Longmenshan Fault Research Articles

Deep deformation of the Longmenshan fault zone related to the 2008 Wenchuan earthquake

This paper reviews studies of the past ten year related to the crustal deformation of the Longmenshan fault zone (LMSFZ) which was struck by the M 8.0 Wenchuan earthquake on May 12, 2008. The tectonics of the LMSFZ located at the eastern margin of the Tibetan Plateau is complex, with complicated fault geometry and a heterogeneous velocity structure. The Wenchuan earthquake rupture extended for about 300 km along the middle and northern segments of the LMSFZ, with a very complicated rupture process involving multiple sub-events. The Wenchuan slip distribution is characterized by two major slip patches near Hongkou/Yingxiu and Beichuan with peak slip of 5.0–15.5 m and extended from near the surface to below 20 km depth. Prior to the Wenchuan earthquake, the LMSFZ had been seismically quiet for several centuries and there were no hints that suggested that such an M 8.0 earthquake might strike the area. The long-term geological investigations and short-term geodetic measurements before the Wenchuan earthquake generally agree that the horizontal slip rate along the LMSFZ is no more than 3 mm per year. The low slip rate observed at the surface around the LMSFZ may not reveal the real state of accumulated strain at depth where the devastating Wenchuan earthquake nucleated. Rates of aseismic slip at depth derived from seismological investigation of repeating microearthquakes were found to be approximately twice as large as the interseismic rates inferred from surface GPS and geological data. Most of the clusters of repeating microearthquakes are located at the edge of locked areas where large coseismic slips were observed during the Wenchuan earthquake, suggesting a close relationship between microearthquakes and impending large earthquakes. A two-dimensional viscoelastic finite-element model produces a depth-related slip rate pattern around the LMSFZ that is consistent with that revealed by the seismological observation of repeating earthquakes. The measured in situ deep slip rates increase with depth and vary from 3.5 to 9.6 mm/a over a depth range of 4–18 km.The seismological observations of deep slip rates and microseismicity in the three decades before the Wenchuan earthquake reveal that the LMSFZ is indeed not as “quiet” as traditionally assumed in comparison with its neighboring fault systems. Considering the deep slip rates (3.5−9.6 mm/a) from the repeating microearthquakes and coseismic peak offsets of 5.0–15.5 m, the recurrence interval of Wenchuan-like events is estimated to be about 500−4500 years. The estimated recurrence interval based on the deep slip rates is much smaller than those estimates using the same coseismic displacements divided by GPS-derived or geological slip rates. Slip rate increases with depth were also recognized in the Parkfield section of the San Andreas fault zone and in the northeastern Japan subduction zone before the ruptures of the 2004 M w6.0 Parkfield earthquake and the 2011 M w9.0 Tohoku-oki earthquake, respectively. Accelerated slip is thought to have preceded a number of recent large subduction zone earthquakes and the 2008 Wenchuan earthquake, and repeating earthquakes may document short-term precursory slip at depth. Alternatively, the rapid slip rates indicated by the repeating microearthquakes may represent transiently accelerated slip preceding the Wenchuan mainshock. We suggest that slip rates at seismogenic depths are of critical importance in seismic hazard analysis. Repeating earthquakes can be regarded as “deep creepmeters” that measure the in - situ deep slip rate on otherwise aseismically slipping faults. For less well defined and widespread faults within the continents, it is essential to reveal a fault’s or region’s seismic history over different time scale. Combining a better understanding of earthquake diversity with modern technology is the key to effective and comprehensive hazard mitigation practices. The potential earthquake hazard of locked faults with unusually high inferred deep slow slip rates should be paid more attention. The 2008 Wenchuan earthquake is the best-studied continental earthquake to date with a large number of scientific publications enabled by the vast collected data sets. The research community efforts have provided first-order information about this unexpected event regarding its coseismic slip distribution and fault geometry. However, there are many remaining questions about the basic nature of this earthquake to be further constrained with multiple data sets, including the relationship of its rupture with prior coupling and interseismic creep, the varying dynamical rupture processes with depth, the frequency dependence of seismic radiation across the fault zone, and the role of multiple postseismic deformation processes.

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.

