Table of Content

20 December 2024, Volume 46 Issue 6

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PETROGENESIS OF THE MOST RECENT VOLCANISM IN MAINLAND CHINA: EVIDENCE FROM THE ISOTOPIC CHARACTERISTICS OF ASHIKULE VOLCANIC ROCKS

MAO Xiang, BAI Xiang, YU Hong-mei, ZHAO Bo, CHEN Hui-zhi

2024, 46(6):  1227-1247.  DOI: 10.3969/j.issn.0253-4967.2024.06.001

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The Ashikule Volcanic Cluster(AVC), located in the western Kunlun region of the northwestern Tibetan plateau, represents the most recent volcanic activity on Mainland China. This volcanic cluster, which erupted continuously from the Pleistocene to the Holocene, predominantly produced trachyandesites and trachytes, with minor occurrences of phonotephrites, basaltic trachyandesites, and rhyolites. In this study, we present zircon U-Pb-Lu-Hf and whole-rock Sr-Nd-Pb isotopic analyses for volcanic rock samples from AVC. By integrating these data with petrographic, geochronological, and geochemical findings from Yu et al.(2020), we propose further constraints on the petrogenesis of the volcanic rocks and the geodynamic evolution of the western Kunlun region from the Pleistocene to the Holocene.

Zircon U-Pb-Lu-Hf isotopic analyses were conducted on five samples: Two trachyandesitic (515-01 and 518-14), two trachytic (521-1 and 521-4), and one rhyolitic(517-B-03). Together with previous ⁴⁰Ar/³⁹Ar dating, the magmatic zircon grains reveal negative ε_Hf(t) values ranging from -8.8 to -4.4 for the trachyandesitic samples, -8.6 to -5.7 for the trachytic samples, and -9.1 to -6.7 for the rhyolitic sample, suggesting an enriched magma source. The trachyandesitic samples also contain Paleozoic to Mesozoic zircons (165-2 352Ma) with characteristics such as small oval shapes or core-rim structures, indicating that they are inherited zircons. These inherited zircons display ε_Hf(t) values from -3.1 to 9.8, suggesting the involvement of metasedimentary components in the magma source.

Whole-rock Sr-Nd-Pb isotopic analyses were conducted on eight samples(four trachyandesitic, three trachytic, and one rhyolitic), revealing ⁸⁷Sr/⁸⁶Sr ratios of 0.709 395-0.711 441 and ¹⁴³Nd/¹⁴⁴Nd ratios of 0.512 154-0.512 355. In the ¹⁴³Nd/¹⁴⁴Nd-⁸⁷Sr/⁸⁶Sr diagram, these samples plot to the right of the EM Ⅰ region in the fourth quadrant, indicating a relationship with EM Ⅱ-type magmatism. The samples exhibit ²⁰⁷Pb/²⁰⁶Pb ratios of 15.652-15.673 and ²⁰⁶Pb/²⁰⁴Pb ratios of 18.681-18.754, aligning with EM Ⅱ-type and lower crust-derived magmatism on the ²⁰⁷Pb/²⁰⁴Pb-²⁰⁶Pb/²⁰⁴Pb diagram.

In the Rb/Nd-Rb diagram, the Ashikule volcanic rocks display an oblique distribution, indicating processes of partial melting or magma mixing, which is further supported by their alignment with the mixing trend on the 1/V-Rb/V diagram. Geochemical modeling results suggest that the Ashikule volcanic magmas formed primarily through a magma mixing process. Previous electron probe microanalysis studies have identified reverse zoning in plagioclase and orthopyroxene phenocrysts, providing additional evidence for magma mixing in the magma chamber. Consequently, these data reveal that Ashikule volcanic magmas originated from a mixing process between EM Ⅱ-type mantle-derived basic magmas and intermediate to acidic magmas from partial melting of ancient continental materials. Considering the tectonic setting of the Tibetan plateau, we propose that Ashikule volcanic activity likely formed in a subduction-dominated environment from the Pleistocene to the Holocene.

RECONSTRUCTION OF THE PALEOCONE MORPHOLOGY OF CHANGBAISHAN TIANCHI VOLCANO

MA Chen-yu, CHENG Tao, WAN Yuan, PAN Bo, ZHOU Bing-rui, YAN Li-li

2024, 46(6):  1248-1262.  DOI: 10.3969/j.issn.0253-4967.2024.06.002

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Calderas, large basin-shaped landforms created by massive explosive eruptions, leave behind “pot-like” structures that can provide essential insights into the history and processes of volcanic development and associated hazards. The Changbaishan Tianchi caldera, located on the Sino-North Korean border in eastern Jilin Province, China, is one of the best-preserved large Cenozoic composite active volcanoes in China. This caldera, close to the Wangtiane and Baotaishan volcanoes to the south and southeast, sits atop a basalt plateau, reaching a peak elevation of 2 749m. Its formation involved multiple phases of overflow eruption activities, followed by caldera collapse due to explosive eruptions and pressure loss within the crustal magma chamber during the late Pleistocene. Over time, glaciers and flowing water have sculpted its surroundings, creating U-shaped valleys along the caldera rim. The structure and formation processes of its paleocone have thus attracted significant attention.

In this study, we drew from reconstruction techniques applied to similar calderas globally. Starting with a focus on the volcanic cone profile, we identified large-scale stratovolcanoes with symmetrical cone shapes akin to Changbaishan Tianchi for comparison. Using high-resolution stereo imagery, we extracted a Digital Elevation Model(DEM)with remote sensing software. From these DEMs, we performed detailed topographic analysis, calculating and statistically modeling geomorphological parameters, which allowed us to develop a three-phase empirical model of cone topography. Applying a moving surface algorithm in MATLAB, we generated surface equations for each volcano profile, revealing quantitative relationships between pixel position, coordinates, and elevation in 3D geographic space. We then used ArcGIS's Kriging interpolation method to create a DEM of the reconstructed cone of Changbaishan Tianchi volcano, allowing us to approximate the original cone structure.

