Lushan Ms7 Research Articles

Reactive Transport Process of Earthquake‐induced Hydro‐chemical Changes in Guanding Thermal Spring, Western Sichuan, China

AbstractEarthquake‐related hydrochemical changes in thermal springs have been widely observed; however, quantitative modeling of the reactive transport process is absent. In the present study, we apply reactive transport simulation to capture the hydrochemical responses in a thermal spring following the Wenchuan Ms 8.0 and Lushan Ms 7.0 earthquakes. We first constrain deep reservoir geothermal fluid compositions and temperature by multicomponent geothermometry, and then a reactive geochemical transport model is constructed to reproduce the hydrochemical evolution process. The results show that the recharge from the shallow aquifer increases gradually until it reaches a peak because of the permeability enhancement caused by the Lushan earthquake, which may be the mechanism to explain the earthquake‐related hydrochemical responses. In contrast to the postseismic effect of the Wenchuan earthquake, the chemical evolution can be considered as hydrochemical anomalies related to the Lushan earthquake. This study proves that the efficient simulation of reactive transport processes is useful for investigating earthquake‐related signals in hydrochemical time series.

Slip distribution inversion of seismic sub-fault dip iteration using gradient based optimizer algorithm

This paper considers setting different dips for different sub-faults to fit the actual rupture situation based on the fault rupture of the 2013 Lushan MS7.0 earthquake. Meanwhile, combined with the coseismic GNSS data of the Lushan earthquake, the source parameters and sliding distribution of the Lushan earthquake fault are inversed. Firstly, we use the gradient based optimizer (GBO) in nonlinear inversion to obtain the source parameters of this seismic fault. The inversion results indicate that the strike of the fault is 206.52°, the dip is 44.10°, the length is 21.92 km, and the depth is 12.79 km. To refine the sliding distribution of the seismic fault, the seismic fault is divided into 3 × 3 sub-faults. Then, we fix the central sub-fault dip of 44.10°; the dip of other sub-faults is obtained by iteration. After that, the model is further divided into a fault layer model composed of 23 × 19 sub fault slices, and using the Matlab fitting function is used to fit the dip of the 23 × 19 sub faults. Finally, the Lushan seismic fault plane is established as a shovel structure with steep upper and gentle lower, steep south and gentle north. The slip distribution inversion results indicate that the depth of the slip peak is 13 km, the corresponding maximum slip momentum is 0.67 m, the seismic moment is 1.10 × 1019 N·m and the corresponding moment magnitude is MW6.66. The results above are consistent with the research results of seismology.

Progress and prospect of the time-varying gravity in earthquake prediction in the Chinese Mainland

This paper mainly introduces the application and progress of the time-varying gravity in earthquake research in the Chinese Mainland. Since the Xingtai earthquake in 1966, China has begun mobile gravity monitoring, trying to explore the relationship between gravity changes and seismic activities. The gravity changes before and after the Haicheng MS7.3 earthquake in 1975 and the Tangshan MS7.8 earthquake in 1976 were observed. In 1981, a high-precision metal spring gravimeter was introduced to carry out high-precision mobile gravity observation in the key earthquake monitoring areas in western Yunnan. The gravity anomaly changes near the epicenters of the Lijiang MS7.0 earthquakes in 1996 were observed. In 1998, a high-precision absolute gravity survey was introduced to carry out the overall scale gravity field monitoring in the Chinese Mainland, and the large-scale gravity change information before Wenchuan MS8.0 and Yutian MS7.3 earthquakes in 2008 was obtained, and the effective prediction opinions were given. After the Wenchuan MS8.0 earthquake in 2008, the integration of the national network and the regional network accelerated, forming the whole gravity observation network in the Chinese Mainland, which made a relatively successful medium-term prediction for a series of earthquakes with MS6.0 or above (such as Lushan MS7.0, Menyuan MS6.4, and Jiuzhaigou MS7.0) in recent years and played an important role in the study of the earthquake mechanism and earthquake prediction level in China. Finally, the existing problems in time-varying gravity monitoring in China are pointed out, and the prospect of earthquake research using time-varying gravity monitoring data is put forward.