Electron spin resonance (ESR) dating of pre-Quaternary faults in the Sichuan basin, SW China

The electron spin resonance (ESR) dating method is thought to be limited to the Quaternary period. In this study, we attempt to expand the ESR dating range to the pre-Quaternary using three fault-zone samples collected from faults in the Sichuan basin and its surroundings. We adopt a γ-ray radiation dose of up to 2 × 104 Gy, which is much greater than that typically employed, and find that the ESR intensity of the E1′ center (a type of signal for ESR dating) in quartz continuously increases with the γ-ray dose and is slightly influenced by the radiation dose rate. This indicates the potential of ESR to date pre-Quaternary faults. By simulating environmental radiation under geological burial conditions, we use ESR to date the latest extension event in the western Hubei and Hunan thrust belt to 9.17 ± 0.76 Ma and the thrusting of the Longmenshan fault zone toward the Sichuan basin to approximately 6.14 ± 0.73 Ma. Conversely, the saturated ESR intensity of fault-zone samples from the Daba fault zone suggests a lack of strong tectonic activity in the southern Daba fault zone in the last 10 Ma. Our results are consistent with fission track ages and the regional tectonic evolution history. Therefore, ESR is a practical method for dating pre-Quaternary faults in structural and petroleum geology applications.

Thermochronological Constraints on the Late Cenozoic Morphotectonic Evolution of the Min Shan, the Eastern Margin of the Tibetan Plateau

AbstractStrain distribution inferred from rock exhumation history could provide significant insights into the geodynamic models proposed for explaining late Cenozoic outward growth of the Tibetan Plateau. In this work, we present a new thermochronological data set to constrain the exhumation history of the eastern margin of the Tibetan Plateau (i.e., the Min Shan and adjacent areas) and the long‐term dip‐slip rates of boundary faults, including the Huya fault in the plateau margin and the Minjiang fault in the hinterland. The data set shows evident age diachroneity between different sides of the faults, with dramatically younger ages in their hanging walls than footwalls, suggesting differential exhumation across the faults. Age‐elevation plots and inverse thermal history modeling for a vertical profile data set from the plateau margin (west of the Huya fault) indicate the differential exhumation started at late Miocene time (~10 Ma), synchronous with the timing at other sites of the eastern Tibetan Plateau. The magnitude of the differential exhumation is constrained as >0.6 and ~0.2 km/m.y. across the Huya and Minjiang faults, as estimated from age‐elevation relationships and one‐dimensional modeling of exhumation, providing unique constraints for the dip‐slip rates along the faults. These two N‐S striking faults, together with the Tazang fault (NWW‐striking and left‐lateral slipping) to the north and Longmen Shan faults (a set of NE‐striking reverse faults with right‐lateral components) to the south, forms a large‐scale fault system to accommodate the late Cenozoic northeastward upper crustal shortening along a deep‐seated hinterland‐ward dipping detachment.

Structure of the Crust and Mantle Down to 700 km Depth beneath the Longmenshan From P Receiver Functions

AbstractSince the Ms 8.0 Wenchuan earthquake occurred in 2008 in the Longmenshan at the eastern margin of Tibet, this region has attracted much attention. In order to investigate the deep structure of the region, an analysis of P receiver functions was carried out, using data from an array of 80 broadband seismic stations operated in 2012 and 2013, and straddling the Longmenshan fault zone around the epicenter of the Wenchuan earthquake. Positive Ps conversions from 5 to 10 km depth indicate the base of the Mesozoic sediments in the Sichuan basin and the base of the Songpan‐Ganzi flysch deposits in Tibet. Later positive Ps conversions represent the Moho, which occurs at 40–50 km depth below the Sichuan basin and deepens to the NW to 55–70 km depth under Tibet. An important finding of this study is that whereas in the NE, the Moho deepening from SE to NW is smooth, in the middle and SW of the region the Moho deepening is abrupt with, in some places, a possible overlap. Negative Ps conversions at stations in Tibet represent the top of low shear velocity zones in the middle and lower crusts below Tibet. Beneath the Sichuan basin, a negative phase about 1 s earlier than the Moho Ps conversion is to a large extent, almost certainly due to multiples from discontinuities in the upper crust. Finally, positive Ps conversions can be observed from the 410 and 660‐km discontinuities at the top and bottom of the mantle transition zone, respectively.