The results estimate the original Changbaishan Tianchi cone reached a height of 4, 100m, with a crater diameter of about 390m and a depth of 170m. The cone displayed a funnel-like structure at the summit, with slopes characteristic of stratovolcanoes. The inner edge of the cone had a relatively uniform slope, while the upper outer edge was steep, averaging 27°, and the lower outer slope angle decreased to an average of 18.5°. These parameters align with typical stratovolcano profiles. The explosive eruptions and subsequent cone collapse are estimated to have led to a volume loss of approximately 28.92km³.

This paleocone reconstruction of Tianchi volcano enhances our understanding of the history of the development and evolution of Tianchi volcano, contributing valuable data for reconstructing similar caldera cones and examining eruption mechanisms within the Changbaishan volcanic field. Moreover, this study provides critical information for analyzing the geological history of Tianchi volcano, including the formation of glacial landforms and processes related to eruptions and natural disasters.

SURFACE RUPTURE OF THE FEBRUARY 6, 2023 M_W7.5 ELBISTAN EARTHQUAKE IN TURKEY

YU Jing-xing, REN Zhi-kun, ZHANG Hui-ping, LI Chuan-you, WANG Shi-guang, GONG Zheng, ZHOU Xiao-cheng, XU Yue-ren, LIANG Peng, MA Zi-fa, LI Jun-jie

2024, 46(6):  1263-1279.  DOI: 10.3969/j.issn.0253-4967.2024.06.003

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On February 6, 2023, two destructive earthquakes struck southern and central Turkey and northern and western Syria. The epicenter of the first event(M_W7.8)was 37km west-northwest of Gaziantep. The earthquake had a maximum Mercalli intensity of Ⅻ around the epicenter and in Antakya. It was followed by a M_W7.7 earthquake nine hours later. This earthquake was centered 95km north-northeast from the first one. There was widespread damage and tens of thousands of fatalities. In response to these catastrophic events, in March 2023, a seismic scientific expedition led by China Earthquake Administration(CEA)was promptly organized to investigate the surface ruptures caused by these earthquakes. Here, we focus on the surface ruptures of the second earthquake, known as the Elbistan earthquake. The post-earthquake field survey revealed that the Elbistan earthquake occurred on the East Anatolian fault zone's northern branch(the Cardak Fault). This event resulted in forming a main surface rupture zone approximately 140km long and a secondary fault rupture zone approximately 20km long, which is nearly perpendicular to the main rupture.

We combined the interpretation of high-resolution satellite imagery and geomorphic investigations along the fault to determine the fault geometry and kinematics of the second earthquake event. The Elbistan earthquake formed a main surface rupture zone approximately 140km long, which strikes in an east-west direction along the Cardak Fault. The main rupture zone starts from Göksun in the west and extends predominantly eastward until the western end of the Sürgü Fault. It then propagates northeast along the southern segment of the Malatya fault zone. The entire Cardak Fault and the Malatya fault zone's southern segment are considered seismic structures for this earthquake. The overall surface rupture zone exhibits a linear and continuous distribution. Secondary ruptures show a combination of left-lateral strike-slip or left-lateral oblique-thrust deformation. Along the rupture zone, a series of en echelon fractures, moletracks, horizontal fault striations, and numerous displaced piercing markers, such as mountain ridges, wheat fields, terraces, fences, roads, and wheel ruts, indicate the predominance of pure left-lateral strike-slip motion for most sections. The maximum measured horizontal displacement is(7.6±0.3)m. According to the empirical relationship between the seismic moment magnitude of strike-slip faulting earthquakes and the length of surface rupture(SRL), a main rupture zone of 140km in length corresponds to a moment magnitude of approximately 7.6. Based on the relationship between the seismic moment magnitude and the maximum coseismic displacement, a maximum coseismic displacement of(7.6±0.3)m corresponds to a moment magnitude of about 7.5. The magnitudes derived from the two empirical relationships are essentially consistent, and they also agree with the moment magnitude provided by the USGS. Besides the main surface rupture zone, a secondary fault rupture zone extends nearly north-south direction for approximately 20km long. Unfortunately, due to the limited time and traffic problem, we did not visit this north-south-trending secondary fault rupture zone.

According to the summary of the history of earthquakes, it is evident that the main surface rupture zone has only recorded one earthquake in history, the 1544 M_S6.8 earthquake, which indicates significantly less seismic activity compared to the main East Anatolian Fault. Moreover, the “earthquake doublet” will inevitably significantly impact the stress state and seismic hazard of other faults in the region. Seismic activity in this area remain at a relatively high level for years or even decades to come. The east-west striking fault, which has not been identified on the published active fault maps at the western end of the surface rupture zone, and the north-east striking Savrun Fault, which did not rupture this time, will experience destructive earthquakes in the future. It remains unknown why the east-west striking rupture did not propagate to the Sürgü Fault this time. More detailed paleoearthquake studies are needed to identify whether it is due to insufficient energy accumulation or because this section acts as a barrier. If the Sürgü Fault, about 40km long, was to rupture entirely in the future, the magnitude could reach 7 based on the empirical relationship.

Considering the distribution of historical earthquakes along the East Anatolian fault zone, as well as the geometric distribution of the surface ruptures from the recent “earthquake doublet” and the surrounding active faults, it is believed that the future earthquake hazards in the northeastern segment of the East Anatolian fault zone, the northern segment of the Dead Sea Fault, and the Malatya Fault deserve special attention.