Insights into the Landslides Triggered by the 2022 Lushan Ms 6.1 Earthquake: Spatial Distribution and Controls

On 1 June 2022, a magnitude Ms 6.1 (Mw 5.8) earthquake, named the 2022 Lushan earthquake, struck the southern segment of the Longmenshan fault zone, with an epicenter at 30.395°N, 102.958°E and a focal depth of approximately 12.0 km. To gain insight into the landslides triggered by this event and the characteristics of coseismic landslides in the Longmenshan fault zone, we collected multitemporal satellite images and carried out field investigations. The results reveal that the 2022 Lushan event triggered at least 1288 landslides over an affected area of 1470 km2. The total landslide area is 5.33 km2, and the highest landslide concentration reaches 22.3 landslides/km2. The landslide distribution has a hanging wall effect, and the right bank area of the Qingyi River, featuring deep-cutting gorges, is part of an area with obvious concentrated landslides; this area consists mainly of intrusive rocks, including granite, gabbro and hornblende. The coseismic landslides in the Longmenshan fault zone have hanging wall effects, and the landslides triggered by the 2022 Lushan event are distributed in higher and steeper areas.

Microscopic Characteristics of Fault Gouge in Minor-Surface-Rupture Faults: A Case Study in the Longmenshan Fault Zone, Eastern Tibetan

Active faults with potential earthquake magnitude of 6–7 are often incorrectly identified as non-Holocene active faults by traditional geological methods because their co-seismic displacements were very small or even did not reach to surface, and are defined as minor-surface-rupture faults. Geological studies associated with the 2013 Lushan Ms 7.0 earthquake show that the Dachuan–Shuangshi fault (DSF) in the southern segment of the Longmenshan fault zone (LFZ) is a minor-surface-rupture fault. This study focuses on the microstructure and mineral composition of the fault gouge in the DSF using optical microscopy, a scanning electron microscope (SEM), and X-ray diffraction (XRD) methods; then, we compare our results with the previous achievements in the Beichuan–Yingxiu fault (BYF), a major seismogenic fault of the 2008 Wenchuan earthquake, in the middle–northern segment of the LFZ. The results show that the microscopic characteristics of the fault gouge of the DSF are obviously different from those of the BYF in the following aspects: 1) the thickness of the fault gouge produced by one fault event is less than 5 mm; 2) under the microscope, no obvious micro-cracks were examined in surrounding rocks around the fault gouge, and discontinuous micro-cracks and untypical S-C fabrics in the fault gouge were observed; 3) under the SEM, reworked fragments were rare in the fault gouge; 4) the XRD mineral analysis reveals that the total clay content is less than 50%, the content of kaolinite is obviously higher than that of clinochlore, and the content of illite/smectite mixed layer is less than 30%. A contrastive analysis reveals the differences between the microscopic features of the fault gouge of the DSF and the BYF, which are systematical. Therefore, the abovementioned microscopic characteristics identified from the fault gouge in the DSF may be used as auxiliary indicators to identify minor-surface-rupture faults.

Coseismic fault slip inversion of the 2013 Lushan Ms 7.0 earthquake based on the triangular dislocation model

The 2013 Lushan Ms 7.0 earthquake occurred on the Longmenshan thrust tectonic zone, a typical blind reverse-fault type earthquake that caused the death of nearly 200 people. The investigation of the fault geometry and fault slip distribution of this earthquake is important for understanding the seismogenic tectonic type and seismic activity mechanism of the Longmenshan Fault Zone. In this paper, for the fault geometry of the Ms 7.0 earthquake in Lushan, the geometric parameters of the planar fault are inverted based on the rectangular dislocation model using GPS coseismic displacement data, and on this basis, a curved fault steeply-dipping on top and gently-dipping at depth is constructed by combining the aftershock distribution. The GPS and leveling data are used to invert the slip distribution of the curved fault for the Lushan Ms 7.0 earthquake. The results show that the fault is dominated by reverse slip with a small amount of sinistral rotation, and there is a peak slip zone with a maximum slip of 0.98 m, which corresponds to a depth of ~ 13.50 km, and the energy released is 1.05 × 1019 N/m with a moment magnitude of Mw 6.63. Compared with the planar rectangular dislocation model, the curved fault model constructed by using triangular dislocation elements can not only better approximate the fault slip, but also better explain the observed surface displacement, and the root mean square error of the GPS and leveling data fitting is reduced by 1.3 mm and 1.9 mm, respectively. Both the maximum slip and moment magnitude of the fault based on the inversion of the curved structure are slightly larger than the results based on the planar structure.