Present-day crustal deformation characteristics of the southeastern Tibetan Plateau and surrounding areas by using GPS analysis

The southeastern Tibetan Plateau and surrounding areas comprise a typical tectonic belt in mainland China, characterized by a complex geological background and intense tectonic activity. We study present-day crustal deformation characteristics of this region based on GPS data for the periods 1999–2007 and 2009–2011. We first analyze the variations in crustal motion and strain rate, and then discuss the 3D crustal motion and local sub-block activities, as well as the correlation between the intensity of crustal activity and the strain rate distribution. Finally, we explain the present-day geodynamic characteristics of the region. Our results indicate that the entire region shows overall clockwise motion with respect to the stable Eurasian Plate. The tectonic boundary belts between this region and the South China Block display significant compressional strain, accompanied by associated extensional strain. Relatively high maximum shear strain and the transition zones of the significant plane strain gradients are also mainly concentrated along the Ganzi–Yushu–Xianshuihe, Anninghe–Zemuhe–Xiaojiang, Lijiang–Xiaojinhe, and Red River faults, as well as the western and southern Yunnan Province. 3D crustal velocities further reflect significant differences in tectonic activity between different structural belts. We conclude that the regions showing higher shear strain and the transition zones of the significant plane strain gradients correspond to the areas with frequent earthquakes. According to the crustal deformation and strain characteristics, we infer that the present-day geodynamic setting of the region is related to the ongoing India–Eurasia collision and the associated resistance of the stable Alashan, Ordos, and South China blocks, resulting in the extrusion of the southeastern Tibetan Plateau crustal material with an overall clockwise flow around the Eastern Himalayas and significant compressional strain along the tectonic boundary belts. Furthermore, notable compressional strain and enhanced sub-block motions occurred around the Longmenshan fault area following the Wenchuan earthquake.

Plate interactions, crustal deformation and magmatism along the eastern margins of the Tibetan Plateau

In order to investigate the deep structure and dynamics of the triangular colliding system of the eastern Tibetan Plateau (TP), as well as the Yangtze Platform and the Qinling-Dabie Fold Belt, a large number of P- and S-wave arrival times from both local and teleseismic earthquakes were used to generate high-resolution tomographic images beneath the eastern margins of TP. The results of this study show that the western margin of the Minjiang fault is characterized by low velocity (low-V) anomalies in sharp contrast to the high velocity (high-V) anomalies that are derived from the eastern part. This means that the Minshan uplift zone is imaged as high-V and acts as a barrier to the eastward escape crustal flow, which leads to obvious segmentation of the Longmenshan fault (LMSF) zone between velocity belts that are concatenated into the Minjiang fault. Because high-V anomalies are clearly imaged in this study to extended depths of ca. 450 km, we consider the Yangtze Platform (containing the Sichuan Basin) to comprise craton-like lithosphere. This study reveals the presence of a low-V belt extending to a depth of ca. 250 km between the Sichuan basin and the Ordos Block which probably represents the migration pathway of upwelling asthenosphere materials from Tibet. At the same time, low-V zone with a downdip depth of ca. 500 km accompanied by an approximately 400 km rectangular high-V body is imaged under the Tengchong volcano. We interpret the high-V body and the low-V zone as evidence for the subducted Indian plate and the volcanic magma source, respectively. The results of this study reveal a number of new features of structural heterogeneities relative to deep coupling between the Tibet and the Yangtze platform, as well as the relationship between the Sichuan Basin and the Ordos Block, and the original source of the Tengchong volcano. The results presented here enable us a better understanding of the plate interaction, crustal deformation, and magmatism of the Tengchong volcano along the eastern margins of TP.