LATE QUATERNARY DEFORMATION RATE OF THE WENSU FORELAND THRUST BELT IN THE SOUTHERN TIANSHAN MOUNTAINS

ZANG Ke-zhi, WU Chuan-yong, ZHANG Jin-shuo, GAO Zhan, YUAN Si-hua, YUAN Hai-yang, YU Xiao-hui, WANG Xue-zhu

2024, 46(6):  1280-1294.  DOI: 10.3969/j.issn.0253-4967.2024.06.004

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In response to the ongoing India-Eurasia collision in the late Cenozoic, the Tianshan orogenic belt was reactivated and experienced rapid uplift. Strong uplifted topography results in that the mountains propagate from the range front toward the foreland basin to form several fan-shaped foreland thrust belts both on its north and south sides. These foreland thrust belts accommodate the most north-south convergence strain and control the regional deformation pattern. However, in contrast to the well-studied foreland thrust belts, the kinematics and deformation rate of the transition area between different foreland thrust belts have not been well-documented. Therefore, it is still unclear how the crustal shortening in the foreland basins changes along the east-west direction. Further, the deformation characteristics and seismic hazard in this region are poorly understood because quantitative information on active deformation is lacking.

The Wensu foreland thrust belt is located in the Kalpin and Kuqa foreland thrust systems' transition areas. In contrast to the Kuqa and Kalpin foreland thrust belts at its east and west sides, the Wensu foreland thrust belt propagated approximately 20km toward the basin and only developed one row of active thrust fault-anticline belts, namely the North Wensu thrust fault-anticline belt. The North Wensu structural belt shows clear evidence of tectonic solid activity because the late Quaternary sediments and river terraces have been faulted. However, this structural belt's kinematics and late Quaternary deformation rate remain poorly constrained. This study quantifies its deformation mode based on field geological mapping across the anticline. Our results indicate that the North Wensu structural belt is a fault-bending fold. Based on interpretations of detailed high-resolution remote sensing images and field investigations, five levels of river terraces can be identified along the Kekeya River valley. By surveying of the displaced terraces with an unmanned drone, the crustal shortening values of ~20.7m、 ~35.3m and ~46.9m are determined for the T3, T4 and T5 terraces, respectively. Our optically stimulated luminescence(OSL)dating yields a depositional age of(9.02±0.55)ka for the T3 terrace, (24.23±1.58)ka for the T4 terrace, and(40.43±3.07)ka for the T5 terrace. Thus, we estimate a crustal shortening of ~1.31mm/a in the late Quaternary(since approximately 40ka), and approximately 2.29mm/a in the Holocene for the North Wensu structural belt. Our results indicate that the deformation rate of the North Wensu structural belt exhibits an obvious increase in the Holocene. This phenomenon indicates that the strong earthquake activity on the North Wensu thrust belt has increased significantly in the Holocene, implying an irregular activity habit of the strong earthquake recurrence cycle on this tectonic belt. The propagation deformation toward the basin of the Wensu foreland thrust belt is very limited. Therefore, we suggest that the foreland thrust belt is a thick-skinned nappe structure and is dominated by high-angle thrust faulting. The tectonic deformation in the Wensu region seems to be characterized by considered vertical growth. Although the deformation rate is small, the uplift amplitude is significant in this region.

STUDY ON THE SLIP RATE OF THE BAISHA RIVER SEGMENT IN THE YINGXIU-BEICHUAN FAULT IN THE LONGMENSHAN FAULT ZONE

HUA Chun-yu, SHI Feng, WEI Zhan-yu

2024, 46(6):  1295-1313.  DOI: 10.3969/j.issn.0253-4967.2024.06.005

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The tectonic belt stretches approximately 400km from Lushan County to Wenchuan County in an east-west direction. The Longmenshan fault zone can be geometrically divided into several sections, including the Houshan Fault, the Central Fault, the Qianshan Fault, and the Foreland Basin(Chengdu Plain)Deformation Zone. The Central Fault is the main segment of the active tectonic belt in the Longmenshan region, and the Yingxiu-Beichuan Fault is one of the most active segments within this central section. The Yingxiu-Beichuan Fault has experienced numerous moderate and strong earthquakes throughout its history, including the Wenchuan earthquake 2008. The 2008 Wenchuan earthquake was ahigh destructive natural disaster that profoundly impacted the Chinese mainland, leading to significant economic losses and casualties. This earthquake caused extensive building collapses, leading to the loss of tens of thousands of lives, and triggered severe secondary geological disasters such as landslides, rockfalls, and mudflows, severely affecting the normal operation of transportation and communication infrastructure. The Yingxiu-Beichuan Fault was one of the key surface rupture zones during this earthquake. However, there is still some uncertainty about the slip rate of this fault.

The Baisha River segment examined in this paper is located in the southern part of the Yingxiu-Beichuan Fault, measuring approximately 14km long. This area contains 14 fractures of varying lengths and complex geometric structures, forming a fracture zone that reaches a maximum width of nearly 300m. The overall orientation of the rupture zone is about 50 degrees; however, the orientation of each small secondary rupture varies, with differences ranging from 0 to 90 degrees. The coseismic displacement along the Baisha River section displays complexity and diversity. The thrust movement primarily occurs on the northern and western walls, with some local thrust faults. Additionally, the strike-slip motion is predominantly right-lateral, exhibiting a maximum horizontal displacement of approximately 4.8m, although some local areas show left-lateral displacement.