Hydrogeochemical Characteristic of Geothermal Water and Precursory Anomalies along the Xianshuihe Fault Zone, Southwestern China

Hydrogeochemical changes in association with earthquakes are considered as a potential means of identifying earthquake precursors. The Xianshuihe fault zone (XSHF) is considered one of the most active seismic fault zones in China; 43 hot springs were sampled and analysed in the laboratory for major elements, silica, stable isotopes (δD and δ18O) and strontium isotopes were investigated from 2008 to 2021. The meteoric water acted as the primary water source of the hot spring in the XSHF, and recharged elevations ranged from 1.9 to 4.8 km. The geothermometers method was used to estimate the region of thermal storage temperature and its temperature between 8 and 142 °C. And the circulation depth ranged from 0.1 to 6.9 km. Most of the hot spring water was immature water with a weak degree of water-rock reaction. However, the degree of water-rock reaction and the depth of hot spring water circulation were high in part of the Kangding and Daufu segments, which also had the highest reservoir temperature and the most frequent strong earthquakes. Temporal variations of hydrogeochemical showed that Na+, Cl− and SO42− decreased obviously following the 12 May 2008 Wenchuan Ms8.0 and existed abnormal value fluctuations from the 20 April 2013 Lushan Ms7.0 to 22 November 2014 Kangding Ms6.3 occurred and after 20 July 2017 returned to the normal levels. And the ion concentrations in hot springs increased by 5% to 35% three months before 22 November 2014 Kangding Ms6.3 with the obvious precursor anomaly. Hydrogeochemical anomalies could be useful for predicting an earthquake in the study area.

High-Quality Signal Denoising and Deep Feature Representation and Its Application to the ENPEMF

The Earth’s natural pulsed electromagnetic field (ENPEMF) signal is released by the instantaneous disturbance of the Earth’s naturally varying magnetic field, contains a large amount of information about the changing geological structures and their kinetic principles. The ENPEMF signals acquired by the equipment are often accompanied by a great deal of noise, and effective noise reduction is required for accurate signal analysis. We propose a new noise reduction method for a very low signal-to-noise ratio (SNR), called the Blaschke unwinding adaptive Fourier decomposition, and use synthetic signals to demonstrate the high accuracy of the proposed noise reduction method. Meanwhile, the Root-phase deep feature representation (RPDP) of the ENPEMF signal acquired during the Lushan Ms7.0 earthquake is explored using the roots obtained by Blaschke unwinding. The results show that the time-based RPDP can accurately reveal the deep feature of the ENPEMF signal before and after the earthquake, and extract the anomalous changes, potentially revealing the electromagnetic anomaly information contained in the ENPEMF signal.

Study on Seismic Precursory Anomaly Signals Based on Critical Slowing Phenomena

This paper applied the principle of critical slowing down to the processing of radon concentartions observation data, calculated the autocorrelation coefficient and variance which can characterize critical slowing down by taking Wenchuan Ms8.0 earthquake on May 12, 2008, Lushan Ms7.0 earthquake on April 20, 2013, Minxian-Zhangxian Ms6.6 earthquake on July 22, 2013, Jiuzhaigou Ms7.0 earthquake on August 8, 2017 as examples. The result indicated that the water radon concentration at several stations has obvious critical slowing phenomenon before server earthquakes. The formation mechanism of radon concentration anomalies in seismic underground fluids was discussed in the context of the focal mechanism solution of several earthquakes. It is considered that the phenomenon of critical slowing down before server earthquakes is related to the earthquakes occurrenced combined with the hydrodynamic mechanism of radon anomaly and the structure of the abnormal points.

The spatial distribution characteristics of coseismic landslides triggered by the Ms7.0 Lushan earthquake and Ms7.0 Jiuzhaigou earthquake in southwest China.

