Shallow Seismic Profiles Research Articles

Discovery of a Buried Active Fault to the South of the 1679 M8.0 Sanhe–Pinggu Earthquake in the North China Plain: Evidence from Seismic Reflection Exploration and Drilling Profile

Abstract This study focuses on the key structural locations to the south of the 1679 M8.0 Sanhe–Pinggu earthquake. In conjunction with prior deep seismic reflection exploration in the area, we conducted four shallow seismic investigations to the south of Sanhe–Pinggu seismic area to delineate the exact structure of identified faults and to ascertain the precise location, characteristics, and activity levels of active faults within the region. By analyzing the burial depth of the fault’s breakpoint as revealed by high-precision shallow seismic profiles, we postulate that the fault has been active since the middle and late Pleistocene epochs. In addition, we conducted a high-density borehole investigation in tandem with composite drilling profile at the corresponding sites of shallow breakpoints. Using chronological data from neighboring boreholes and accounting for the ages of samples acquired from these boreholes and staggered strata, the fault manifests as a Holocene active fault within the composite borehole–geological section. This study contradicted the previous conception that to the south of 1679 Sanhe–Pinggu seismic area contained no active faults. This new discovery not only has significant application value for evaluating the risk of large earthquakes in the southern part of the capital circle and understanding the earthquake disaster risk in Beijing but also has scientific significance for studying the development and evolution of faults and their deep–shallow coupling characteristics in North China since the late Cenozoic.

The latest activity of the slope fault zone (Pearl River Mouth) in northern South China Sea and implications for earthquake hazard assessments

In the northern region of the South China Sea, located at the intersection of the continental shelf and slope, there have been recorded four earthquakes with magnitudes M ≥ 6.0. This pattern of seismic activity suggests the presence of a previously unidentified active submarine fault. Notably, no active faults have been documented in this area prior to our investigation, with earlier studies predominantly focusing on the structural morphology and sedimentary basin evolution associated with the South China Sea's spreading phenomenon. Through the examination of seismic profiles and historical earthquake records, we have identified a newly discovered fault, termed the Slope Fault Zone (SFZ). The SFZ exhibits diverse structural components, including a listric normal fault in the western sector and a strike-slip fault zone in the eastern sector. Analysis of shallow seismic profiles and borehole data has revealed that the SFZ intersects with strata from the late Pleistocene epoch, confirming its classification as an active fault. The derived fault parameters indicate that the western segment of the SFZ has the potential to generate earthquakes of considerable magnitude, ranging from M 6.7 to 7.2. Furthermore, given the significant occurrence of submarine landslides along the southern boundary of the SFZ, there exists a risk that such seismic events could trigger slope failures and subsequent tsunamis. In summary, our research has unveiled the presence of an active fault capable of precipitating submarine earthquakes, landslides, and tsunamis.

Seismogenic Structure of the 1605 Qiongshan M7½ Earthquake and Its Holocene Activity History in Northern Hainan Island, China: Evidence from Cross-Section Drilling and Shallow Seismic Profile

Seismogenic Structure of the 1605 Qiongshan M7½ Earthquake and Its Holocene Activity History in Northern Hainan Island, China: Evidence from Cross-Section Drilling and Shallow Seismic Profile

Characteristics and Environmental Indications of Grain Size and Magnetic Susceptibility of the Late Quaternary Sediments from the Xiyang Tidal Channel, Western South Yellow Sea