Previous studies have employed various techniques, such as geology and geodesy, resulting in a wide range of slip rate estimates from 0.07mm/a to 1.1mm/a. The slip rate of fault is a crucial factor for analyzing the characteristics of fault activity and for studying regional kinematics and dynamic mechanisms. According to river terrace longitudinal profiles estimates, the fault has a vertical slip rate of about 0.3mm/a to 0.6mm/a. Estimates based on displaced landforms indicate a vertical slip rate between 0.07mm/a and 1.1mm/a. According to GPS observations, the horizontal slip rate in the Longmenshan fault zone has a limit of 2mm/a, but the slip rate of individual faults is lower than 0.7mm/a.

In recent years, remote sensing techniques have been extensively utilized to study surface rupture zones, particularly during significant seismic events. This paper primarily employs aerial and QuickBird satellite images captured before and after the earthquake. The resolution of the aerial images is nearly 1m, while the QuickBird satellite images have a resolution of 0.6m, both of which allow for precise interpretation of tectonic landforms. River terraces consist of terraced units, including terraced surfaces, steep terraces, terrace fronts, and terrace backs. As geomorphic markers that are relatively easy to identify and measure, river terraces are among the most essential geomorphic units in the quantitative study of active tectonics. They also serve as crucial geological relics documenting Quaternary tectonic movements and climate changes. By examining river terraces and their deformations, researchers can discuss the timing and scale of tectonic activity, making this a long-term area of research.

This paper focuses on the Baisha River section, situated in the southern part of the Yingxiu-Beichuan Fault. We employed geological and geomorphological methods along with optically stimulated luminescence dating, remote sensing interpretation, field investigations, and data analysis to assess the slip rate of the Baisha River section of the Yingxiu-Beichuan Fault within the Longmenshan fault zone. Additionally, we analyze the spatio-temporal variation characteristics of this slip rate. This study constrains the slip rate of the Baishahe segment of the Yingxiu-Beichuan Fault in the Longmenshan fault zone using 10 terrace cross-sections and terrace ages. The results indicate that the Yingxiu-Beichuan Fault Baisha River segment has a vertical slip rate since the Late Pleistocene ranges from(0.10±0.02)mm/a to(0.30±0.05)mm/a. Considering that only one event, the 2008 Wenchuan earthquake, is associated with the T1 terrace, we believe the calculated rate based on the dislocation and age of the T1 terrace may significantly deviate from reality. If we exclude the sliding rate of the T1 terrace, the vertical slip rate since the late Quaternary ranges between(0.10±0.03)mm/a and(0.30±0.05)mm/a. The linear fitting results indicate that the average vertical sliding rate since the late Quaternary is approximately 0.19mm/a.

These findings provide fundamental data for understanding the seismogenic structure of the Wenchuan earthquake and the overall characteristics of the Longmenshan fault zone, as well as for assessing its long-term seismic hazard.

CHARACTERISTICS AND SEISMIC STRUCTURE ANALYSIS OF THE M_S5.9 AND M_S4.7 EARTHQUAKE SEQUENCES IN ALUKEERQIN BANNER, INNER MONGOLIA

WANG Xin, ZHANG Ke, WANG Yue

2024, 46(6):  1314-1331.  DOI: 10.3969/j.issn.0253-4967.2024.06.006

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The Alukeerqin Banner region in Chifeng city, Inner Mongolia, experienced two notable earthquakes, with magnitudes of M_S5.9 and M_S4.7, in 2003 and 2021, respectively. These were the most significant seismic events in the area in recent years, however, neither resulted in surface rupture. The distribution of aftershocks also did not align with known fault lines, and both the characteristics and mechanisms of these seismic activities remain unclear. To address this, we utilized data from the Inner Mongolia Seismic Monitoring Network to reposition the M_S5.9 and M_S4.7 earthquake sequences in Alukeerqin Banner.

The results indicate that the M_S5.9 and M_S4.7 earthquake sequences occurred on the western and eastern sides of the Shuiquanzigou Tianshankou fault, respectively, aligning in a northwest-southeast(NW-SE)direction. The main shocks are approximately 45km apart, with focal depths of 2-12km and 8-22km for aftershocks. The main earthquakes are situated in the southeastern portion of the aftershock sequences, which also trend NW-SE. Depth-profile analysis of the aftershock zones shows relatively simple structures, with clusters oriented in a NW trend and inclined southwest(SW). The 2003 M_S5.9 earthquake sequence exhibits a fault plane with a dip angle of approximately 60°, while the 2021 M_S4.7 earthquake sequence has a nearly vertical fault plane.

Using the CAP method and P-wave first-motion polarity analysis, we derived focal mechanism solutions and source depths for earthquakes of M_L≥1.5 in the region. The focal mechanism solution indicates that the M_S4.7 main shock primarily involved left-lateral strike-slip motion at a source depth of 19.9km, which closely matches the initial rupture depth of 21km obtained from relocation. Other significant earthquakes in the series similarly exhibit left-lateral strike-slip characteristics, with most developing along a NW-SE strike plane, consistent with the seismogenic fault characteristics identified in the relocated series. Previous research also shows that the main shock of the M_S5.9 earthquake sequence involved left-lateral strike-slip motion, with the B-node plane orientation(NW direction)aligning with the distribution of fine-located events and the long axis of the macroscopic intensity isoseismal line.

The temporal-spatial distribution and focal mechanism analyses of the M_S5.9 and M_S4.7 earthquake sequences suggest that their primary faults are consistent in strike and mechanical properties with the Shuiquanzi-Tianshankou Fault, trending NW but located at different positions. This confirms that the seismogenic structure of the M_S5.9 earthquake is likely a left-lateral strike-slip secondary fault on the western side of the Shuiquanzi-Tianshankou Fault, trending SW. The seismogenic structure of the M_S4.7 earthquake may be a concealed fault nearby with similar characteristics to the Shuiquanzi-Tianshankou Fault.