GPS-Derived Fault Coupling of the Longmenshan Fault Associated with the 2008 Mw Wenchuan 7.9 Earthquake and Its Tectonic Implications

Investigating relationships between temporally- and spatially-related continental earthquakes is important for a better understanding of the crustal deformation, the mechanism of earthquake nucleation and occurrence, and the triggering effect between earthquakes. Here we utilize Global Positioning System (GPS) velocities before and after the 2008 Mw 7.9 Wenchuan earthquake to invert the fault coupling of the Longmenshan Fault (LMSF) and investigate the impact of the 2008 Mw 7.9 Wenchuan earthquake on the 2013 Mw 6.6 Lushan earthquake. The results indicate that, before the 2008 Mw 7.9 Wenchuan earthquake, fault segments were strongly coupled and locked at a depth of ~18 km along the central and northern LMSF. The seismic gap between the two earthquake rupture zones was only locked at a depth < 5 km. The southern LMSF was coupled at a depth of ~10 km. However, regions around the hypocenter of the 2013 Mw 6.6 Lushan earthquake were not coupled, with an average coupling coefficient ~0.3. After the 2008 Mw 7.9 Wenchuan earthquake, the central and northern LMSF, including part of the seismic gap, were decoupled, with an average coupling coefficient smaller than 0.2. The southern LMSF, however, was coupled to ~20 km depth. Regions around the hypocenter of the 2013 Mw 6.6 Lushan earthquake were also coupled. Moreover, by interpreting changes of the GPS velocities before and after the 2008 Mw 7.9 Wenchuan earthquake, we find that the upper crust of the eastern Tibet (i.e., the Bayan Har block), which was driven by the postseismic relaxation of the 2008 Mw 7.9 Wenchuan earthquake, thrust at an accelerating pace to the Sichuan block and result in enhanced compression and shear stress on the LMSF. Consequently, downdip coupling of the fault, together with the rapid accumulation of the elastic strain, lead to the occurrence of the 2013 Mw 6.6 Lushan earthquake. Finally, the quantity analysis on the seismic moment accumulated and released along the southern LMSF show that the 2013 Mw 6.6 Lushan earthquake should be defined as a “delayed” aftershock of the 2008 Mw 7.9 Wenchuan earthquake. The seismic risk is low along the seismic gap, but high on the unruptured southwesternmost area of the 2013 Mw 6.6 Lushan earthquake.

Joint Inversion of 1-D Magnetotelluric and Surface-Wave Dispersion Data with an Improved Multi-Objective Genetic Algorithm and Application to the Data of the Longmenshan Fault Zone

Magnetotellurics and seismic surface waves are two prominent geophysical methods for deep underground exploration. Joint inversion of these two datasets can help enhance the accuracy of inversion. In this paper, we describe a method for developing an improved multi-objective genetic algorithm (NSGA–SBX) and applying it to two numerical tests to verify the advantages of the algorithm. Our findings show that joint inversion with the NSGA–SBX method can improve the inversion results by strengthening structural coupling when the discontinuities of the electrical and velocity models are consistent, and in case of inconsistent discontinuities between these models, joint inversion can retain the advantages of individual inversions. By applying the algorithm to four detection points along the Longmenshan fault zone, we observe several features. The Sichuan Basin demonstrates low S-wave velocity and high conductivity in the shallow crust probably due to thick sedimentary layers. The eastern margin of the Tibetan Plateau shows high velocity and high resistivity in the shallow crust, while two low velocity layers and a high conductivity layer are observed in the middle lower crust, probably indicating the mid-crustal channel flow. Along the Longmenshan fault zone, a high conductivity layer from ~ 8 to ~ 20 km is observed beneath the northern segment and decreases with depth beneath the middle segment, which might be caused by the elevated fluid content of the fault zone.

Role of pre-existing structures in controlling the Cenozoic tectonic evolution of the eastern Tibetan plateau: New insights from analogue experiments

Pre-existing weakness due to repeated tectonic, metamorphic, and magmatic events is a fundamental feature of the continental lithosphere on Earth. Because of this, continental deformation results from a combined effect of boundary conditions imposed by plate tectonic processes and heterogeneous and anisotropic mechanical strength inherited from protracted continental evolution. In this study, we assess how this interaction may have controlled the Cenozoic evolution of the eastern Tibetan plateau during the India–Asia collision. Specifically, we use analogue models to evaluate how the pre-Cenozoic structures may have controlled the location, orientation, and kinematics of the northwest-striking Xianshuihe and northeast-striking Longmen Shan fault zones, the two most dominant Cenozoic structures in eastern Tibet. Our best model indicates that the correct location, trend, and kinematics of the two fault systems can only be generated and maintained if the following conditions are met: (1) the northern part of the Songpan–Ganzi terrane in eastern Tibet has a strong basement whereas its southern part has a weak basement, (2) the northern strong basement consists of two pieces bounded by a crustal-scale weak zone that is expressed by the Triassic development of a northwest-trending antiform exposing middle and lower crustal rocks, and (3) the region was under persistent northeast–southwest compression since ∼35 Ma. Our model makes correct prediction on the sequence of deformation in eastern Tibet; the Longmen Shan right-slip transpressional zone was initiated first as an instantaneous response to the northeast–southwest compression, which is followed by the formation of the Xianshuihe fault about a half way after the exertion of northeast–southwest shortening in the model. The success of our model highlights the importance of pre-existing weakness, a key factor that has been largely neglected in the current geodynamic models of continental deformation.

