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Abstract: What are the geodynamic processes that caused these deadly earthquakes? Why have these earthquakes caused so much damage? What are the key lessons that we have learned from these devastating earthquakes? Answers to these questions will significantly enhance not only our understanding of earthquake occurrence but also our ability to reduce seismic hazard. Under the framework of bi-lateral cooperation on earthquake sciences between China and USA, the Second Bi-Lateral Workshop on Earthquake Sciences was held in Chengdu, Sichuan Province, China, from April 22 to 25, 2011. Among the goals of this workshop was a review of recent advances in the study of great earthquakes and the exchange of ideas on earthquake disaster reduction. The principle theme of the workshop was “Great Earthquakes in the 21st Century and Geodynamics.” This Special Issue contains a total of 24 papers presented during the workshop. The contributions cover a wide-range of topics associated with the theme. This preface summarizes the main points of the papers presented in this issue. [Copyright &y& Elsevier]

Abstract: The Longmen-Shan fault zone, at the eastern margin of the Tibetan Plateau, is one of the most extensively studied areas in the world, yet the deformation model and earthquake-generating mechanism remain subjects of vigorous debate. This paper presents a new three-dimensional (3-D) velocity model determined using a large volume of seismic data and two-dimensional (2-D) magnetotelluric (MT) profiles from previous studies, to investigate the mechanisms of crustal deformation and earthquake generation along the reverse-thrust and strike-slip fault zone. It has been observed that low-velocity, and low-resistivity anomalies related to the Sichuan foreland basin, is in sharp contrast to high-velocity and high-resistivity anomalies in the Songpan-Ganze block in the upper crust. The tomographic model presented here reveals two crustal bodies with low-velocity and high-conductivity anomalies underneath the Longmen-Shan fault zone, separated into three contrasting segments by the two crustal bodies. The two low-velocity and low-resistivity bodies have been interpreted as being associated with extrusion of either fluids or products of partial melting from the lower crust and/or the upper mantle. This suggests strong variations in the rheological strength of the rock along the fault zone. This finding implies that coupling between these presumably fluid-bearing bodies and earthquake generation could be extremely complex and that there is dramatic variation from the southwestern area to the northeastern segment along the fault belt. It is suggested here that this complex and variable deformation system along the fault zone played a principal role in controlling seismic generation and rupturing during the 2008 Wenchuan earthquake (Ms 8.0) and that it will do so again during possible future earthquakes in the region. [Copyright &y& Elsevier]

Abstract: Trade-off between the depth of an interface and the average velocity above the interface is well known in receiver function inversion. To resolve model ambiguity, surface wave dispersion data, which are sensitive to vertical shear velocity average, are commonly combined with P receiver functions in the inversion. In this paper, we design a joint inversion scheme using the neighborhood searching algorithm to solve shear velocity structure of the crust and uppermost mantle with receiver function and surface wave dispersion data. Two sets of stretched splines are implemented in the inversion to obtain smooth velocity profiles, separated by a specifically defined Moho discontinuity. We apply this method to the dense Hi-CLIMB seismic array in the central Tibetan Plateau (TP). The Indian crust is observed to subduct and underplate the Tibetan crust between the Yarlung-Zangbo suture and the Bangong-Nujiang suture, at the latitude of about 31.5°N. The crust thickens from about 50km below the Indian foreland to over 75km south of Bangong-Nujiang suture and turns shallower at around 65km under the Qiangtang block. We identify two zones with complicated Moho structure beneath the northern Lhasa block and southern Qiangtang block, indicating highly deformed Moho morphology, and beneath the Himalaya block associated with the underthrusting of the Indian Plate, respectively. A low velocity zone is present in the mid-crust in most parts of the profile under the TP with up to more than 10% velocity reduction. The low velocity structure is not continuous, forming discontinuous patches rather than channels, calling for a revision of the channel flow model. [Copyright &y& Elsevier]

Abstract: The Xianshuihe–Xiaojiang fault zone (XFZ) forms a prominent linear NW–SE trending tectonic feature along the southeastern margin of the Tibetan Plateau. The spatial and temporal evolution of the predominantly strike–slip movement along the fault remains unclear, as estimates for the initiation age of its different segments range from 5 to 13Ma. The Jinhe–Qinghe fault (JqF; the south part of the Longmen Shan thrust belt) provides a key for understanding the tectonic role and deformation history of the Xianshuihe–Xiaojiang fault zone in terms of timing, magnitude of displacement, and spatial–temporal evolution. This paper presents research on the structure and chronology of the JqF. The results of a geometric and kinematic study of the Jinhe–Qinghe fault show it to be a thrust fault with a left-lateral strike slip component; The strike slip offset extends at least 13–18km to southwest, based on the shortening of its folds and faulting, and based on stream deflection. Ten apatite fission track apparent ages show a pronounced change in age/elevation gradient at ~17±2Ma, consistent with an abrupt increase of exhumation due to uplift of the hanging wall of the JqF at that time. Combined with the age of deformed Tertiary strata in the footwall of the JqF and the geometric relationships of the JqF with the Chenghai fault and the XFZ, this indicates that the JqF was active between 17 and 5Ma. Based on the fault chronology, geometry, and movement, we suggest that the fault was a transpressional structure at the south end of the XFZ during the period from 17 to 5Ma. [Copyright &y& Elsevier]