To reveal the characteristics and environmental indications for the combination of the grain size and magnetic susceptibility of coastal sediments, we provided a necessary basis for further study on their genetic mechanisms. Based on the data of grain size and magnetic susceptibility of the 36.10 m long core 07SR01 sediments in the Xiyang tidal channel of western South Yellow Sea, we analyzed their variations and correlations and further revealed their environmental indications and corresponding regional sedimentary evolution via the combination of the aforementioned analysis results, the reinterpretation results of the sedimentary sequence and the age of core 07SR01 and shallow seismic profiles, and the findings of climate and glacial–eustatic cycles during Late Quaternary. The three stages of the sedimentary evolution of the Xiyang tidal channel between marine isotope stage (MIS) 7 and MIS 5 were summarized as follows: First is the stage of marginal bank and riverbed developments in the tidal estuary under a relatively high sea level and strong hydrodynamic conditions during MIS 7 (core section: 36.10–26.65 m). The sediments deposited in this stage were mainly affected by the paleo-Changjiang River and characterized by a coarse grain size (mean: 4.02 Φ) and relatively high magnetic susceptibilities (mean: 27.06 × 10−8 m3·kg−1), with small fluctuations which were strongly and positively correlated with the sand component. Second is the stage dominated by fluviolacustrine and littoral environments with the weak hydrodynamics during MIS 6–5, in which the climate changed from cold and dry to warm and humid as the sea level rose after a drop (core section: 26.65–15.77 m). The sediments deposited in this stage were characterized by a fine grain size (mean: 5.27 Φ) and low magnetic susceptibilities with minor variations (mean: 10.83 × 10−8 m3·kg−1) which were weakly and positively correlated with the coarse silt component. Third is the stage of delta front in the tidal estuary with a relatively high sea level and strong hydrodynamics during MIS 5 (core section: 15.77–0 m). The sediments deposited in this stage were strongly influenced by the paleo-Yellow River and characterized by a relatively coarse grain size (mean: 4.86 Φ), and high magnetic susceptibilities (mean: 37.15 × 10−8 m3·kg−1) with large fluctuations which were weakly and positively correlated with the sand and coarse silt components.

Mechanism and genesis of Sanmen Bay as a sink of Holocene Yangtze River sediment, Zhejiang Province, China

Introduction: The Holocene mud deposits that extend from the Yangtze River mouth to the Taiwan Strait along the Zhejiang–Fujian coast, East China Sea (ECS), have attracted considerable research attention. However, there is a lack of consensus regarding the redistribution flux of Yangtze River sediment after it enters the sea. There are many bays along the Zhejiang and Fujian coasts, but their role in the source–sink process of Yangtze River sediment is not well-documented owing to limited data.Methods: In this study, data from shallow seismic profiles, core samples, and surface sediment samples were analysed to assess the role of Sanmen Bay, the second largest bay in Zhejiang Province, in the source–sink process of Yangtze River sediments.Results: The results showed that Sanmen Bay is an important sink for Yangtze River sediments. The characteristics of the clay minerals indicate that the fine-grained sediments in Sanmen Bay are mainly derived from Yangtze River sediments. The thickness of Holocene deposits in Sanmen Bay ranges from 8.2 to 32.0 m, with an average of 23.4 m and a total sediment volume of 5.06 × 104 Mt, accounting for 1% of Yangtze River sediment in this time period. The Bayesian age-depth model based on borehole accelerator mass spectrometry (AMS) 14C dating shows that a high sedimentation rate occurred in the early and late Holocene but decreased significantly in the mid-Holocene, which is similar to that of the inner shelf of the East China Sea (ECS).Conclusion: The variation in the Holocene sedimentation rate in Sanmen Bay is related to the southward transport process and mechanism of Yangtze River sediments. The results of this study provide a basis for understanding the source–sink process of sediment in large rivers.

Application of Horizontal-to-vertical Spectral Ratio with a Dense Linear Array on Prospecting Urban Underground Active Faults: a Case in The Mingguang City, Anhui Province

Reliable knowledge of sedimentary structures is beneficial for the study of active faults covered by sediments and can also provide important information for seismic hazard assessment, earthquake emergency response and urban planning. Based on the dense seismic array with a spacing of ∼ 60 m between stations and a length of ∼ 8 km, the horizontal-to-vertical spectral ratio (HVSR) curves were obtained at 133 stations with the ambient noise, and the two-dimensional sedimentary structures of the Quaternary and Neogene strata were imaged over two main faults of the Tanlu fault zone in the eastern Jiashan Basin by frequency-depth conversion. The variations (∼ 130 m) in the depth of the Quaternary and Neogene sedimentary basement directly beneath the fault zone agree well with the spatial characteristics of the fault revealed by the shallow seismic reflection profile with active sources at the same locations.