We also analyzed 71 earthquakes of magnitude 2 and above in the southeastern segment of the Daxing'anling uplift since 2012, based on observation data from the China Earthquake Networks Center. Using comprehensive focal mechanism inversion, we determined the regional P-axis distribution, finding that the primary compressive stress direction in the southeastern Daxing'anling uplift is predominantly NW and nearly east-west(EW). In the vicinity of Alukeerqin Banner, the P-axis orientation is mainly EW, reflecting a relatively simple stress field. The focal mechanisms of the 2003 M_S5.9 and 2021 M_S4.7 earthquakes are consistent with this regional stress field, suggesting that these earthquakes were likely caused by faulting influenced by the nearly EW-oriented regional principal compressive stress.

Research paper

FRICTIONAL PROPERTIES ASSOCIATED WITH ACOUSTIC EMISSION CHARACTERISTICS OF SIMULATED GRANITE FAULT GOUGES: EFFECTS OF NORMAL STRESS AND WATER CONTENT

SHAO Kang, LIU Jin-feng, XIE Bin, ZHU Min-jie

2024, 46(6):  1332-1356.  DOI: 10.3969/j.issn.0253-4967.2024.06.007

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The Xinfengjiang Water Reservoir in Guangdong, China, is one of the reservoirs that has triggered earthquakes of magnitudes greater than 6. Numerous earthquakes have occurred since the reservoir's impoundment, making it one of the most active seismic zones in Guangdong. Disturbance in effective stress and water content in existing fault zones upon the water reservoir may play a critical role in induced earthquakes.

Here, we report 17 friction experiments performed on simulated granite fault gouges(28%quartz, 25% albite, and 44%microcline, particle size <0.25mm, 2mm thickness)collected from fault zones near Xinfengjiang Reservoir to investigate the effect of effective normal stress and water content. This was achieved using a direct shear apparatus associated with two acoustic emission(AE)sensors. Microstructure observation was also performed on post-deformed samples to discuss the possible mechanism of reservoir-induced earthquakes. Regarding the analysis method of acoustic emission data, we adopt a crack cumulative summation curve method, following the RA-AF crack classification method, which can successfully inverse and distinguish the possible different microcracking processes in deformed rock from the pure tensile microcrack development(k=1)to the pure shear microcrack development (k=-1) according to the variance of curve slope k value(i.e., -1~1). The velocity stepping(1-10-50~100μm/s)and slide-hold-slide(at a constant sliding velocity of 1 mm/s with hold intervals of 10-30-100-300-1 000-3 000 seconds)experiments were conducted at the fixed normal stresses of 0.5~20MPa(10%water content)effective normal stress and 0%~25%(10MPa effective normal stress)water content under drained conditions at room temperature. Acoustic emission signals were also recorded. The shear displacement of both types of experiments is~7mm.

(1)Velocity stepping experiments show that the wet samples with 10%water content exhibit a transition from velocity weakening to velocity strengthening at the normal stress of 10MPa, and the apparent dilatancy was observed for the samples showing velocity strengthening utilizing the changes in axial displacement during sliding. At the normal stress of 10MPa, the room dry sample also shows velocity strengthening accompanied by dilatancy. Still, the wet sample with 25%water content shows velocity weakening, though dilatancy was also observed. This may indicate the negative effect of water on velocity strengthening. In addition, AE analysis indicates the development of tensile microcracks(60%~80%)dominated for velocity weakening and the appearance of strongly developed shear microcracks dominated for velocity strengthening, which was consistent with the microstructural observation performed on the deformed samples. It clearly illustrates that the tensile fracture(T) zones developed in the samples showed velocity weakening, while the Ridel shear fracture(R) zones developed in the samples showed velocity strengthening.

(2)Slide-hold-slide experiments show a nonlinear relation between transient peak healing(Δμ_pk) and the logarithm of hold time(log(t_h)), or “non-Dieterich” healing behavior. By contrast, postpeak weakening(Δμ_w, a stress drop measured as the difference between the peak frictional strength and the steady-state friction of reshearing)shows a direct, near-linear relation with log(t_h). In general, Δμ_w increased with increasing log(t_h) and water content but reduced with increasing normal stress. This may indicate the lower normal stress, the higher water content and the longer hold time the more likely for the unsteady slip of the faults. AE analysis showed that the stress drop is positively correlated with the proportion of tensile cracks.

Based on the effects of the effective normal stress and water content on frictional properties, we suggest that the effect of hydrochemical weakening and increase of stress drop after healing may be the mechanisms responsible for a transition from velocity weakening to velocity weakening for granite gouges. The CNS model can well explain the “non-Dieterich” healing behavior of granite gouges. Combined with reservoir-induced earthquakes, it is indicated that the fault zone under Xinfengjiang Reservoir may have unstable slip below 5MPa effective normal stress and 25%water content, resulting in reservoir-induced earthquakes. Importantly, the weak rate weakening behavior under high water content is conducive to generating slow slip seismic events. The experimental results have certain indicative significance for reservoir-induced earthquakes.