In recent years, a series of earthquakes have occurred in Sichuan province, southwest China. On April 20, 2013, 1013 landslides were triggered in the Lushan earthquake within the research area of 1215.87km2. On August 8, 2017, 821 landslides were induced in the Jiuzhaigou earthquake within the study area of 541.61km2. The slope, aspect, elevation, distance to the river, distance to fault, stratum lithology, curvature, and PGA as evaluation factors were chosen to research the distribution characteristics of coseismic landslides. The relationship between influence factors and distribution of coseismic landslides were analyzed using landslide area density (LAD), defined as the ratio between the coseismic landslides area and the total area under the specific classification of eight factors, and landslide number density (LND), interpreted as the number of landslides per square kilometer affected by an earthquake. Both the LAD and LND curves show the common correlations between distribution characteristics of coseismic landslides and the classification of eight factors. The high density of coseismic landslides was found at locations where the valley evolves from U-shaped to V-shaped, along slopes with an inclination between 20 and 50°. Due to the effect of river cutting erosion and human engineering activities, massive coseismic landslides appeared within the scope of 200m from the river system. Eight hundred sixty small shallow coseismic landslides were found less than 3km from the seismogenic fault in both earthquakes. Both concave and convex terrain has the same effect of promoting coseismic landslides. The strata of the Carboniferous and Cretaceous are more prone to coseismic landslides occurrence. In the event of another earthquake of similar magnitude in southwest China in the future, the research results can quickly predict the distribution of coseismic landslides and provide a scientific basis for emergency rescue.

Directional seismic response to the complex topography: A case study of 2013 Lushan Ms 7.0 earthquake

Azimuthal variations in site response can provide a good insight into the site amplification and seismic conditions of geohazard occurrences. In this study, multiple directional site response methods, including D-Arias (Directional-Arias), D-SER (Directional- Shaking energy ratio), D-HVSR (Directional-Horizontal to vertical spectral ratio) and D-SSR (Directional — Standard spectral ratio), are adopted to analyse seismic data of the 2013 Lushan Ms 7.0 earthquake captured by the self-established Lengzhuguan (LZG) station which consists of the complex topography of isolated ridge, large mountain and some typical micro-reliefs. The results show that the isolated ridge could cause stronger site responses than the large mountain, and whose pronounced response direction is roughly perpendicular to its ridgeline. With the growth of elevation, the site resonant frequency decreases. The different microreliefs on the mountain cause different site responses, which present as protruding slope > linear slope. The site response mainly exists on the surficial layer of the mountain and shows that with the increase of the distance to mountain surface, the site response gets weaker, the site resonant frequency gets higher, and the pronounced response direction is perpendicular to its ridgeline.

Establishment of a seismic topographic effect prediction model in the Lushan Ms 7.0 earthquake area

SUMMARYThe seismic topographic effect is one of the debated research topics in seismology and earthquake engineering. This debate is due to the discrepancy between the observed amplification and the amplification underestimation in numerical simulations. Although the numerical simulation of ground motion, which began in the 1970s, has been an important and effective way to study topographic effects, the quantitative mathematical model of topographic amplification is urgent. The actual influences on ground motion due to the topography depends on multiple topographic features, such as the topographic slope, topographic geometrical scale. To date, no definite conclusions regarding the main influencing factors and how to express the influencing factors have been made. In this paper, by introducing the back-propagation (BP) neural network technique, a set of mathematical parameters are determined to establish a quantitative topographic effect prediction model. These parameters are the elevation, the first gradient of the elevation and the higher order gradient in two orthogonal directions. Theoretically, the set of mathematical parameters is directly related to the simple topographic features, such as the elevation, topographic slope and height-to-width ratio. Furthermore, their combinations indirectly denote the complex topographic geometrical features, such as the different topographic geometrical scales, designated by the elevation (large-scale variable), the first gradient (middle-scale variable), the second-order gradient (small-scale variable) and so on (smaller scale variable), and the hill ridges that correspond to the sites with the first gradient of the elevation equal to zero and an elevation larger than its surrounding. In 2013, an earthquake of Ms 7.0 occurred in the Lushan area of Sichuan Province in Western China, where the topography sharply fluctuates. At station 51BXD, an acceleration was recorded close to 1.0 g, while at station 51BXM (14 km away from station 51BXD), the acceleration was recorded at only 0.2–0.3 g. In this paper, the spectral element method (SEM) is used to simulate the ground motion in the Lushan Ms 7.0 earthquake area. Then, the topographic amplification ratio of the simulated ground motion is calculated. Furthermore, a BP topographic amplification prediction model is established and compared based on different parameters. A rms of less than or close to 10 per cent between the BP model prediction results and topographic amplification ratio calculated using the simulated ground motion suggests that the parameters of the topographic elevation, the first gradient of the elevation and the second-order gradient in two orthogonal directions are enough to provide the acceptable topographic effect model in the Lushan area. Finally, using the prediction model, the topographic spectral ratio at stations 51BXD and 51BXM is predicted, and the topography amplification due to the scattering of seismic waves by the irregular topography at 51BXD is found to be 1.5–2 times that of 51BXM. The most important highlights of this paper identify the main factors of the topographic effect for the first time and provide an effective method for establishing a quantitative topographic effect prediction model.