Geological hazards in the Oujiang estuary in the Zhejiang Province, China: type, distribution, and origin

Introduction: As our understanding of the ocean and its uses continues to advance, countries worldwide with ocean access are implementing new marine strategies. For example, understanding marine geological disasters can help develop the use of ocean resources and oceanic engineering.Methods: Therefore, this study used recent data from the Oujiang Estuary, a nearshore area in China, to determine its geological hazards.Results: The geological hazards were classified as active or restricted based on topography, geomorphology, shallow seismic profile, single-channel seismic profile, and geological drilling data. Active geological hazards primarily include shallow gas and active sand waves, whereas restrictive geological hazards include irregularly buried bedrock, erosion channels, steep submarine slopes, and buried paleochannels. We also evaluated the distribution characteristics and scope, such as the vertical distribution of shallow gas based on the seismic profiles, drilling rock facies, methane, and carbon dioxide contents in the top air, and the isotope values. We found that shallow gas was vertically distributed among multiple layers. The main gas-bearing layers were the clayey silt and sandy (silt) clay layers of the early Holocene and late Pleistocene strata. The shallow gas content was relatively low in the coarse sediment layer at the bottom of the late Pleistocene succession. Generally, the Holocene and late Pleistocene deposits do not contain gas, and the gas content in the middle and late Pleistocene strata (at greater depths) is relatively low.Discussion: The combined effects of the regional geological structure, sea level changes, modern hydrodynamics, and human activities have formed the geological environment of the Oujiang Estuary.

Characteristics and mechanism of Jiaocheng ground fissure in Taiyuan Basin, China

The Fenwei Basin, which has 612 developed ground fissures, is the most concentrated and severe area in China and even the world for ground fissure hazards. The Jiaocheng ground fissure in the Taiyuan Basin is known for being the longest length and causing the most damage and impact in China. To discover the origin of the Jiaocheng ground fissure, surveying, trenching, and geophysical exploration were used to study its geological basis, developmental characteristics, and genetic processes. With a total length of 46 km and an impact bandwidth of 80–120 m, it is one of the longest ground fissures found in the world. The fissure is located on the hanging wall of the Jiaocheng fault, and the NE strike is quite consistent with this fault. Houses, roads, fields, and other structures have all been damaged to varying degrees by the fissure. The most common movements are vertical slip, horizontal tension, and right-lateral slide. According to trenching and shallow seismic profiling, this fissure has synsedimentary features. The Jiaocheng ground fissure was formed due to the interaction of several forces. First, the Jiaocheng fault shifted because of regional extension, creating a rupture system in the surface strata. Pumping activity contributed to the formation of the current ground fissure. This research has significant implications for understanding fissure mechanics and preventing and mitigating ground fissures.

Late Pleistocene to Holocene facies architecture and sedimentary evolution of the Zhejiang coast, East China Sea

The Holocene mud wedge that extends from the Yangtze River mouth to the Taiwan Strait along the Zhejiang–Fujian coast, East China Sea, has attracted considerable research attention; however, the pre-Holocene sedimentary evolution of the same area has been less well documented owing to limited data. We conducted an integrated analysis of 7000 km of high-resolution shallow seismic profiles and two ∼60–80-m-long borehole cores from the Zhejiang coast of eastern China to improve our understanding of the formation of the mud wedge in the context of the post-Late Pleistocene sedimentary evolution of the coastal area. Five major seismic units (numbered SU 1 to SU 5 in descending order) and various subunits that have formed since Marine Oxygen Isotope Stage (MIS) 5 were identified in the study area. SU 5 is recognizable only in the southeastern part of the study area and is interpreted to comprise subtidal to inner-shelf deposits (SU 5b) that were formed during MIS 5 and littoral to tidal-flat deposits (SU 5a) that were formed during the overall sea-level fall of MIS 4 and early MIS 3 (∼60–40 ka). These two subunits are divided by an intervening erosional surface that was likely generated by subaerial exposure of the study area during the sea-level lowstand of MIS 4. During the period from late MIS 3 to MIS 2, the study area was gradually exposed subaerially owing to further sea-level fall, with numerous small rivers incising the underlying strata to form a subaerial unconformity surface (T4). During the early postglacial transgression between ∼15.0 and ∼ 14.0 ka, the study area was progressively flooded, and the river channels were transformed into small estuaries (tidal rivers) that then coalesced into a large-scale, wave-dominated estuary during sea-level rise from ∼14.0 to ∼10.0 ka. The sandy fluvial and/or tidal-river deposits above T4 constitute SU 4. The wave-dominated estuary included a central estuarine bay on the landward side and an estuary-mouth complex on the seaward side, corresponding to fine-grained (muddy) deposits (SU 3a) and coarse-grained (sandy) deposits (SU 3b), respectively. The estuarine deposition concluded with the formation of a transgressive ravinement surface by erosional shoreface retreat during the rising sea level in the early MIS 1 from ∼13.5 ka in the southeasternmost part of the study area to ∼10.0 ka in the area closer to the shore. During the following transgressive phase, the study area was under coastal to shelf sedimentary environments during deposition of SU 2a. Tidal sand ridges (SU 2b) in the eastern part of the study area were formed at ∼11.5 ka and have been active since then. The maximum flooding surface was formed at ∼7.6 ka, and a mud wedge (SU 1) in the study area (except in the eastern part) has been developing since that time and is interpreted to have resulted from the deposition of sediments derived predominantly from the Yangtze River under the modern current system established since the maximum flooding in the Middle Holocene. The study provides new insights into the paleoenvironmental and paleogeographic evolution and facies architecture of the Zhejiang coast since MIS 5, which were controlled by preexisting geomorphological conditions and changes in sea level, hydrodynamics, and sediment supply.