Abstract: Although many studies of the 2008 Mw 7.9 Wenchuan earthquake have described the ground deformation features, rupture mechanism, and structural features of the seismogenic fault zone associated with this event, debate remains concerning the total length of the co-seismic surface rupture zone and whether the earthquake ruptured the Qingchuan Fault in the northeastern segment of the Longmen Shan Thrust Belt (LSTB), China. In this paper, we present new field evidence that the Qingchuan Fault was ruptured by the 2008 Wenchuan earthquake and that the total length of the co-seismic surface rupture zone is up to 285–300km. Field investigations reveal that the earthquake produced a ≥60-km-long surface rupture zone along the pre-existing Qingchuan Fault, with the offset being mainly right-lateral strike–slip and a distinct component of vertical slip. Co-seismic surface ruptures are characterized by faults and extensional cracks. Field measurements indicate co-seismic right-lateral strike–slip displacements along the Qingchuan Fault of 0.3–0.6m and vertical offsets of 0.2–0.5m, which differs to the displacements observed along the central and southwestern segments of the Wenchuan surface rupture zone in the displacement amount and sense. The change in slip sense from thrust-dominated slip in the central and southwestern segments of the LSTB to right-lateral strike–slip-dominated displacement along the Qingchuan Fault (northeastern segment of the LSTB) reflects a change in the orientation of compressive stress along the LSTB, associated with eastward extrusion of the Tibetan Plateau as it accommodates the ongoing penetration of the Indian Plate into the Eurasian Plate. [Copyright &y& Elsevier]

Abstract: In this paper, the 3D Finite Element Method was combined with a Discontinuous Deformation Analysis (DDA+FEM) to study the influence of the Wenchuan earthquake on the stability of the tectonic blocks system of the Qinghai–Tibet Plateau under the background of northward extrusion of the Indian Plate and obstruction of the strong crust of the Sichuan basin. With constraints from GPS data and focal mechanisms, we first calculated the velocity and stress fields of the region. Then, we numerically simulated the rupture process of the seismogenic fault of the 2008 Wenchuan earthquake in overthrust form with a right-lateral component. We then studied the movement and deformation of the tectonic blocks that was caused by the earthquake and its influence on the stress state of the boundary faults. The numerical simulation indicates that the large earthquake causes the most tectonic blocks in the study zone at approximately a 1-mm deviation toward the northeast; in particular, the Wenchuan seismogenic fault obviously deviated eastwards, and the maximum deviation reaches 1.5m. It seems that the large earthquake causes the Bayankala block at the west side of the Longmenshan fracture zone to extend farther eastward. The Wenchuan earthquake causes the Coulomb failure stresses on the boundary faults of the tectonic blocks in the study zone to change to different extents. The maximum change of the Coulomb failure stress reaches about 0.2MPa at both ends of the Wenchuan seismogenic fault, the south section of the Xianshuihe fracture zone and part of the southeast section of the Dongkunlun fracture zone in the upper crust. Additionally, the Coulomb failure stresses also increase to different extents between 0.01 and .02MPa in regions such as the southernmost end of the Xianshuihe fracture zone, part of the southeast section of the Dongkunlun fracture zone, and so on. [Copyright &y& Elsevier]

Abstract: A N–S-trending active fault zone, which includes the Anninghe, Zemuhe, and Xiaojiang faults, forms the tectonic boundary of the Tibetan Plateau on its southeastern margin, marking the limit of the eastward extrusion of the Tibetan Plateau. In this paper, we document the Late Pleistocene–Holocene activity of the Zemuhe Fault, which is the central segment of the fault zone. We have been able to determine slip rate and the average recurrence interval of large earthquakes upon the fault, based on aerial photograph interpretations, field investigations, trench excavations, and radiocarbon age data. The Zemuhe Fault is mainly characterized by left-lateral offsets with a horizontal slip rate of ~10–12mm/yr, and a vertical slip component of ~2.0–2.4mm/yr. Trench investigations and 14 C ages reveal that at least four large earthquakes occurred during the last 1100–1500years, indicating an average recurrence interval of ~300–400years for M 7.0–7.5 earthquakes, consistent with the documented history of the region. Our results confirm that the Zehume Fault plays an important role as a tectonic boundary in releasing the strain energy caused by the southeastwards motion of the Tibetan Plateau. The energy is released in the form of repeated large earthquakes. [ABSTRACT FROM AUTHOR]

Abstract: Rising as “the roof of the world” the Tibetan plateau is now underlain with the thickest continental crust on Earth. How and when was this crust formed, which would have exerted pivotal controls to the formation of the plateau, has long been an issue of hot debates. This paper reports zircon U–Pb ages and Hf isotope data for postcollisional (~30–9Ma) adakites in the southern Lhasa terrane, southern Tibet. A comparative analysis of whole-rock rare earth element geochemistry and zircon Hf isotopes between the adakites and associated Gangdese igneous rocks suggests that the Tibetan crust underwent a major phase of tectonic thickening between ca. 45 and 30Ma in the region. The lower part of the thickened crust consisted prevailingly of mafic lithologies, which we argue to have resulted from intense basaltic underplating and subsequent remelting that took place during the Late Cretaceous and Eocene time related to the Neotethyan subduction processes including breakoff of the subducted slab at ca. 50Ma in the early stage of the India–Asia collision. These processes were responsible for not only the juvenile crust formation but also for the creation of a thermally softened lithosphere in the southern Lhasa terrane. The indentation of India, consequently, caused distributed lithospheric thickening with formation of an orogenic root beneath southern Tibet. Root foundering during the Oligocene gave rise to the adakitic magmatism, regional topographic uplift, and onset of northward underthrusting of the Indian plate that has since played a key role in forming the entire Tibetan plateau. [Copyright &y& Elsevier]