DISCRETE ELEMENT SIMULATION STUDY OF FAULT STICK-SLIP INSTABILITY PATTERNS AT DIFFERENT MICROCRACK DENSITIES

ZHAO Qian-bai, ZHAO Yong, YANG Tian-hong, TENG Long, WANG Shu-hong, LIU Yi-long

2024, 46(6):  1357-1373.  DOI: 10.3969/j.issn.0253-4967.2024.06.008

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This study examines the impact of in-situ stress and mining-induced disturbances on fault stability, specifically focusing on the influence of micro-crack density on the fault stick-slip instability process. The rock masses on the hanging wall and footwall of a fault are often characterized by micro-cracks, which alter their load-bearing structural system and thus influence the fault's sliding behavior, leading to various failure modes. Therefore, understanding the mechanical behavior and damage patterns associated with fault stick-slip instability under different micro-crack densities is essential. Acoustic emission(AE)monitoring, a critical tool for studying fault stick-slip failure, provides valuable information on fault activation. However, the fault structure constrains AE wave propagation paths and intensity, limiting insights into the interaction between fault planes and surrounding rock masses.

The study uses the Particle Flow Code(PFC), a discrete element method(DEM)simulation governed by force-displacement laws and Newton's second law, to model and analyze fault stick-slip instability. PFC simulates the motion and interaction of rigid particle assemblies, representing material fracture, damage, and crack propagation. In this framework, particle contact models define the mechanical properties of particle assemblies. The study constructs discrete element numerical models under varying micro-crack densities to simulate the stick-slip process of faults. By monitoring mechanical behavior at particle contacts, the study provides insights into the AE characteristics and evolutionary patterns of fault stick-slip. The AE system, constructed via the moment tensor method, reveals micro-fracture interactions within the rock mass and enables identification of fracture types and the spatio-temporal distribution of AE events. During fault stick-slip instability, the moment tensor represents the displacement generated by contact forces on particles, akin to the effect of body forces. By tracking displacement and force changes during particle-bond fracture, the moment tensor is calculated based on contact forces within the fracture region.

Key findings reveal that different micro-crack densities significantly influence the fault stick-slip instability process. The results detail stress-strain relationships(e.g., stick-slip event frequency, onset stress, stress drop at onset, and maximum stress drop)and AE signal evolution patterns. The fault stick-slip instability process can be divided into four stages, with numerous tensile micro-cracks generated near the fault surface. As micro-crack density increases, structural damage within the rock mass intensifies, reducing the fault's self-locking effect. This, in turn, affects fault stick-slip instability. Increased micro-crack density generally leads to a larger maximum stress drop, while onset stress drop tends to decrease. High micro-crack density also correlates with a higher frequency of minor stress drops in the latter stages of stick-slip.

In conclusion, this study provides valuable insights into the effects of micro-crack density on the fault stick-slip instability process, presenting a novel numerical simulation approach for examining fault activation mechanics. These findings offer a reference for laboratory AE experiments on fault stick-slip and contribute to field-based microseismic monitoring research.

S-WAVE VELOCITY STRUCTURE IN SHANGHAI AND ADJACENT AREAS BASED ON AMBIENT NOISE TOMOGRAPHY

FENG Ce, SONG Xiu-qing, WANG Ren-tao, LIU Hao-lan

2024, 46(6):  1374-1390.  DOI: 10.3969/j.issn.0253-4967.2024.06.009

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The Shanghai region is located south of the Yangtze River estuary, at the eastern edge of the Lower Yangtze Platform formed during the late Proterozoic Jinning cycle. The East China region is currently in the late stage of the second phase of seismic activity response, with heightened seismic activity expected to continue. The study area is characterized by numerous faults, including significant structures like the Xiaoshan-Qiuchuan fault zone(Shanghai segment)and many smaller NE- and NW-trending shallow fault zones, which are intertwined and cross each other horizontally and vertically. Background noise tomography technology relies on seismic background noise data to compute the corresponding Green's functions through cross-correlation, using the results to image phase velocities and surface waves. This method is widely applied due to its high resolution, low cost, and independence from seismic source data.

This study collected continuous vertical background noise data from 58 broadband mobile seismic stations in Shanghai and its neighboring areas over a year. Using background noise cross-correlation technology combined with a fast marching method(FMM)for imaging, we obtained the phase velocity structure of surface waves in the crust and upper mantle for 5s to 30s. Finally, using a least-squares linear inversion method, we derived a high-resolution 3D S-wave velocity structure from 5km to 30km below the study area.

The results indicate that the Shanghai area's S-wave velocity structure exhibits specific lateral and vertical heterogeneity. The distribution of velocity anomalies in the shallow layer and upper crust at depths of 5km to 10km shows a correlation with local topography and the distribution of major faults, notably influenced by the Huzhou-Suzhou Fault. In the middle to lower crust at depths of 15km to 20km, velocity anomalies are demarcated by the Fengjing-Chuansha area, where velocities gradually increase from northern Jiangsu to Hangzhou Bay, with significant structural uplift along the Fengjing-Chuansha fault in Shanghai. At depths of 25km to 30km, the distribution of velocity anomalies reflects a “west-deep, east-shallow” burial state in the lower crust and upper mantle.

From the analysis of three longitudinal profiles(AA', BB', CC'), we draw the following conclusions:

(1)The underground structure of Shanghai and its vicinity shows a gradual change, with overall crustal thickness decreasing from west to east, characterized by a “thick west, thin east” profile and a topography of “low west, high east with a central uplift.” The shallow S-wave velocity significantly correlates with local topography; the velocity values in the middle to upper crust are predominantly controlled by major faults, especially the Huzhou-Suzhou fault. The middle to lower crust beneath Shanghai exhibits an uplifted structural form, with a “low west, high east” fluctuation in the lower crust and upper mantle.

(2)The velocity anomalies and seismic activity are closely related to fault zones, with low-speed anomalies accompanying major faults. The distribution of earthquakes tends to cluster near low-speed bodies and fault zones, notably at the prominent uplift junctions beneath Taicang and Qingpu. The area's predominant occurrence of shallow-source earthquakes is likely due to stress imbalance between the fractured middle to the upper crust and the stable lower crust.

