Longmenshan Fault Zone Research Articles

Seismic potential in the southeastern margin of Bayanhar block after 2017 Jiuzhaigou earthquake, China – insight from stress evolution and earthquake probability estimation

The southeastern margin of the Bayanhar block (BHB) is located at the junction of the Bayanhar block and the southern China block (SCB). Strong earthquakes occurred frequently in this region because of the intensive tectonic movement. The stress changes caused by strong earthquakes and the observed background seismicity can be used to forecast strong earthquake probability in this region. Therefore, we construct a 3D (three Dimension) viscoelastic finite element model that includes the complex fault system, inhomogeneous medium, tectonic stress, and gravity to simulate the evolutionary stress field when using the observed Global Positioning System (GPS) vectors as the boundary conditions. In addition, we calculate the strong earthquake probability based on the calculated stress and a Poisson probability distribution. The stress evolution result shows the Xianshuihe fault (XSHF), the Huya fault (HYF) and the southern segment except for the Yaan County of Longmenshan fault (LMSF) are located in the stress enhancement area indicating high seismic potential. However, the Minjiang fault (MJF) is located in the stress shadow area. After the Lushan and Jiuzhaigou earthquakes, there is still a high possibility of strong earthquakes in the MJF, HYF, and LMSF zone, according to the strong earthquake probability result.

Terrestrial heat flow measurement and numerical simulation of lithospheric thermal structure in western Sichuan, China

The western Sichuan in the eastern Qinghai-Tibet plateau has significant geothermal potential. Heat flow is one of the most important parameters in geothermic, its measurement can help obtain a better understanding of the regional lithospheric thermal structure. In this study, we reported 4 heat flow data based on the continuous steady-state temperature logging and rock thermal conductivity test, including two class-A and two class-D data. Furthermore, we calculated the lithospheric thermal structure by a 2D steady-state thermal conduction equation in COMSOL, analyzing the lateral differences of lithospheric thermal structure in the eastern Qinghai-Tibet plateau. Our study has determined that the heat flow values in the western Sichuan range from 94.7 to 116.2 mW/m2, with a corresponding geothermal gradient of 32.1–37.3 °C/km. Moreover, numerical simulation results indicate significant differences in lithospheric thermal structure between western Sichuan and Sichuan Basin, where the Longmen Shan fault zone is a crucial regional tectonic boundary and a significant temperature gradient zone. Specifically, the Moho temperature in the western Sichuan region ranges from 1000 to 1200 °C with an average temperature of 1060 °C, while the Sichuan Basin ranges from 600 to 700 °C. Besides, the thermal lithosphere thickness in the western Sichuan and the Sichuan Basin is 88–97 km and 105–110 km, respectively. These observations suggest that the western Sichuan region is characterized by high thermal anomaly and provide theoretical evidence for the geodynamic and geothermal development in this area. Finally, we suggest that the high thermal anomaly in western Sichuan is the result of a combination of factors, such as thickened crust, high radioactive heat production, and shear frictional heating from deep fault and so on.

Phylogeography of 912 Cherry Accessions Insight into Independent Origins of Fruiting Cherries and Domestication Footprints of Cultivated Chinese Cherry (Prunus pseudocerasus Lindl.).

The subgenus Cerasus (Rosaceae) contain numerous fruit trees and ornamentals with high economic values. The origin and genetic divergence among various types of fruiting cherries always remain a perplexing issue. We employed three plastom fragments and ITS sequence matrices derived from 912 cherry accessions to elucidate the phylogeographic structure and genetic relationship among fruiting cherries, as well as the origin and domestication of cultivated Chinese cherry. The integration of haplotype genealogies, Approximate Bayesian computation (ABC) approach and estimation of genetic differentiation within and between different groups and lineages has facilitated the resolution of several previously unresolved questions. Firstly, distant phylogenetic relationships between Cerasus and Microcerasus accessions, as indicated by both nuclear and chloroplast data, suggested independent origins and evolution for these two taxa. Moreover, two distinct geographic origin centers (Europe and China) have been confirmed, with significant phylogeographic signals and high genetic differentiation observed between cherries from these regions. This may be attributed to long-term geographic isolation caused by Himalaya-Hengduan Mountains. Our phylogeographic analyses and ABC analysis suggested that cherries inhabiting in China may have undergone multiple hybridization events during the glacial refugia of the eastern edge and southern Himalaya-Hengduan Mountains, followed by rapid radiation throughout their current habitats during interglacial period. The discrepancy between nuclear and chloroplast data may be attributed to hybridization events and incomplete lineage sorting. Furthermore, we speculated that the domesticated Chinese cherries were derived from wild accessions in Longmenshan Fault Zones approximately 2600 years ago. We have also traced the domestication processes and dispersal routes of cultivated Chinese cherries.

