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The occurrence of large earthquakes is not only related to the fault geometry and state of stress accumulation, but also closely related to the structure of surrounding rocks. With the continuous enrichment of observation methods and development of techniques in recent decades, various studies have been conducted to explore the fine structure of large earthquake source regions and link the observations to the mechanism of large earthquakes. Seismologists generally use the asperity model to explain the generation mechanism of large earthquakes, that is, the process from locking to sudden slip of the seismogenic faults. The detailed geometrical shape of the fault planes and accurate state of stress accumulation are usually tough to depict during the process, while the structure features of the surrounding rocks are available through seismic tomography and magnetotelluric studies. In this paper, we aim to systematically summarize the results of structure studies in the focal areas of large earthquakes, and discuss the relationship between structure features and asperities. We therefore made statistics on the studies of asperity in the source regions of worldwide large earthquakes with magnitude larger than 6.0. There are totally 123 events with different focal mechanism, among which 54 events are intraplate earthquakes, while the other 69 events are interplate earthquakes. These large earthquakes are well studied by structure and/or source rupture process researches. Structure studies carried on 17 intraplate and 14 interplate earthquakes suggest the presence of high strength (tomographic high-velocity and/or magnetotelluric high-resistance) bodies surrounding the seismogenic faults. These anomalous bodies are considered to be the asperities, which play an important role in the generation of large earthquakes. Among the 31 large earthquakes, there are 4 events that took place on subducted seamounts, which are characterized as patches of anomalous structure features as well as elevated effective normal stress and are asperities with high strength in the evolution of these large earthquakes. The structure studies provide direct evidence for the important role of asperity in the generation of large earthquakes. The indirect evidences come from the source rupture studies of 92 large earthquakes, which show large coseismic slip zones near the seismogenic faults, manifesting the existence of asperities with high strength. The predominance of unilateral rupture in earthquakes has also been revealed, which has a close relationship with the structural heterogeneity. Through statistical analysis of asperity mechanism studies on large earthquakes, we find the asperity characterized by high velocity and resistivity plays an important role in the generation of large earthquakes. The asperities are strong in mechanical strength and could accumulate tectonic stress more easily during the long frictional locking periods, large earthquakes are therefore prone to generate in these areas. If the close relationship between asperity and high-velocity and high-resistivity bodies is valid for most of the large earthquakes, it can be used to predict potential large earthquakes and estimate the seismogenic capability of faults in light of structure studies, which will provide important clues for earthquake prevention and disaster reduction. Furthermore, the asperity model is irrelevant to focal mechanism and plays a same important role in thrusting, normal and strike-slip faults.

Combining the advantages of Glaciology and seismology, Glacier seismology formes a young interdisciplinary subject. Icequakes refer to the vibration generated during the movement and rupture of a glacier, ranging from small creaks to sudden rupture or sliding equivalent to an earthquake (MW7). According to the location and mechanism of icequakes, the icequakes can be divided into five types: surface crevasses, stick-slip motion, iceberg calving, subglacial water flow and hydrofracturing. In addition to traditional seismological methods, icequake research can be used combing with multidisciplinary methods such as GPS, numerical simulation, and glacier physical properties. Icequake research can further explore the occurrence process and risk assessment of ice avalanches. We reviewed the research progress of glacier seismology. Moreover, we introduced part of the glacial earthquake research work carried out by domestic researchers in the Tibetan Plateau. Finally, we comprehensively discussed the enlightenments from glacier earthquakes to natural earthquake research.

Abstract Detailed crustal structure of large earthquake source regions is of great significance for understanding the earthquake generation mechanism. Numerous large earthquakes have occurred in the NE Tibetan Plateau, including the 1920 Haiyuan M8.5 and 1927 Gulang M8 earthquakes. In this paper, we obtained a high-resolution three-dimensional crustal velocity model around the source regions of these two large earthquakes using an improved double-difference seismic tomography method. High-velocity anomalies encompassing the seismogenic faults are observed to extend to depths of 15 km, suggesting the asperity (high-velocity area) plays an important role in the preparation process of large earthquakes. Asperities are strong in mechanical strength and could accumulate tectonic stress more easily in long frictional locking periods, large earthquakes are therefore prone to generate in these areas. If the close relationship between the aperity and high-velocity bodies is valid for most of the large earthquakes, it can be used to predict potential large earthquakes and estimate the seismogenic capability of faults in light of structure studies.