(3)Low-speed anomalies have been consistently observed at depths of 5km to 25km in the Changshu region. Based on related literature and imaging results from this study, we hypothesize the potential presence of thermal material upwelling in the Changshu area.

(4)The longitudinal profile results indicate that the thickness of the upper crust in the study area is approximately 12km, the middle crust around 10km, and the lower crust about 8km. Distinct layering within the crust is evident, with a gradual change in the middle to lower crust, while the upper crust shows significant heterogeneity. The influence of the faulting in the area controls the distribution of low-speed anomalies.

PERIOD-DEPENDENT NONLINEAR CHARACTERISTICS OF SURFACE/BOREHOLE SPECTRAL ACCELERATION RATIOS

WANG Lin, WANG Yu-shi, LI Xiao-jun, LIU Yan-qiong, DING Yi

2024, 46(6):  1391-1407.  DOI: 10.3969/j.issn.0253-4967.2024.06.010

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Weak overburden layers at shallow surfaces would substantially affect the ground motion characteristic parameters of ground motion, such as spectral accelerations, and these influences regularly showed obvious nonlinearity with the increase of ground motion strength. Despite the differences between motions down the borehole and ground motions on bedrock, the surface/borehole spectral ratios could reflect the seismic site effect to some extent and overcome the shortcomings of the lack of reference bedrock stations in the standard spectral ratio method and the assumption of seismic point source in the generalized inversion method. In recent years, more and more scholars have studied the nonlinear dynamic characteristics of the soil body and its change trend based on the strong motion data by comparing the surface/borehole spectral acceleration ratios under different motion strengths. However, the research conclusions, especially the quantitative characterization of the nonlinear characteristics, still need to be improved due to the insufficient statistical sample size.

It was found that the surface/borehole spectral acceleration ratio curves at the same station exhibited high similarity under different motion strengths. However, these spectral acceleration ratios significantly decreased as motion strength increased and shifted towards longer periods at certain period ranges. The surface/borehole spectral acceleration ratios at different motion strengths corresponding to the same period point cannot adequately reflect their nonlinear characteristics. It is more reasonable to characterize these features by the corresponding relationships between the points on the surface/borehole spectral acceleration ratio curve shapes at different motion intensities. Although some scholars have proposed methods for correcting the dominant frequency offset, there remains a lack of effective methods for fixing the period shift of the surface/borehole spectral acceleration ratio across the wholeentire period range, as well as for the quantitative characterization of nonlinear characteristic parameters throughout the wholeentire period range. Dynamic Time Warping(DTW)algorithm is a nonlinear alignment method that combines time alignment and measurement matching. The core idea of the Dynamic Time Warping(DTW)algorithm is to find the optimal mapping between two-time series by calculating the distance between their respective discrete points and identifying the best path connecting them. This process enables point-to-point matching between the two-time series, and the DTW distance(sum of distances along the best path)can be utilized to assess their similarity.

In this study, we selected 166 700 strong-motion records from 180 stations in Japan's KiK-net network them. We grouped them by ground motion intensity to obtain the surface/borehole spectral acceleration ratios and their average values for each station under different intensity levels. A dynamic time warping algorithm was employed to effectively correct long-period shifts in the spectral acceleration ratio curves, which allowed for the precise extraction of both the period shift and amplitude attenuation values at various period points for each seismic station. Subsequently, a detailed statistical analysis was conducted to assess how these values varied about different levels of ground motion intensity. In addition, nonlinear characteristic parameter curves, dependent on both the period and seismic intensity changes, were derived for different site categories, accompanied by the establishment of corresponding empirical relationships. Furthermore, a novel method was proposed to predict the surface-to-borehole spectral ratio under conditions of strong seismic motion, utilizing the spectral acceleration ratio data obtained from weaker ground motion intensity scenarios. This approach is intended to offer more precise and detailed data support for adjusting nonlinear site effects in China's seismic design codes and seismic hazard zoning maps. Ultimately, the goal is to provide a more refined and comprehensive basis for enhancing the nonlinear site response adjustments in China's seismic design specifications and seismic motion parameter zoning maps.

The quantitative analysis of period offset characteristics of the surface/borehole spectral acceleration ratios indicated that the period offsets were more significant under larger motion strength. Meanwhile the period offsets at different periods were not consistent, and the relative period offsets at the same period exhibited linear relationships with motion strength under the double-logarithmic coordinates. The relative period offsets could be reliably expressed by the period offset coefficient defined empirically. The period offset coefficients obtained at each station and on each site class (Ⅰ, Ⅱ, Ⅲ, and Ⅳ) of Chinese standards were all related to the period, which showed a trend of initially increasing and subsequently decreasing with the increase of the period. Furthermore, the period offset coefficients increased with the thickness and softness of the overburden layers, which indicated more significant period offsets on sites of thick and soft overburden layers. The quantitative analysis of the amplitude decay characteristics of the surface/borehole spectral acceleration ratios indicated that the spectral ratio amplitude was lower under more considerable motion strength. Meanwhile, the amplitude rates at different periods were not consistent, and the spectral ratio amplitude at the same period exhibited fine linear relationships with motion strength under the semi-logarithmic coordinates. The amplitude decay rate could be reliably expressed by the amplitude decay coefficient defined empirically. The amplitude decay coefficients obtained at each station and on each site class (Ⅰ, Ⅱ, Ⅲ, and Ⅳ) of Chinese standards were all related to the period. They approximately conformed to a Gaussian function under the semi-logarithmic coordinates, indicating that the amplitude decay rates were lower in the middle period range(the platform segment of spectral accelerations)compared to the shorter or longer period ranges. Moreover, the amplitude decay coefficients (<0) decreased with the thickness and softness of the overburden layers, which indicated faster amplitude decay rates on sites of thick and soft overburden layers. Despite the differences between surface/borehole spectral acceleration ratios and soil surface/bedrock surface spectral acceleration ratios, the period offset coefficients and amplitude decay coefficients derived statistically could reflect the nonlinear characteristics of seismic site effects to some extent. According to the empirical relationships for relative period offsets and amplitude decay rates of surface/borehole spectral acceleration ratios on different site classes (Ⅰ, Ⅱ, Ⅲ and Ⅳ) of Chinese standards, the surface/borehole spectral acceleration ratios under stronger motions could be predicted reliably by surface/borehole spectral acceleration ratios under weaker motion.