Spatial and Temporal Variations in the Moment Tensor Solutions of the 2008 Wenchuan Earthquake Aftershocks and Their Tectonic Implications

AbstractFollowing the mainshock of the 2008 M8 Wenchuan Earthquake, there were more than 300 ML ≥ 4.0 aftershocks that occurred between 12 May 2008 and 8 September 2010. We analyzed the broadband waveforms for these events and found 160 events with sufficient signal‐to‐noise levels to invert for seismic moment tensors. Considering the length of the activated fault and the distances to the recording stations, four velocity models were employed to account for variability in crustal structure. The moment tensor solutions show considerable variations with a mixture of mainly reverse and strike‐slip mechanisms and a small number of normal events and ambiguous events. We analyzed the spatial and temporal distribution of the aftershocks and their mechanism types to characterize the structure and the deformation occurring in the Longmen Shan fold and thrust belt. Our results suggest that the stress is very complex at the Longmen Shan fault zone. The moment tensors have both a spatial segmentation with two major categories of the moment tensor of thrust and strike slip; and a temporal pattern that the majority of the aftershocks gradually migrated to thrust‐type events. The variability of aftershock mechanisms is a strong indication of significant tectonic release and stress reorganization that activated numerous small faults in the system.

Upper crustal anisotropy observed around the Longmenshan fault in the 2013 <i>M</i><sub>S</sub>7.0 Lushan earthquake region

Upper crustal anisotropy observed around the Longmenshan fault in the 2013 <i>M</i><sub>S</sub>7.0 Lushan earthquake region

Fault geometrical model of Dujiangyan section in Longmenshan fault zone

Fault geometrical model of Dujiangyan section in Longmenshan fault zone

Comprehensive crustal structure and seismological evidence for lower crustal flow in the southeastern margin of Tibet revealed by receiver functions

The lower crustal flow model has become an interesting proposal to describe the growth and expansion of eastern Tibet. To date, however, the geophysical results are not definitive due to the non-uniqueness of the solutions found through data inversion and the resolution limitations. In this study the polarity of P-to-S converted phases at intra-crustal interfaces are analyzed in order to reveal evidence for low seismic velocity and lower-crustal flow in southeastern Tibet. We are interested in a joint study that is both a mapping of the crustal structure (Moho topography and Poisson's ratio) and an attempt to constrain an ICLVZ. We have determined the crustal thickness and Poisson's ratio from P (PRFs) and S (SRFs) receiver functions acquired in 108 permanent seismic stations deployed in the Sichuan and Yunnan areas. Also, we have obtained 98 measurements of Pms-phase splitting parameters at the array stations. The crustal thickness provided by PRFs is consistent with that given by SRFs. It varies from ~60km in the Songpan-Ganzi (SG) fold system and the northern part of the Sichuan-Yunnan diamond-shaped block (SYDSB), near the Eastern Himalayan Syntaxis, to ~33km in southern Yunnan. The Poisson's ratio varies mostly from 0.24 to 0.32; the highest values 0.28–0.32 are found along the axis formed by the Longmenshan fault, the Lijiang-Jinhe fault and in the Tengchong volcano area. The value of ~0.28 observed in SYDSB and the SG fold system is attributed to the presence of a low-velocity zone in the lower crust, and partly to the great sediment thickness in Sichuan Basin. The crust is clearly anisotropic in the southeastern margin of Tibet: the fast-wave polarization direction exhibits a NW-SE regional bias, while the time delay varies between 0.07s and 1.27s, being the average 0.54s±0.12s, implying the existence a strong shear zone in the lower crust. Based on the negative polarity observed after the direct P-wave in teleseismic receiver functions, we have detected an intra-crustal low-velocity zone (ICLVZ) in a wide region including east and southeast of Tibet and southern Yunnan, so both the shear zone and ICLVZ suggest the existence of lower crustal flow. The reverberation PpPs and PsPs+PpSs phases at the Moho are clearly weaker in SYDSB and the SG fold system than in the Indochina Block and eastern Yunnan, implying that SYDSB has a structure and tectonic history different from Indochina on its west side and eastern Yunnan on its east side. The above results provide seismological evidence for a lower crustal flow spreading from the eastern Tibet and that in its advance southward has invaded SYDSB widely, crossed the Lijiang-Jinhe fault and reached southern Yunnan.