Late Quaternary activity of the Xiadian west concealed fault east to Beijing and its tectonic significance.

Beijing plain is a strong earthquake tectonic area in China. There was a Sanhe-Pinggu earthquake with Ms8 that happened in1679. The seismogenic fault of this earthquake is called Xiadian fault. Our work found fault with a similar strike and opposite dip in the west of the Xiadian fault, which is called the Xiadian west fault in this paper. Six shallow seismic profiles have been constructed to determine the location of the fault in Sanhe city, and the late Quaternary activity of the fault is studied with the method of combined drilling, magnetic susceptibility logging, and luminescence dating. The results of shallow seismic exploration show that the fault is zigzag and generally strikes NE and inclines NW. According to the core histogram and logging curves of ten boreholes and eight effective dating data, the buried depth of the upper breakpoint of the concealed fault is about 12 m, which dislocates the late Pleistocene strata. The effective dating result of this set of strata is 36.52 ±5.39 ka. The vertical slip rate has been about 0.075± 0.023 mm/a since the late Pleistocene and about 0.058 ± 0.030 mm/a since the late period of the late Pleistocene. It can be inferred that the Xiadian west fault is probably a part of the seismogenic structure of the Sanhe-Pinggu Ms8 earthquake that happened in 1679. In a broad sense, the Xiadian fault zone is likely to extend to the southwest along the Xiadian west fault.

Sea level change determined paleochannel development on the continental shelf of the southern East China Sea since MIS 5

Since the late Pleistocene, a large number of paleochannel systems have developed on the shelf of the East China Sea (ECS), and these systems archived important information on sedimentary evolution and environmental changes in the ECS. In this study, we systematically identified the spatial distribution and filling characteristics of the paleochannels in the two stages since Marine Isotope Stage 5 (MIS 5) and investigated the mechanisms on how sea level change responds to paleochannel development and evolution by using sedimentological, geophysical, and chronostratigraphic methods based on high-precision measurements of shallow seismic profiles and borehole data. The results indicate that (1) two stages of paleochannels, named SU2 and SU4 from top to bottom, were identified in the study area in Marine Isotope Stage 2 (MIS 2) and Marine Isotope Stage 4 (MIS 4), respectively; (2) A comparison of the two paleochannel stages reveals that the main stream of SU2 extends longer, than that of SU4; and (3) The paleotopography shows a trend of gradually sloping from the northwestern land area to the southeastern sea area. We conclude that: (1) The sea level fluctuation since the late Pleistocene was the driving force that controlled the development of the paleochannel systems. Understanding this influence can guide the prediction of stratigraphic distribution. (2) The sea level change directly affects the progradation and retreat of coastlines and estuaries, which in turn controls the extension of paleochannels. From this information, the migration range of the coastline can be identified. (3) The SU2 paleochannel may have originated from the Minjiang River, flowed eastward, and merged into the southern tributary of the ancient Yangtze River in the eastern portion of the study area. This study can provide a scientific basis for the southern flow of the MIS 2 ancient Yangtze River system.