A high-resolution two-dimensional Pg-wave velocity model is obtained for the upper crust around the epicenters of the April 20, 2013 Ms7.0 Lushan earthquake and the May 12, 2008 Ms8.0 Wenchuan earthquake, China. The tomographic inversion uses 47235 Pg arrival times from 6812 aftershocks recorded by 61 stations around the Lushan and Wenchuan earthquakes. Across the front Longmenshan fault near the Lushan earthquake, there exists a strong velocity contrast with higher velocities to the west and lower velocities to the east. Along the Longmenshan fault system, there exist two high velocity patches showing an ''X'' shape with an obtuse angle along the near northwest-southeast (NW-SE) direction. They correspond to the Precambrian Pengguan and Baoxing complexes on the surface but with a ~20 km shift, respectively. The aftershock gap of the 2008 Wenchuan and the 2013 Lushan earthquakes is associated with lower velocities. Based on the theory of maximum effective moment criterion, this suggests that the aftershock gap is weak and the ductile deformation is more likely to occur in the upper crust within the gap under the near NW-SE compression. Therefore our results suggest that the large earthquake may be hard to happen within the gap. [ABSTRACT FROM AUTHOR]

A high-resolution 2D (horizontal) seismic velocity model is presented for the upper crust around the epicenter of the 14 April 2010 MS 7.1 Yushu earthquake from aftershock tomography. The velocity model is obtained using a simple Pn-like tomography method including both station and event depth corrections. The most striking result is a fast-velocity anomaly at the epicenter on the Yushu-Garze fault (YGF). The main rupture originated within this fast-velocity anomaly and propagated southeastward into a slow-velocity anomaly at Yushu. The second largest aftershock on 29 May 2010 on a nearby fault also occurred within this fast-velocity anomaly. These results demonstrate a clear example that lateral variation in seismic velocity structures of the upper crust controlled the origination (stress accumulation) and rupture propagation of the 2010 Yushu earthquake and distribution of aftershocks as well. The simple 2D Pg wave travel-time tomography method presented here introduces a new approach utilizing abundant aftershocks data for investigating the rupturing process of a major earthquake. [ABSTRACT FROM AUTHOR]

We present tomographic velocity and anisotropy models of the uppermost mantle beneath the Iran region. A total of 74,375 Pn phase readings from 86 stations of the Iranian network and 133 stations of the International Seismological Centre are used in the investigation. The study uses the Pn travel-time tomography method proposed by Hearn (1996). The tomography results show some interesting anomalies. The average Pn velocity under the Iran region is approximately 8.0 km/s, and the maximum velocity perturbations are approximately 3%-4%. High Pn velocities under the Zagros Mountains and the Caspian Sea may be due to the presence of oceanic crustllithosphere material. Low Pn velocities were found under the Alborz and Caucasus regions and may be due to higher temperatures or partial melting resulting from volcanoes and mid-Cenozoic volcanic/plutonic rocks in these regions. The inversion velocities support the idea that the subduction of the Arabian plate into the mantle beneath the Iranian plateau may have resulted in the upwelling of hot material. The well-resolved Pn anisotropy model is jointly obtained with a velocity model for the areas with good ray-path coverage. In the plate collision regions (Zagros and Alborz), the fast Pn anisotropy direction is oriented parallel to the collision arc and to large reverse faults due to pure shear deformation from cross-fault compression and along-fault extension. Under the Caucasus regions, the Pn anisotropy results indicate that the preferred alignment of olivine crystals is parallel to the plate movement direction; however, the surface fault strike is at an angle of nearly 45° with the crustal movement direction and anisotropy. These differing deformations suggest potential decoupling between the crust and upper mantle. The possible decoupling and differing deformation between the crust and upper mantle are easily enhanced under high temperatures due to volcanoes and supported by low velocities beneath the Caucasus. We validate the existence of Pn anisotropy under these regions by azimuthal averaging of the apparent Pn velocity. [ABSTRACT FROM AUTHOR]