Paleoearthquakes and long-term seismic regime in the Longmenshan fault zone, Southwest China

The collected paleoseismological data about the ancient earthquakes are analyzed for the zone of the Wenchuan earthquake. Four earthquakes with magnitude М = 8.0 occurred over a period of 10000 years and two earthquakes with magnitude 7.5 occurred over 1200 years. The obtained data allowed us to reconstruct the long-term seismic regime within the studied territory by constructing the frequency–magnitude relationship based on the instrumental, historical, and paleoseismological data.

A bridged government–NGOs relationship in post-earthquake reconstruction: the Ya’an service center in Lushan earthquake

In April 2013, a 7.0 Mw earthquake struck along the Longmen Shan fault in the city of Ya’an, Sichuan Province, China, causing serious damage. The invaluable contributions of the non-governmental organizations (NGOs) in dealing with this disaster highlighted the significance of government–NGO relationships in post-disaster reconstruction and the effect this has on the efficiency and quality of post-disaster reconstruction. This paper examines government–NGO relationships (GNR) in the post-Lushan earthquake period as a multi-stakeholder collaborative problem based on organizational lifecycle perspectives through the birth, growth, stability, and saturation stages. Two-year field research was conducted in the affected areas which identified the GNR as a bridged government–NGO relationship (BGNR) during the reconstruction period, with the Ya’an service center acting as the bridging organization. The BGNR’s experience was compared to the previous Wenchuan earthquake, and the general BGNR post-disaster reconstruction situation is discussed on two dimensions: mutuality and organizational identity.

Types of community-focused organisations for disaster risk reduction in the Longmen Shan Fault area

ABSTRACTIn the past 10 years, both the Wenchuan earthquake (2008, Magnitude = 8.0) and the Lushan earthquake (2013, Magnitude = 7.0) struck in the Longmen Shan Fault area, causing extraordinary human and economic losses. After the Wenchuan earthquake, the Chinese government began promoting the Community for Disaster Prevention and Mitigation (CDPM) project nationwide to enhance community-level disaster-resistance capacities. Due to post-earthquake demand, CDPM construction in the Longmen Shan Fault area involved many diverse organisations, each of which had different organisational leadership models, which greatly influenced the CDPM characteristics and mechanisms. From long-term field research in 23 CDPM organisations in Longmen Shan Fault area, four types of CDPM organisations were found, including eight Government-oriented CDPM, six Resident-oriented CDPM, seven NGO-oriented CDPM and two Enterprise-oriented CDPM, forming a multiple organisation-oriented CDPM (M-CDPM) model. As there was only 85 km between the Wenchuan earthquake and the Lushan earthquake epicentres, many of the hardest-hit regions were the same; therefore, most CDPM organisations examined in this study were established after the Wenchuan earthquake and their effectiveness was tested in the Lushan earthquake. Therefore, research on the M-CDPM gives valuable information and provides a practical perspective for community-level disaster risk reduction.