Analysis of Structural Information from Aftershock Clouds—A Case Study of the 2008 Ms 8.0 Wenchuan Earthquake

Abstract We present a method to quantitatively analyze the characteristics of clusters and extract cluster centers in aftershock clouds to study the corresponding structural information. Based on a cluster center approach, a 3D rupture surface is constructed based on the aftershocks of the Wenchuan earthquake. The geometric characteristics of the rupture surfaces show that the Longmenshan fault zone should be divided into seven segments from south to north, and for most of the rupture surfaces, there are two discontinuities at depths of 10 and 20 km. According to the cluster characteristic distance of each rupture branch, the number of aftershocks within the cluster distance is counted. The statistical results show that 22% of the aftershocks of the Wenchuan earthquake occurred on the rupture surface parallel to the main fault and that the rest occurred along other nonparallel rupture surfaces or were scattered. The complex branches of rupture surfaces and the existence of rupture surfaces penetrating the 10 km deep discontinuity layer are two common characteristics of the XI intensity area. The change in tendency of the fracture surface between depths of 10 and 20 km may restrain the continued northward extension of the XI intensity area.

Upper to middle crustal structure beneath the Longmen Shan fault zone: Implications for the growth of eastern Tibet

Upper to middle crustal structure beneath the Longmen Shan fault zone: Implications for the growth of eastern Tibet

Spatial and Temporal Stress Variations before and after the 2008 Wenchuan Mw 7.9 Earthquake and its Implications: A Study based on Borehole Stress Data

AbstractIn situ stress measurement data was analyzed to estimate the temporal and spatial stress variations at shallow depths in the Longmenshan fault zone (LMSF), prior to and following the 2008 Wenchuan earthquake (WCEQ). Analysis of the stress field related to fault strength and behavior is useful for understanding geodynamic processes and conducting hazard assessments. The shallow stress changes after the WCEQ show clear along‐strike variations. Degrees of stress orientation rotations have a negative correlation with the horizontal principal stress ratios and the WCEQ apparently reduced the magnitude difference between horizontal principal stresses. Taking stress magnitudes and orientation distribution relative to the fault strike into account, we propose an intermediate‐strength of LMSF, with a friction coefficient generally constrained between 0.35 and 0.6. In addition, high‐pressure fluids in the fault zone reduce the effective normal stress and to a certain degree weaken the fault strength. The accumulated stress over a certain period following release of the WCEQ indicates the start of another earthquake cycle. The changing crustal stress field makes the LMSF stable or slipping optimally during geodynamic processes. The segmentation feature of the shallow crustal stress field in the LMSF may imply a different tectonic loading and seismic release processes along the fault. The southwestern section to the epicenter of the WCEQ favors the occurrence of future earthquakes, as high μm in a state of critical failure was present in this area, which indicates that the Wenchuan and Lushan earthquakes did not release the accumulated stress to a sufficient extent there.

Upper Cretaceous–Paleocene Magnetostratigraphy of the Longmenshan Foreland Basin and Its Implications for the Tectonic Deformation of the Longmenshan Fault Zone

AbstractKnowledge of the history of the tectonic deformation of the eastern edge of Tibet is important for understanding the evolution of the entire Tibetan Plateau; however, the corresponding tectonic deformation process remains controversial. We conducted a detailed rock magnetic and magnetostratigraphic study of the Cretaceous Jiaguan and Guankou Formations (the Jinmenguan section) within the Longmenshan foreland basin on the eastern edge of Tibetan Plateau. The high‐temperature remanence components separated after thermal demagnetization passed the A‐class reversal test, Bootstrap Cartesian coordinate test, and fold test, at the 95% confidence level. The resulting magnetostratigraphy constrains the age range of the Jinmenguan section to 128.0–64.1 Ma, and that of the Jiaguan Formation to 128.0–83.6 Ma. The resulting chronology identifies two phases of rapid sedimentation (128.0–124.1, 68.8–64.1 Ma) that constrain two phases of tectonic uplift in the Longmenshan Fault Zone on the eastern margin of Tibet during the Cretaceous–early Cenozoic. Our findings suggest that the stress field generated by the subduction and closure of the Bangong‐Nujiang Ocean had already been transferred to the eastern edge of the Tibet in the Cretaceous (at ∼128 Ma).