Detailed shallow structure of the seismogenic fault of the 1976 Ms7.8 Tangshan earthquake, China

In 1976, an Ms7.8 strong earthquake occurred in the Tangshan region of Northeast China within an ancient craton. In this study, we conducted shallow seismic exploration, drilling-based exploration, and exploratory trench excavation of ground fissures in the Tangshan Fault Zone and obtained a high-resolution shallow seismic profile. Through analysis of its lithology and sedimentary cycle, we constructed a composite drilling profile across the faults. Coupled with the shallow fault combination patterns identified from the exploratory trenches, the profile reveals that the Guye-Nanhu Fault was the shallow response fault in the seismogenic structure of the Tangshan earthquake. This fault is a strike-slip fault with a positive flower structure; the reverse fault branches become progressively steeper with depth and converge downward toward the vertical main strike-slip fault. A high-angle thrust fault is located to the west of the main strike-slip fault, and a series of small-scale normal faults appear in front of this fault, owing to local extension. The tectonic form revealed by the deep seismic reflection profiles further supports the superficial tectonic model. The near-NEE orientation of the stress field in North China and the lower strike-slip movement component of the Tanlu Fault Zone facilitate faulting in the Tangshan–Hejian–Xingtai Fault Zone, which lies diagonally in the middle rectangular area of the North China Plain faulted basin. The detailed structural model of the seismogenic fault obtained by various detection methods is of great significance for understanding the seismogenic mechanism of the Tangshan earthquake.

Processing of seismic diffractions from carbonate nodules in clay formations

The problem of localizing small (relative to wavelength) scatterers by diffractions to enhance their use in identifying small-scale details in a seismic image is extremely important in shallow exploration to identify interesting features such as fractures, caves, and faults. The conventional approach based on seismic reflection is limited in resolution by the Rayleigh criterion. In certain acquisition geometries, such as crosswell surveys aimed at obtaining high-resolution signals, the availability of suitable data sets for effective migration depends on the spatial extent of the available source and receiver data intervals. With the aim of overcoming the resolution limits of seismic reflection, we study the detectability, response, and location of meter- and possibly submeter-dimension carbonate concretions (septaria) in the Boom Clay Formation (potential host rocks for radioactive waste disposal) by diffraction analysis of high-frequency signals. We investigate diffraction wavefields by signal separation, focusing, and high-resolution coherency analysis using the multiple signal classification method and semblance. The investigation is performed for two different surveys in Belgium, a shallow and high-resolution reverse vertical seismic profile (RVSP) and a near-offset crosswell application at Kruibeke and ON-MOL-2 sites, respectively. The data analysis is supported by synthetic wavefield modeling. The multioffset RVSP provides the appropriate geometry to observe and investigate the septaria diffractions from depth and the surface. The crosswell approach, calibrated using synthetic data in the analysis of wavefield patterns in 2D, indicates promising imaging results with field data of a selected diffraction zone in the interwell area.

Late Quaternary persistent impacts of permafrost on lake hydrology and landscape evolution on the NE Tibetan Plateau

Permafrost on the Tibetan Plateau (TP) plays an important role on influencing regional to local hydrology of lake systems by thermokarst processes. Rare investigations and research of such process variations and how they affect persistently the water budget and landscape cratering in alpine lake basin through the Late Quaternary are reported. We used Lake Heihai on the north-eastern Tibetan Plateau as an example to reveal how landscape change/collapse under the regime of permafrost and thermokarst processes occurred since the Lateglacial. Five shallow seismic profiles in combination with lake core sediments from the lake centre as well as fossil lacustrine deposits in the catchment were obtained and analysed by multiple proxies. The results show that Lake Heihai did not exist as a terminal lake during the Lateglacial and the beginning of the early Holocene with widespread permafrost in the basin. Permafrost thaw and glacier melt of short duration contributed to lake hydrology and sediment accumulation. Thermokarst development started in the early Holocene with climate warming and increased water storage in the shallow basin, finally enabling the onset of a thermokarst lake with subsidence of lake deposits. Thermokarst thaw contributed reworked materials to the lake during the formation of a growing talik layer beneath the lake. The damming of the previous outflow in the east by glacio-fluvial accumulation initiated a closed system, combined with the accelerated thermokarst subsidence, which promoted a rapid lake level rise during the middle Holocene, mainly supplied by glacier melt. This period was characterized by the withdrawal of permafrost to higher position in the catchment. Lake Heihai became open again after around 5.3 ka and initiated a significant shrinkage culminating at around 4 cal ka BP and lasted until 2.4 cal ka BP. Permafrost returned into the exposed middle Holocene lacustrine sediments which were then subject to strong seasonal thaw-induced erosion and back transport into the lake, while water depth increased to the present level with returned thermokarst processes at the lake margin. Recent occurrences and disappearances of permafrost phenomena near shore, such as lithalsa/pingos, might have existed in the past as well. Our research sheds new light on the long-lasting impacts of permafrost on the hydrological cycle and integrated landscape evolution in a lake basin on the Tibetan Plateau.