Analysis of the Seismic Vulnerability of Buildings in the Lushan Ms7.0 Earthquake in the Sichuan Province of China

ABSTRACT This study is carried out based on field investigation data obtained over more than 20 days after the 20 April 2013 Lushan Ms7.0 earthquake in Sichuan Province, China. The investigation includes detailed seismic damage data from 3,684 buildings in 428 districts. This research summarizes the seismic characteristics of buildings with different structures in this area and systematically analyzes the cause of damage. Based on this analysis, the seismic vulnerability of various structural forms in different intensity regions is systematically assessed. The damage characteristics are summarized for several types of the structures typical in the region: constrained masonry structures designed following current standards, unengineered constrained masonry structures, unreinforced masonry structures, brick-wood structures, brick-concrete structure, traditional Chinese timber frame structures, reinforced concrete frame structures, tall buildings, and industrial plants. Moreover, the causes of damage to these structures are investigated. Furthermore, through statistical analysis, this study develops a vulnerability matrix for the main building types and compares their vulnerabilities of similar buildings in regions with different earthquake intensities. The research results provide an important reference for earthquake engineers to recognize failure modes in different types of structures (especially those reconstructed after the Wenchuan earthquake, Sichuan Province, China), revise seismic design codes, and formulate future disaster-reduction plans.

Spatial prediction strategy for landslides triggered by large earthquakes oriented to emergency response, mid-term resettlement and later reconstruction

To enhance the timeliness and accuracy of spatial prediction of co-seismic landslides, this paper proposes a prediction strategy in three phases: emergency response, mid-term resettlement and later reconstruction in the affected area. Taking the Lushan Ms 7.0 earthquake of 2013 as an example, it is stated as follows: In the emergency response phase, combining with topographic and geological data of the study area, the rapid assessment is conducted by the Newmark method without co-seismic landslide data. In the temporary resettlement phase with partial landslide data available, considering 10 factors including elevation, slope gradient, slope direction, topographic wetness index (TWI), peak ground acceleration (PGA), distances to the epicenter, distances to rivers, distances to roads, lithology and annual precipitation, a logistic regression (LR) model is established, and the spatial prediction of landslides is made. In the reconstruction phase with a complete landslide inventory, the incomplete data is replaced and the latest assessment is conducted based on the LR model. The accuracies of three phases are tested by using complete landslide data. The results show that the area under curve (AUC) values of the three stages is 0.59, 0.84 and 0.87, respectively. Such results can meet the requirement of spatial prediction of seismic landslides in each stage of disaster reduction.

Earthquake-related hydrochemical changes in thermal springs in the Xianshuihe Fault zone, Western China