Source characteristics of the aftershocks of the Wenchuan and Lushan earthquake sequences in the Longmen-Shan fault zone

After the occurrence of the 2008 Wenchuan Mw 7.9 earthquake, the Longmen-Shan fault zone exhibits highly active seismicity. In the years between 2008 and 2022, successive moderate magnitude aftershocks followed in the region. In this study, we used high-quality seismic data from the National Earthquake Data Center of China to estimate the source parameters of 184 moderate magnitude events (Mw 3.9–6.2) in the Longmen-Shan Fault zone. The dataset comprised the aftershocks of two sequences that occurred in the area, including the 2008 Wenchuan earthquake sequence and the 2013 Lushan earthquake sequence. They have been classified according to their focal mechanism. First, our results suggested that the values of apparent stress varied from approximately 0.1–11 MPa, which are higher than the global average. The strike-slip earthquakes have the highest average apparent stress among the types of earthquakes. The above results indicate that the moderately high levels of energy release from earthquakes in this region could result in high ground shaking potential. This behavior may be associated with the high stress accumulation and the strong deformation characteristics in the intraplate environment of the Longmen-Shan Fault zone. The values of the stress drop vary from approximately 0.24–39 MPa and appear to show a slightly increasing trend with the seismic moment. The stress drop level of the Lushan earthquake sequence is higher than the mean value of intraplate earthquakes, while the Wenchuan earthquake sequence is typical of intraplate earthquakes. Second, we investigate the radiated energy enhancement factor (REEF) to quantify the source complexities. The results show that the earthquakes have REEF values between 1.2 and 7.1, and most events are consistent with expectations based on the Brune ω-square model. There exist differences in the source complexity of earthquakes within the source area, and the average ω-square source model could result in an unreliable estimate of the radiated seismic energy of some events. The observation of moderate magnitude earthquakes suggests that the availability of different source parameters will help us to assess the seismic hazard and predict the shaking potential in the earthquake area.

Dynamic Characteristics of Vegetation Change Based on Reconstructed Heterogenous NDVI in Seismic Regions

The need to protect forests and enhance the capacity of mountain ecosystems is highlighted in the U.N.’s Sustainable Development Goal (SDG) 15. The worst-hit areas of the 2008 Wenchuan Earthquake in southwest China were mountainous regions with high biodiversity and the impacted area is typical of other montane regions, with the need for detecting vegetation changes following the impacts of catastrophes. While the widely used remotely sensed vegetation indicator NDVI is available from various satellite data sources, these satellites are available for different monitoring periods and durations. Combining these datasets proved challenging to make a continuous characterization of vegetation change over an extended time period. In this study, compared with linear regression, multiple linear regression, and random forest, Convolutional Neural Networks (CNNs) performed best with an average R2 of 0.819 (leave-one-out cross-validation). Thus, the CNNs model was selected to establish the map of the overlapping periods of two remote-sensing products: SPOT-VGT NDVI and PROBA-V NDVI, to reconstruct a SPOT-VGT NDVI for the period from June 2014 to December 2018 in the worst-hit areas of the Wenchuan earthquake. We analyzed the original and reconstructed SPOT-VGT NDVI in the hard-hit areas of the Wenchuan earthquake from 1999 to 2018, and we concluded that NDVI showed an overall upward trend throughout the study period, but experienced a sharp decline in 2008 and reached its lowest value a year later (2009). Vegetation recovery was rapid from 2009 until 2011 after which, it returned to a pattern of slower natural growth (2012–2018). The Longmenshan fault zone experienced the greatest vegetation damage and initiation of recovery there has caused the overall regional average recovery to lag by 1–2 years. In areas where the land was denuded of vegetation (i.e., effectively all vegetation was stripped from the surface) after the earthquake, the damage exceeded what was experienced anywhere else in the entire study area, and by 2018 it remained unrestored. In the 15 years since the earthquake, the areas that were denuded were expected to recover to the level of restoration equivalent with the NDVI of 2007, as was the case in other earthquake-damaged regions. In addition to the earthquake and the immediate loss of vegetation, the Chinese government’s Grain for Green Policy, the elevation ranges within the region, the forest’s phenological conditions, and human activities all had an impact on vegetation recovery and restoration. The reconstructed NDVI provides a long-term continuous record, which contributes to the identifying changes that are improving predictive forest recovery models and to better vegetation management following catastrophic disturbances, such as earthquakes.