We have obtained a Poisson's ratio image of the uppermost mantle beneath China by performing tomographic inversion of travel-time differences between Sn and Pn. The arrival pairs were selected from the Annual Bulletin of Chinese Earthquakes from 1985 to 2007 and from the International Seismological Center (ISC) data set between 1985 and 2005. The data include 58,663 arrival pairs from 10,486 earthquakes recorded at 204 stations. The average Poisson's ratio is 0.266. The preliminary tomographic results show that (1) the pseudowave velocity is high and the velocity ratio (VP/VS) and Poisson's ratio are low in the stable cratons around the Tibetan Plateau such as the Tarim and Junggar basins, the Ordos craton, and the southern region of the Sichuan basin; (2) a low pseudowave velocity and high velocity ratio and Poisson's ratio exist in the central and northern Tibetan Plateau, the North-South Seismic Zone, and north China; and (3) the high velocity ratio and Poisson's ratio region in the Tibetan Plateau extends to the surrounding cratons, suggesting that the uplift of the Tibetan Plateau results from a high Poisson's ratio or partially melted rocks beneath the plateau and that the softer rocks have intruded into the upper mantle of surrounding cratons. [ABSTRACT FROM AUTHOR]

The fate of the colliding Indian and Asian tectonic plates below the Tibetan high plateau may be visualized by, in addition to seismic tomography, mapping the deep seismic discontinuities, like the crust-mantle boundary (Moho), the lithosphere-asthenosphere boundary (LAB), or the discontinuities at 410 and 660 km depth. We herein present observations of seismic discontinuities with the P and S receiver function techniques beneath central and western Tibet along two new profiles and discuss the results in connection with results from earlier profiles, which did observe the LAB. The LAB of the Indian and Asian plates is well-imaged by several profiles and suggests a changing mode of India-Asia collision in the east-west direction. From eastern Himalayan syntaxis to the western edge of the Tarim Basin, the Indian lithosphere is underthrusting Tibet at an increasingly shallower angle and reaching progressively further to the north. A particular lithospheric region was formed in northern and eastern Tibet as a crush zone between the two colliding plates, the existence of which is marked by high temperature, low mantle seismic wavespeed (correlating with late arriving signals from the 410 discontinuity), poor Sn propagation, east and southeast oriented global positioning system displacements, and strikingly larger seismic (SKS) anisotropy. [ABSTRACT FROM AUTHOR]

Seismic attenuation (Q-value) can be estimated by extracting the amplitude-frequency information contained in seismic waveforms. We apply the attenuation tomography method of Pei et al. (2006) using ML amplitude data to estimate attenuation within the upper crust in Japan. More than 60,000 Sg-wave maximum amplitude readings from 5559 events, recorded by 971 stations, were selected from the dense High-Sensitivity Seismography Network (Hi-net) under the condition that epicentral distance is less than 2° and event depth is less than 10 km. The lateral S-wave Q variations of the upper crust at 1 Hz in Japan were obtained. The results indicate that low Q-values exist in the central Japanese islands, with almost the same distribution as volcanoes, while high Q-values exist mainly between the front of volcanoes and the Japanese east coast. In addition, a low Q was found between the eastern coast and the subducted trench. Most large crustal earthquakes occur in or around zones of low Q or the boundaries between areas of low and high Q, which will be very helpful in estimating the risk of large earthquakes. [ABSTRACT FROM AUTHOR]

Amplitude tomography reconstructs seismic attenuation directly from recorded wave amplitudes. We have applied the tomography to amplitude data reported in the ‘Annual Bulletin of Chinese Earthquakes’ and interpreted the regionally varying crustal attenuation in terms of tectonics. The seismic amplitudes were originally recorded for determining the ML and MS magnitudes. They generally correspond to the maximum amplitudes of the horizontal components of the short-period S waves and intermediate-period Rayleigh waves. Both sets of measurements are sensitive to crustal structure. The peak amplitudes from ML amplitudes spread spherically with significant dispersion and scattering. MS amplitudes show cylindrical spreading with little dispersion. Average crustal Q values for attenuation at 1 Hz are 737 and 505 for ML and MS, respectively, with substantial regional variations. Frequency dependence in the attenuation is also indicated. Regions with the lowest attenuation (high Q values) are beneath the south China Block, Sichuan Basin, Ordos Platform, the Daxinganling and the Korea Craton. These tend to be tectonically inactive regions, which are generally dominated by intrusive and cratonic rocks in the upper crust. Regions with the highest attenuation (low Q values) are beneath Bohai Basin, Yunnan, eastern Songpan-Ganzi Terrain, margins of the Ordos platform and the Qilian Shan. These are predominantly active basins, grabens and fold belts. The continental margin also highly attenuates both S and surface waves. [ABSTRACT FROM AUTHOR]

