with the increase of electrode depth. However, when the electrode depth reaches a certain depth, the SNR does not decrease anymore, with a maximum decrease of about 6dB.
(4)When the borehole electrode is buried at a depth of 100m to several hundred meters, and the observation is closer to the target layer, appropriately reducing pole distance will not affect the monitoring capability. However, electrode distance design requires a combination of factors such as effective detection range, media inhomogeneity and backfill material effects.
The above research results can provide a reference for the design of geoelectrical resistivity observation configuration and selecting observation stations in work.

2.5-D CSAMT FORWARD NUMERICAL SIMULATION BASED ON ANISOTROPIC AND POLARIZED MEDIA

YU Sheng-hong, TANG Xin-gong, XIONG Zhi-tao

2024, 46(4):  972-992.  DOI: 10.3969/j.issn.0253-4967.2024.04.012

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The Controlled Source Audio Magnetotelluric(CSAMT)method is a frequency-domain electromagnetic sounding technique that utilizes an artificial field source. It has been widely used in the exploration of various resources. Due to tectonic stress, temperature variations, material migration, and geological deposition within the Earth, the electrical parameters of underground media may vary with the direction of current, exhibiting electrical anisotropy. Additionally, rocks and ores exhibit complex electrochemical phenomena under the influence of an artificial field source, resulting in resistivity being a frequency-dependent complex number, known as the induced polarization(IP)effect. Electrical anisotropy and the IP effect are common phenomena in Earth's internal media, significantly impacting CSAMT observation data. Traditional CSAMT methods typically assume that the underground medium is electrically isotropic and lacks IP effects when performing forward and inversion numerical simulations. However, the actual underground medium often exhibits electrical anisotropy, the IP effect, or both. Adopting an isotropic model assumption during data processing and interpretation can therefore lead to serious errors in the results. Current research on CSAMT mainly considers the influence of either anisotropy or the IP effect individually. Since the anisotropy and polarization effect model involves multiple parameters, this complexity increases the difficulty of research. Consequently, there are relatively few studies on the combined influence of anisotropy and the IP effect on CSAMT responses. With increasing demands for exploration accuracy, it is imperative to study the response characteristics of the CSAMT electromagnetic field under the combined effects of anisotropy and the IP effect. In this paper, we employ a 2.5-dimensional model to derive the electromagnetic field partial differential equation for a 2.5-dimensional anisotropic medium based on Maxwell's equations with a source. The 2.5-dimensional CSAMT numerical simulation of anisotropy is achieved using the Galerkin finite element method. The real resistivity is replaced by complex resistivity using the Cole-Cole model. We discuss the effects of the Euler angle and principal axis resistivity in electrical anisotropy parameters, as well as three polarization parameters in the Cole-Cole model, on the CSAMT electromagnetic field response. Finally, we study the controllable source electromagnetic field response considering both anisotropy and IP effects.
The results show that electrical anisotropy significantly impacts the response of 2.5-D CSAMT, primarily depending on the Euler angle and principal axis resistivity values. Depending on these values, electrical anisotropy can either increase or decrease the CSAMT response value. Among the three polarization parameters, polarizability has the greatest influence on the calculation results, with the apparent resistivity value gradually decreasing as polarizability increases. The time constant and frequency correlation coefficient have relatively smaller influences on the calculation results, and their effects are consistent. The combined effect of anisotropy and the IP effect on the 2.5-D CSAMT response is more complex. Electrical anisotropy, influenced by the Euler angle and principal axis resistivity, can either increase or decrease the CSAMT response result, while the polarization effect generally decreases the CSAMT response result. Thus, when both factors act simultaneously, their effects can overlap or offset each other depending on the values of the Euler angle and principal axis resistivity. The findings of this paper have significant theoretical implications for improving the accuracy of CSAMT data processing and interpretation.