Research on Recent GPS Crustal Deformation Characteristics in the Northeastern Edge of Qinghai-Tibet Plateau

Based on the GPS data in the northeastern edge of Qinghai-Tibet Plateau, the velocity field and the extension change rate of baseline between stations are analyzed. The crustal movement characteristics and their dynamic changes in this region are discussed. The result indicates that the baseline in the regions demonstrates an overall characteristic of shrinking in the NE direction and extending in the NW direction; The extrusion of Bayan Har Block in SE direction since 2009 has amplified obviously, which leads to the enhancement of correlated movement in SE direction in the NW side of Longmenshan Fault. In addition, Tianzhu empty regions and the area near the epicenter of Menyuan Ms6.4 earthquake in 2016 are right in the boundary of this differentiated movement. Recently, a certain amount of strain energy might have been accumulated in the nearby area of the northern margin of Alkin Fault Zone, west boundary of Erdos Block and west to105°N on Northern Xiqinling Fault. The seismic risk in the west and east section of the intersecting area between Bayan Har Block and Qaidam Block is high. The west section of Northern Xiqinling Fault belongs to the inner ring of clockwise vortex and possesses medium-strong seismic risk.

Postseismic deformation associated with the 2008 Mw 7.9 Wenchuan earthquake, China: Constraining fault geometry and investigating a detailed spatial distribution of afterslip

We reconstruct two types of fault models associated with the 2008 Mw 7.9 Wenchuan earthquake, one is a listric fault connecting a shallowing sub-horizontal detachment below ∼20km depth (fault model one, FM1) and the other is a group of more steeply dipping planes further extended to the Moho at ∼60km depth (fault model two, FM2). Through comparative analysis of the coseismic inversion results, we confirm that the coseismic models are insensitive to the above two type fault geometries. We therefore turn our attention to the postseismic deformation obtained from GPS observations, which can not only impose effective constraints on the fault geometry but also, more importantly, provide valuable insights into the postseismic afterslip. Consequently, FM1 performs outstandingly in the near-, mid-, and far-field, whether considering the viscoelastic influence or not. FM2 performs more poorly, especially in the data-model consistency in the near field, which mainly results from the trade-off of the sharp contrast of the postseismic deformation on both sides of the Longmen Shan fault zone. Accordingly, we propose a listric fault connecting a shallowing sub-horizontal detachment as the optimal fault geometry for the Wenchuan earthquake. Based on the inferred optimal fault geometry, we analyse two characterized postseismic deformation phenomena that differ from the coseismic patterns: (1) the postseismic opposite deformation between the Beichuan fault (BCF) and Pengguan fault (PGF) and (2) the slightly left-lateral strike-slip motions in the southwestern Longmen Shan range. The former is attributed to the local left-lateral strike-slip and normal dip-slip components on the shallow BCF. The latter places constraints on the afterslip on the southwestern BCF and reproduces three afterslip concentration areas with slightly left-lateral strike-slip motions. The decreased Coulomb Failure Stress (CFS) change ∼0.322 KPa, derived from the afterslip with viscoelastic influence removed at the hypocentre of the Lushan earthquake, indicates that the postseismic left-lateral strike-slip and normal dip-slip motions may have a mitigative effect on the fault loading in the southwestern Longmen Shan range. Nevertheless, it is much smaller than the total increased CFS changes (∼8.368 KPa) derived from the coseismic and viscoelastic deformations.

Recurrence interval of the 2008 Mw 7.9 Wenchuan earthquake inferred from geodynamic modelling stress buildup and release

The destructive 2008 Mw 7.9 Wenchuan earthquake ruptured the Longmen Shan (LMS) fault zone and devastated cities in Sichuan province, China. Estimates of the recurrence interval of the large earthquake is important to understand the feature of seismic activity and to analyze the seismic hazard in the fault area. In this research we introduce the method of geodynamic modelling to estimate the recurrence interval of the Mw7.9 earthquake based on the basic physics of earthquakes-stress buildup and release on fault. The inter-seismic stress accumulation prior to the 2008 Wenchuan earthquake is extracted from our previous study, which developed 3D finite element visco-elastic lithospheric model of the LMS fault zone. The co-seismic stress release and the post-seismic stress relaxation of the earthquake are simulated through dislocation source models. The recurrence interval, which is the duration needed to accumulate the magnitude of the stress drop of the Mw7.9 earthquake, is estimated to be about 4200–6500 years. Sensitivities of the estimated recurrence interval relying on model dependent parameters, such as the viscosity and slip models, are discussed. This research provides a preferred method to estimate recurrence interval of large earthquake in fault zone.