Thermochronological constraints on Eocene deformation regime in the Long-Men Shan: Implications for the eastward growth of the Tibetan Plateau

Understanding the spatio-temporal distribution of strain during Cenozoic growth of the Tibetan Plateau is important for constraining the geodynamic processes underpinning plateau formation. Offset Quaternary landforms and historic earthquake data suggest an along-strike change in deformation style for the eastern margin of the Tibetan Plateau, characterized by a transition from ESE-verging shortening to right-lateral shear from the southern to northern segment of the Long-Men Shan fault zone within a distance of ca. 500 km. When and how this along-strike deformation pattern formed is central to understanding the uplift history and spatio-temporal distribution of strain in the eastern margin of the Tibetan Plateau, and the underpinning geodynamics. To address this, we report a suite of low-temperature thermochronology data from the northern segment of Long-Men Shan fault zone that show a contrast in post late Cretaceous cooling and exhumation histories between the hinterland (west of the marginal Yingxiu-Beichuan fault) and foreland sides (east of the fault). Prior to the Eocene (ca. 40 Ma), the hinterland experienced significant exhumation in contrast to minor exhumation on the foreland side, but, post Eocene exhumation accelerated on the foreland side. This change reflects a switch in the deformation regime from shortening to strike-slip-dominated. This switch reduced hinterland rock uplift and tectonic and topographic loading over the foreland basin, leading to accelerated foreland exhumation through isostatic adjustments. A compilation of fault deformation history for the eastern Tibetan Plateau shows a second tectonic transition in the late Miocene, characterized by formation of the south-striking Huya and Minjiang faults. Our results highlight the importance of progressive late Eocene and late Miocene tectonic transitions in shaping the eastern margin of the Tibetan Plateau. • The hinterland and foreland sides of the eastern Tibetan Plateau margin experienced differential Eocene exhumation. • The differential evolution indicates a coeval tectonic transition from shortening-dominated to strike-slip-dominated. • A second tectonic transition is characterized by the late Miocene formation of the Huya and Minjiang faults. • The eastern margin of the Tibetan Plateau is shaped by progressive late Eocene and late Miocene tectonic transitions.

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.

Rapid report of June 1, 2022 MW 5.9 Lushan earthquake, China with geodetic and teleseismic data

Timely response to earthquake characterization can facilitate earthquake emergency rescue and further scientific investigations. On June 1, 2022, MW 5.9 earthquake occurred in the southern area of the Longmenshan fault zone. This event also happened at the south end of the Dayi seismic gap and is the largest earthquake that has occurred in this seismic gap since the 1970 M 6.2 event. The slip-distribution model constrained by the seismic waveforms suggests a thrust-dominated faulting mechanism. The main slip occurs at a depth of ∼14 ​km, and the cumulative energy is released in the first 6 ​s. The variations of Coulomb stress caused by the mainshock show a positive change in the southwest area of the Dayi seismic gap, indicating possible activation of future earthquakes. In addition, we emphasize the importance of rapid estimation of deformation for near-field hazard delineation, especially when interferometric radar fails to image coseismic deformation in a high relief terrain.