Research paper

GROUND MOTION SIMULATION CONSIDERING VELOCITY DISPERSION AND ITS IMPLICATIONS FOR SEISMIC HAZARD ASSESSMENT

ZHANG Yan, JI Zhi-wei, ZHAI Hong-yu, WU Chun-hao

2024, 46(6):  1408-1425.  DOI: 10.3969/j.issn.0253-4967.2024.06.011

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Seismic hazard assessment is crucial for determining engineering fortification levels, guiding urban planning, mitigating earthquake disasters, and addressing secondary hazards such as landslides and mudslides triggered by earthquakes. Energy attenuation during seismic wave propagation is influenced by multi-angle scattering, physical dispersion, and geometric spreading. When conducting numerical simulations of post-earthquake ground motion, accounting for these factors significantly affects the accuracy of hazard assessments.

This paper examines the physical dispersion characteristics of seismic waves in viscoelastic media. Through simulations of velocity dispersion and seismic wave time distribution in a simple one-dimensional model, we explore the impact of dispersion on the spatial distribution of seismic motion and its implications for seismic hazard assessment. A case study of the 2021 Yangbi M_S6.4 earthquake further illustrates the importance of considering physical dispersion in seismic hazard analysis.

In contrast to traditional ground motion prediction equations(GMPE), physics-based simulations of ground motion provide more reliable estimates of seismic hazard levels and enhance the accuracy of hazard assessments. It is well-established that, excluding site effects, peak ground motion parameters on bedrock decrease with increasing epicentral distance. However, considering the wave field dispersion characteristics reveals that peak ground motion parameters do not always decrease monotonically with distance; in some cases, they may even slightly increase. This highlights the complexity of seismic wave propagation through viscoelastic media. Further validation of these findings through refined, scenario-based numerical simulations is necessary.

Additionally, with increasing epicentral distance, the amplitude of ground motion time histories decreases, while their duration increases. This low-frequency, long-duration seismic motion may be amplified under specific site conditions, such as in basins. The influence of the non-uniform viscoelastic medium results in varying attenuation rates for horizontal peak ground motion parameters at different angles. The findings of this study have important implications for national security, critical infrastructure, unconventional energy development, and secondary hazards such as landslides and mudslides. The integration of source physics, seismic wave theory, medium structure imaging, and structural stress analysis is essential for improving the accuracy and reliability of seismic hazard assessments.

CURRENT TECTONIC ACTIVITY OF THE CANGSHAN-NISHAN FAULT AND ITS IMPACT ON THE RESIDENTIAL BUILDING DAMAGE

WANG Yi-ying, SUN Xi-xi, ZHU Meng, WANG Ji-qiang, SHI Pi-long, WANG Hua-lin

2024, 46(6):  1426-1443.  DOI: 10.3969/j.issn.0253-4967.2024.06.012

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The NW-trending Cangshan-Nishan(Cang-Ni) Fault, extending 170km, exhibits significant tectonic deformation and seismic activity in the late Quaternary, attracting considerable attention. This study investigates the relationship between residential building damage in Guanzhuang village, Baiyan town, Pingyi county, Shandong Province, and the activity of the Cang-Ni Fault. Based on field investigations, geomorphological measurements, fault activity identification, surface structure analysis of quartz fragments in fault gouge(SEM), particle size distribution analysis, InSAR time-series analysis, historical research on the 179BC M7 Qichu earthquake, and previous earthquake damage studies, the following conclusions were drawn: 1)Severe structural damage with left-lateral displacement of 0.5-2cm was observed in buildings along the main Cang-Ni Fault, with cracks originating at foundations and decreasing in scale upward. In contrast, buildings along secondary faults experienced relatively minor damage. Foundation cracks in the main fault zone were wide and numerous, propagating upwards to narrower wall fissures, indicative of structural failure. 2)Damaged buildings in Guanzhuang village align with the left-lateral strike-slip zone of the Cang-Ni Fault, coinciding with the epicenter of the 179BC M7 Qichu earthquake. This area represents the hub of late Quaternary fault activity, characterized by significant displacement and recent faulting strata. 3)SEM analysis of quartz fragments suggests that the Cang-Ni Fault's latest activity occurred in the early Holocene, with a combination of stick-slip and creep-slip mechanisms. Particle size analysis revealed a high proportion(36%-43%)of ultra-fine particles(<0.001mm)and fractal dimension values of 2.817-2.857, favoring fault creep. 4)InSAR data confirm ongoing sinistral strike-slip movement of the Cang-Ni Fault, indicating surface creep deformation. 5)Damage to residential buildings is attributed to the sinistral strike-slip and reverse-thrust creep deformation of the fault, stratigraphic and structural differences across fault walls, uneven stress distribution, and swelling effects of fault gouge. This research provides a case study of building damage caused by active fault creep, emphasizing the importance of precise identification and spatial mapping of active faults for seismic design and site selection in engineering projects. The findings offer valuable insights for mitigating seismic hazards in fault zones.