Geometric and Dynamic Patterns of the Golmud Segment in the Southern Marginal Fault of the Qaidam Basin

The southern marginal fault of the Qaidam Basin (SMQBF) is a block-bounding border fault that has played a key role in the structural evolution of the Kunlun Fault. However, its geometric and dynamic deformation patterns since the Late Pleistocene have not been clearly observed. Field investigations, combined with high-resolution imagery and shallow seismic profiles, show that the SMQBF is a thrust fault with a sinistral strike-slip component composed of several secondary faults. Its Late Quaternary deformation pattern is characterized by piggyback thrust propagation, and the frontal fault may not be exposed to the surface. Due to the flexural slip of the hanging strata of the secondary fault, sub-parallel faults with widths of thousands of meters have formed on high terraces; these are important when assessing the seismic hazard of this area. Based on high-resolution topographic data obtained using an unmanned erial vehicle and optically stimulated luminescence chronology, the slip rates of several secondary faults were obtained. The vertical and strike-slip rates of the SMQBF were determined to be 0.96 ± 0.33 mm/a and 2.66 ± 0.50 mm/a, respectively, which may be the minimum rates for the fault. Considering that the SMQBF is composed of several secondary faults, these rates possibly correspond to minimum deformation only. The evident sinistral strike-slip of the SMQBF indicates that although the sinistral slip of the Kunlun Fault system is concentrated in main fault of this system, the branch faults have a significant influence on the lateral extrusion of the Qinghai-Tibet Plateau.

Accumulation characteristics and geological hazard effects of gravity flow on northwestern lower continental slope in the South China Sea

The northwestern lower continental slope margin in the South China Sea is the main fluid property conversion area of the debris flow and turbidity current, where accumulations of typical and special gravity flows are developed. In the upper strata of the research area, there are at least four stages of debris flow accumulations. Several 1-3 m high pressure ridges are developed on the seafloor surface and on the top of underlying accumulations. A large quantity of slide accumulations or extrusion deformation structures is developed on the slope and at the base of the slope. The slides accumulation surface can be clearly seen, and the accumulations are still moving downward slowly and extruding continuously, resulting in a series of geological hazard effects. The study shows that the sediments are mainly composed of silt and clayey silt on the northwestern lower continental slope in the South China Sea. The sediments are deposited at the foot of the slope where the energy of gravity flow begins to reduce rapidly. The cores and shallow seismic profiles of upper strata on the lower continental slope as described in this paper have proven that there are three different accumulation processes, and the geological hazard effects incurred are completely different so that we shall further enhance the research on the seafloor geological hazard effects.

Tectonic activity and earthquake risk in the Chengnanhe fault zone in Weihai city, Shandong province, China, obtained by using an integrated prospecting technique in geophysics and geology

For city planning and reducing potential earthquake risk, it’s necessary to detect the information of the buried faults in an urban area especially, including the location and activities. An integrated technique with geophysical and geological methods, including the shallow seismic reflection profile, electrical resistivity measurement, geologic borehole section, and exploration trench, was used to detect the Chengnanhe fault, which is one of the two main faults passing through the Weihai urban area in Shandong province, China. The results show that it is a normal fault striking with E-W direction, and it is relatively inactive and stable. By using the thermoluminescence (TL) dating, we found that the Chengnanhe fault initiated in mid-Pleistocene and there was no offset after late Pleistocene. Such an integrated technique with multiple geological and geophysical methods provides a significant assessment of earthquake risk for city planning in urban areas.