Validation of physics‐based regional‐scale ground‐motion simulations of the 2008 Mw 7.9 Wenchuan earthquake for engineering applications

AbstractThe 2008 Mw 7.9 Wenchuan earthquake, one of the largest continental intraplate events instrumentally recorded, struck the central part of Sichuan Province in southwestern China causing great destruction and loss of life but also providing a wealth of seismological data, geodetic measurements, and tectonic observations. The Wenchuan earthquake ruptured two northwest‐dipping imbricate oblique reverse faults along the middle segment of the Longmenshan fault zone—a northeast‐trending thrust belt located at the boundary between the Tibetan Plateau and the Sichuan Basin. In this study, a hybrid approach that combines deterministic modeling at low frequencies with stochastic modeling at high frequencies is used to simulate broadband ground motions at 52 strong‐motion stations and 506 geodetic sites in the vicinity of the causative fault. The low‐frequency components of the synthetic ground motion are simulated using an extended kinematic source model embedded in a layered medium, whereas the high‐frequency components are generated using a stochastic finite‐fault model. The two independently derived ground‐motion components are then combined using matched filtering at a crossover frequency of 0.8 Hz to generate broadband ground‐motion time histories and response spectra. The temporal and spectral characteristics of the synthetic and recorded ground motions at the 52 strong‐motion stations are compared and the effect of soil nonlinearity on the simulated ground motions is investigated through 1‐D nonlinear site response analysis. Finally, the simulated permanent ground displacements at the 506 geodetic sites are evaluated against geodetic observations and the peak amplitudes of the synthetic ground motions at the same locations are compared with predictions of empirical ground‐motion models.

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.

Three dimensional P-wave velocity structure underneath the southeastern margin of the Tibetan Plateau and the deep tectonic significance

The deep velocity structure underneath the southeastern margin of the Tibetan Plateau is the key to understand the tectonic evolution of the Tibetan Plateau. In this study, 145,254 relative travel-time residuals were measured from the records of 1360 teleseismic events using the waveform correlation method, we determined the three-dimensional (3-D) P-wave velocity structures in the upper mantle using a large-scale dense seismic array. The western boundary of the Yangtze Craton extends from the Longmenshan fault zone in the north to the Red-River fault in the south, our imaging results show that in a depth range of 50–150 km, obvious differences in physical properties of the medium exist across the Longmenshan fault zone. The Red-River fault and the Xiaojiang fault are the western and eastern boundaries of Chuan-Dian diamond block, respectively. Low-velocity anomalies are visible from the north of the Red-River fault to the east of the Xiaojiang fault and their intersection region. The high-velocity anomaly below the Sichuan Basin is thinning westward, with the thickness of 350 km below eastern Sichuan Basin. We obtained a low-velocity anomaly extending 400 km below the Tengchong volcanic field. A high-velocity anomaly, extending down to the mantle transition zone, is thought to be related to the subduction of the Indian slab. Our results also reveal an obvious low-velocity anomaly exists to the south of 26°N, we speculate that the flow of the upper mantle material beneath the Tibetan Plateau affects the velocity structure in this region.

Acceleration of Deep Slip Along the Longmenshan Fault Plane Before the 2008 M8.0 Wenchuan Earthquake

The first step toward earthquake forecasting is the identification of variables whose spatio-temporal variation can be connected with pre-seismic crustal deformation. Four periods of campaign Global Positioning System (GPS) observations around the Longmenshan fault zone (LFZ) provide important insights in crustal deformation and deep fault slip before the 2008 M8.0 Wenchuan earthquake. By using TDEFNODE to invert the coupling fraction and dynamic slip deficit rate of the Longmenshan fault plane before the Wenchuan earthquake, we show that under a background of strong coupling, the compressive slip deficit rate perpendicular to fault increased slightly at first and then decreased; a value of ∼1.5–3.6 mm/a along the central–southern segment from 1999 to 2001 increased to ∼1.9–3.9 mm/a from 2001 to 2004, and then decreased to ∼1.1–2.6 mm/a from 2004 to 2007. The dextral slip deficit rate parallel to fault was ∼6 mm/a from 1999 to 2004, before increasing significantly to ∼9.5 mm/a from 2004 to 2007, when the compressive slip deficit rate decreased. At the same time, large-scale GPS velocity profiles show that compressive shortening deformation in the Eastern Tibet Block, perpendicular to fault, increased slightly from 2001 to 2004, and then decreased from 2004 to 2007. Meanwhile, the dextral shear deformation parallel to fault near the LFZ increased significantly from 2004 to 2007. These findings are in good agreement with those calculated using repeating earthquake sequences, indicating that the deep slip rate of the Longmenshan fault plane may have accelerated several years before the Wenchuan earthquake. Our results demonstrate that GPS data can record pre-seismic preparatory processes, and have the potential for use in medium-term large earthquake forecasting.