Hydrochemical changes in three hot springs (GD, LTG and EDQ) were studied by following three large near-field earthquakes (2008 Wenchuan Ms 8.0, 2013 Lushan Ms 7.0 and 2014 Kangding Ms 6.3 earthquakes) in the Xianshuihe Fault zone, Western China. Each earthquake caused significant hydrochemical and isotopic changes in the three thermal springs. Hydrochemical ions (such as Ca2+ and HCO3−) decreased sharply following the Wenchuan earthquake and recovered after the Lushan and Kangding earthquakes. Although δD and δ18O data from before the Wenchuan earthquake are lacking, we find that δD and δ18O after the Lushan and Kangding earthquakes shifted toward the local meteoric water line (LMWL). Both the hydrochemical and isotope data indicate changes in the mixture of different waters, CO2 degassing and geothermal reservoir temperatures. Combining this information with the noble gas analysis, we interpret that new flow paths were created after the Wenchuan earthquake and were reclogged following the Lushan and Kangding earthquakes. More groundwater with low HCO3− and Ca2+ from the granite aquifer entered the hydrothermal system, and more CO2 escaped from the water following the Wenchuan earthquake, while less water from the granite aquifer entered the system following the Lushan and Kangding earthquakes. This study indicates that permeability can either increase or decrease in the near field following different earthquakes. Such changes should be considered significant factors in the management of groundwater supply and earthquake precursor monitoring in seismically active areas.

A Matching Pursuit-Based Method for Cross-Term Suppression in WVD and its Application to the ENPEMF

The earth’s natural pulse electromagnetic field (ENPEMF) signal, which is released by the instantaneous disturbance of the earth’s natural changing magnetic field, contains a large amount of information about the changing geological structures and their kinetic principles. The analysis based on the time-frequency (TF) representation can provide significant interpretations for the ENPEMF signal. Wigner–Ville distribution (WVD) is one of the typical TF representations and has a high degree of TF concentration but is accompanied by severe cross-term interference. Hence, we propose a new TF representation called matching pursuit-based double WVD (MP-DWVD) to suppress cross-term interference, and synthetic signals are used to demonstrate the high TF concentration of the proposed TF analysis method. Furthermore, we apply MP-DWVD to explore the TF characteristics of the ENPEMF signal collected during the Lushan Ms7.0 earthquake. The results show that MP-DWVD can reveal accurate TF distributions of the ENPEMF signal before and after the earthquake and extract anomaly TF components, which have the potential to reveal electromagnetic anomaly information contained in the ENPEMF signal.

Pre-seismic deformation detected from regional GNSS observation network: A case study of the 2013 Lushan, eastern Tibetan Plateau (China), Ms 7.0 earthquake

GNSS (Global Navigation Satellite System) observation is complicated by the presence of noise, local benchmark motion, un-modeled atmospheric and other effects. More subtle signals have existed the individual GNSS time series but have undetected due to its low signal-to-noise ratio (SNR). We introduce here a new method to extract spatially and temporally coherent signals by analyzing data from entire GNSS networks simultaneously using free-net solutions. Daily GNSS network is performed a similarity transformation by constraining one location and one direction. The deformation signal is extracted by analyzing the change of the GNSS geometry net-form integrating the location, baseline length and direction information of all GNSS stations from entire networks. Combined with area strain, the deformation characteristic are further interpreted. We apply the method for the 2013 Lushan Ms 7.0 earthquake and find a significant pre-seismic deformation anomaly with the locked status and negative acceleration in the first shear strain and direction change time series, implying a strong sinistral shear tectonic forces have happened in seismic source area and can intensify the pregnancy of the earthquake. The phenomena can be well supported and explained by the experiment results of rock fracture in the laboratory. It's inferred that a processes from stable locked status to negative acceleration change may be the most characteristic of pre-seismic deformation.

Subsurface structure and spatial segmentation of the Longmen Shan fault zone at the eastern margin of Tibetan Plateau: Evidence from focal mechanism solutions and stress field inversion