3D geometry of the Lanliao Fault revealed by seismic reflection profiles: Implications for earthquake clustering in the Dongpu Sag, North China

Six destructive earthquakes occurred in the Dongpu Sag and surrounding regions from 1493 to 1983, and more recent earthquakes have also been concentrated in these areas. Relocation results show two active earthquake clusters in and around Dongpu Sag, one in the north and one in the south. However, the dynamic mechanisms leading to these earthquake clusters have not previously been established. In this study, we use seismic reflection profiles to develop a three-dimensional geometric model of the Lanliao Fault, which marks the eastern boundary of the Dongpu Sag. This model shows that the Lanliao Fault is a listric strike-slip fault on a regional scale. The northern fault segment is steep (>45°) at >8 km depth, while the southern segment becomes a gentle detachment below 8 km. Shallow seismic reflection profiles combined with analysis of borehole cores indicates that the northern segment of the fault appears to have reverse kinematics at <70–80 m depth. Earthquakes that occurred from 2008 to 2018 in the northern Dongpu Sag are controlled by the Lanliao Fault. A reduction in accommodation space and the presence of a restraining bend along the fault appear to be the main dynamic mechanisms associated with the northern earthquake cluster. The focal depths of the earthquake cluster near Heze City are deeper than those in the northern cluster, and relocated earthquakes and focal mechanism solutions show that the two clusters occur on different seismogenic faults. This study has important implications for the dynamics of seismogenic strike-slip faults and related earthquake hazards.

Revealing the geological materials properties by a shallow seismic method for investigating slope site effects: a case study of Qiaozhuang town, Qingchuan County, China

This article presents a description of the lithology and weathered features of the subsurface under Qiaozhuang Town through a 2-D interpretation based on a shallow seismic reflection geophysical survey. To obtain the overburden, phyllite, and limestone geological parameters for the dynamic effect assessment of the slope site, twelve shallow seismic profiles were collected in the target area at slope sites where intensive ground damage was caused by the Wenchuan earthquake. To expose the geological features of rock masses with different lithologies, six shots field recordings for each profile were performed. The results showed reflection signals of a seismic wave that exhibited four to five weathered layers within a shallow depth range. The interpretation of velocity analysis of the geological imaging shows that the elastic velocities of the reflection layers increase with depth. The overburden, the first layer of slope surface, had velocities in the range of 724 m/s to 1024 m/s with the thickness changing in a large domain from 5 to 25 m, which was partially revealed in an exploration shaft and in a borehole. In deeper bedrock, the limestone was characterized by a mean velocity of 1470 m/s to 3760 m/s and the phyllite was characterized by a mean velocity of 1440 m/s to 3580 m/s. Moreover, the geological imaging of partial profiles showed that a weak and discontinuous reflection seismic wave may be interpreted as a fractured zone. The interpretation results reveal that the elastic velocity changing with lithology material is clear presence, leading to the impedance contrast varies from the slope inside to the surface. In particular, the presence of lower velocity layer, such as the overburden, which big material impedance contrast (> 1.0) likely formed strong slope site effects. Therefore, the geological material property of the study area was probably another factor causing the slope failure during the Wenchuan earthquake.

Development characteristics and formation analysis of the Liangjia Village earth fissure in the Weihe Basin, China

Liangjia Village earth fissure, one of the 79 earth fissures along the Kouzhen-Guanshan fault located in the northern Weihe Basin, causes severe damage to buildings and farmlands. Since the late 1950s, 40 earth fissures have occurred in a similar east-west (EW) direction parallel to the Kouzhen-Guanshan fault, and a further 39 earth fissures have occurred with north-west, east-north, or north-east orientations intersecting the fault. In this study, a case study of Liangjia Village earth fissure was conducted to investigate the mechanisms of fissure generation in detail. Geotechnical and geophysical methods including measurements, trenching, drilling, and seismic exploration were used to reveal the basic characteristics, geological background, and origin model of the Liangjia Village earth fissure. This earth fissure, with a total length of 800 m in 2014, runs EW parallel to the Kouzhen-Guanshan fault, and it has damaged buildings and farmlands by forming sinkholes, gullies, subsidence, and a scarp. The trenching results indicated that this fissure underwent multi-phase activity. Analysis of geological drilling and shallow seismic profiling results showed that the fissure also possessed characteristics of a synsedimentary fault. Regarding its genesis, the fissure was formed through the combined actions of three factors: an earthquake created the fissure, the Kouzhen-Guanshan fault controlled its development, and loess erosion and groundwater runoff subsequently enlarged the fissure. Regional extension first generated many buried faults along the hanging wall of the Kouzhen-Guanshan fault before seismic activity caused the buried faults to propagate to the surface, where loess erosion and ground-water runoff promoted the formation of the current earth fissure.