Numerical Simulation of Fluid Pore Pressure Diffusion and Its Mechanical Effects during Wenchuan Aftershocks

The Ms 8.0 Wenchuan earthquake occurred on 12 May 2008, in the Sichuan Province of China, and it was accompanied by a series of strong aftershocks. The mechanisms contributing to the triggering of the Wenchuan aftershocks have attracted international attention. In this paper, based on previous analysis of spatiotemporal distribution of aftershocks regarding pore pressure diffusion of deep fluid, we established a three-dimensional hydraulic–mechanical coupling model and investigated the influence of fluid migration and its mechanical effects in the Longmenshan fault zone by using FLAC3D software. We obtained the characteristics of the pore pressure diffusion and fault reactivation within 70 days in an area NA. The results show that the pore pressure significantly increases up to 80 MPa during fluid intrusion into the fault plane. The pore pressure increase along the fault dip is greater than that along the fault strike, with a maximum difference of 3.18 MPa. The increase in pore pressure along the fault reduces the effective stress and leads to fault reactivation. The evolution of the fault reactivation area calculated in the model is compared with the spatiotemporal characteristics of the aftershocks. This study is meaningful for furthering the understanding of the role of deep fluids in fault dynamics and aftershocks triggering.

Magnetic Structure and its Tectonic Implication Around Longmenshan Fault Zone Revealed by EMAG2v3

The formation of magnetic minerals is bound up with the tectonic evolution history, whereupon the distribution of magnetic anomalies has great meanings for regional tectonics. In this study, we use the latest global lithospheric magnetic field model EMAG2-v3, processed by various techniques including reduction to the pole (RTP), upward continuation, derivations, Euler deconvolution, estimation of total magnetization direction, and Curie point depth (CPD), to unveil the tectonics around Longmenshan fault zone (LFZ). LFZ is clearly displayed as a positive and negative anomaly transition zone in RTP anomalies and acts as a magnetic basement boundary. The Sichuan Basin (SB), located to the east of LFZ, is marked by strong magnetic basement and NE-strike banded induced positive anomalies which are associated with the Neoproterozoic magmatic activity. The banded shape, absence of radial pattern of anomalies, and existence of fossil subduction zone supports that the magnetic basement was formed in arc environment. The CPD in SB estimated by radial average power spectral is 30–51 km, which allows magnetic minerals in deep crust or even in lithospheric mantle to exhibit high magnetizations. The Songpan-Ganzi fold belt (SGFB), in contrast, is located to the west of LFZ and covered by thick and low-susceptibility Triassic deposits of flysch, manifests as weak negative anomalies caused by relatively shallow CPD and widespread remanent magnetization. Significant positive anomalies, appearing around the Manai and Rilonguan granitic massifs, indicate a strong magnetic basement beneath SGFB, which is conjectured as westward extension of the Yangtze Block at depth.

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.

Seismic Velocity Variations over Four Years (2013–2016) along the Longmenshan Faults in Sichuan, China

ABSTRACT Seismic velocity is a critical geophysical property that reflects the state of the Earth’s interior. Tectonic movements and seismic activities may affect variations in seismic velocity. Here, we report on the spatial and temporal seismic velocity changes along the Longmenshan faults in Sichuan, China, which were inferred from ambient seismic noise from 2013 to 2016, during which the Ms 7.0 Lushan earthquake occurred and introduced clear velocity changes along the southern segments of the Longmenshan faults. The Longmenshan fault zone consists of complex geological structures that were formed by the compression of the Bayan Har block and South China block. The spatial–temporal seismic velocity changes define three segments on the Longmenshan faults, which are consistent with the distributions of the three subblocks of the Bayan Har block. We also calculate the displacement field and analyze the seismicity in the same area to further understand the seismic velocity changes. The strong compression effect of the Bayan Har block and South China block caused the Lushan earthquake, and the relative movements of the blocks changed the seismic velocities and seismicity. Combined with the seismic velocity changes, seismicity, and displacement field results, the evidence suggests that the seismic velocity changes may reflect the interactions among the blocks, and earthquakes may be prone to occur on the edges of the blocks, which are the areas between the positive and negative seismic velocity changes.