In this paper, a 2D velocity structure of the crust and the upper mantle of the northern margin of the Tarim Basin (TB) has been obtained by ray tracing and theoretical seismogram calculation under the condition of 2D lateral inhomogeneous medium using the data of seismic wide angle reflection/refraction profile from Baicheng to Da Qaidam crossing the Kuqa Depression (KD) and Tabei Uplift (TU). And along the Baicheng to Da Qaidam profile, 4 of the 10 shot points are located in the northern margin of the TB. The results show that the character of the crust is uniform on the whole between the KD and TU, but the depth of the layers, thickness of the crust and the velocity obviously vary along the profile. Thereinto, the variation of the crust thickness mainly occurs in the middle and lower crust. The Moho has an uplifting trend near the Baicheng shot point in KD and Luntai shot point in TU, and the thickness of the crust reduces to 42 km and 47 km in these two areas, respectively. The transition zone between the KD and TU has a thickest crust, up to 52 km. In this transition zone, there are high velocity anomalies in the upper crust, and low velocity anomalies in the lower crust, these velocity anomalies zone is near vertical, and the sediment above them is thicker than the other areas. According to the velocity distributions, the profile can be divided into three sections: KD, TU and transition zone between them. Each section has a special velocity structural feature, the form of the crystalline basement and the relationship between the deep structure and the shallow one. The differences of velocity and tectonic between eastern and western profile in the northern margin of the Tarim Basin (NMTB) may suggest different speed and intensity of the subduction from the Tarim basin to the Tianshan orogenic belt (TOB). [ABSTRACT FROM AUTHOR]

We present a layered shear-wave velocity structure of the crust and uppermost mantle of China and the surrounding area. We apply an adaptive moving window (Sun, Li, et al., 2004) to construct the S-wave velocity model from high-quality body-wave phase data extracted from the Annual Bulletin of Chinese Earthquakes (ABCE). More than 350,000 S-wave arrivals are used, spanning from 1990 to 2004. The study area is represented by a 1° geographic grid consisting of 2338 points. At each point, 1D S-velocity-depth profiles are determined independently from the surface to the uppermost mantle; this is accomplished by performing a Monte Carlo random search of optimum layer parameters (thickness and velocity) via minimizing travel-time misfits for fixed earthquake locations. Each profile contains a four-layer crust and a one-layer uppermost mantle. A final 3D model is obtained by combining and smoothing the 1D models. The obtained S-wave model has a good correlation with the previously published P-wave model using the same method (Sun, Li, et al., 2004) and reveals key tectonic features such as the low velocity crust beneath Tibet. Our S-wave model is generally consistent with the existing regional/local models constructed from body-wave travel-time tomography, and provides more detailed structure in both horizontal and vertical directions compared with the model derived from surface wave inversion. Based on our P- and S-wave models, the generated synthetic seismograms fit well with the observed seismograms recorded at broadband stations. [ABSTRACT FROM AUTHOR]

We have selected 10,899 ML amplitude readings from 1732 events recorded by 91 stations, as reported in the Annual Bulletin of Chinese Earthquakes (ABCE), and have used tomographic imaging to estimate the lateral variations of the quality factor Q0 (Q at 1 Hz) within the crust of Northern China. Estimated Q0 values vary from 115 to 715 with an average of 415. Q0 values are consistent with tectonic and topographic structure in Eastern China. Q0 is low in the active tectonic regions having many faults, such as the Shanxi and Yinchuan Grabens, Bohai Bay, and Tanlu Fault Zone, and is high in the stable Ordos Craton, Q0 values are low in several topographically low-lying areas, such as the North China, Taikang-Hefei, Jianghan, Subei-Yellow Sea, and Songliao basins, whereas it is high in mountainous and uplift regions exhibiting surface expressions of crystalline basement rocks: the Yinshan, Yanshan, Taihang, Qinlin, Dabie and Wuyi Mountains, and Luxi and Jiaoliao Uplifts. Quality-factor estimates are also consistent with Pn- and Sn-velocity patterns. High-velocity values, in general, correspond with high Q0 and low-velocity values with low Q0. This is consistent with a common temperature influence in the crust and uppermost mantle. [ABSTRACT FROM AUTHOR]