As the eastern margin of the Tibetan Plateau, the Longmen Shan fault zone (LMSFZ) is characterized by a complex structural style and strong seismicity, such as the 2008 Wenchuan Ms8.0 and 2013 Lushan Ms7.0 earthquakes. In order to better understand the subsurface geometry of the LMSFZ we inverted 391 focal mechanism solutions (FMS) of earthquakes (M ≥ 3.5) that occurred between 2009 and 2016 in the Longmen Shan region using seismic waveforms. Subsequently, we calculated the stress field based on the FMSs. The results show that the LMSFZ is dominated by a thrust faulting stress regime with subhorizontal NW-SE-trending maximum principal stress σ1 and subvertical minimum principal stress σ3, indicating a lateral compression due to the southeastward movement of the Bayan Har block. Changes in fault dips at ~ 10 km depth are inferred as the result of a shallow level detachment layer, which may also promote shortening of the upper crust and uplift of the Longmen Shan. Based on differences in subsurface fault geometry, stress field and geomorphological evidence, we divide the LMSFZ into two segments by Beichuan County: the southwestern segment has a listric fault geometry, while the northeastern segment is dominated by subvertical transpressional strike-slip faults. According to the FMSs results, we suggest that the Lixian fault is a tear fault accommodating different thrusting rates in the eastern margin of the Bayan Har block. The normal faulting earthquakes in the Lixian area are caused by local extension resulting from the sinistral shear movement of Lixian fault in a strike-slip faulting stress regime. It is suggested that under the continuous southeastward movement of the Bayan Har block and the obstruction of the rigid Sichuan basin, stress is accumulating along the southwestern segment of the LMSFZ, to which accordingly a high seismic hazard is ascribed and which should be carefully monitored.

Geodynamic simulation of the WenChuan Ms8.0 and Lushan Ms7.0 earthquakes

The Lushan Ms7.0 earthquake, occurred on April 20th, 2013, is another strong earthquake that occurred on Longmen Mountain Faults after the Wenchuan Ms8.0 earthquake. In this paper we construct a finite element model depicting fault frictional mechanism to study the geodynamics of these two strong earthquakes. The locations of the initial rupture points and the dislocation forms of the Wenchuan earthquake and Lushan earthquake are simulated to find out the potential relationship between the two earthquakes. Simulative results show that the elevation, fault geometry, and the different rheological strengths between the Sichuan basin and Tibetan plateau play an important role in the earthquake dynamics. The dynamic simulation shows the initial rupture points are located at Yingxiu county and the rupture process is mainly along the northeast direction for the Wenchuan earthquake. In particular, the different frictional strengths caused by the fluid pressure decrease between the southern and northern segments of Longmenshan faults after the Wenchuan earthquake have affected the initial rupture point and the fault dislocation form of the Lushan earthquake, when considering the thrust of Tibetan plateau to Sichuan basin as the major dynamic source.

Deformation Response of Seismogenic Faults to the Wenchuan MS 8.0 Earthquake: A Case Study for the Southern Segment of the Longmenshan Fault Zone

The spatiotemporal deformation response of a seismogenic fault to a large earthquake is of great significance to understanding the nucleation and occurrence of the next strong earthquake. The Longmeshan fault, where the 2008 Wenchuan MS 8.0 earthquake and 2013 Lushan MS 7.0 earthquake occurred, provides an opportunity for us to study this important issue. Based on the GPS observations, we exploit the deformation response of the Southern Segment of the Longmenshan Fault (SSLMF) to the Wenchuan earthquake. The results are as follows: (1) during the co-seismic and post-seismic processes of the Wenchuan earthquake, the deformation is dominated by a continuous pattern in the SSLMF, which is different from the rupture pattern in the middle-northern segment of the Longmenshan Fault (LMF). Quantitatively, the compressive strain present between 2008 and 2013 was equal to the strain accumulation of 69 years during the interseismic period in the SSLMF. If the statistics scope is restricted to the eastern region of the Anxian-Guanxian Fault (AGF), which covers the Lushan source area (Abbr.: Eastern Region), the value is about 25 years; (2) After the Wenchuan earthquake, the strain accumulation pattern changes significantly. First, the deformation adjustment (especially the shear deformation) in the region that crosses the Maoxian-Wenchuan Fault (MWF) and Beichuan-Yingxiu Fault (BYF) (Abbr.: Western Region) is significantly greater than that in the Eastern Region. Furthermore, the crustal shortening is significant in the Eastern Region with minor adjustments in shear deformation. Second, the azimuth angles of the principal compressive strain rate in both regions show significant adjustments, which change fast in the first year of the observation period and then turn into the stable state. In general, the deformation responses of the SSLMF reveal that the Wenchuan earthquake promotes the occurrence of the Lushan earthquake. Their differences in the spatiotemporal domain can be attributed to the influence of afterslip, viscous relaxation of the lithosphere, mechanical parameters and block movement.