The evolution and deformation of the continental crust in the continental collision zone between the Indian and Eurasian plates have been debated over the past several decades, as well as the pattern of Indian slab subduction. More and more observations have indicated obvious heterogeneous structure long the collision zone, and it has been recognized that the comparison among structure profiles located in different parts of the collision zone is a crucial means to understand the development of several N-S trending rifts in southern Tibetan Plateau and Himalaya, and tearing Indian slab. Unlike receiver function images show Indian lower crust underplating beneath southern Lhasa, a deep seismic reflection profile west of the city Xigaze suggests Indian lower crust subducting into upper mantle beneath the Yarlung-Zangbo suture. To explore the cause of the discrepancy, we deployed a ~450 km long wide-angle reflection/refraction profile to observe the seismic velocity structure and use it to constrain crustal composition beneath southern-central Tibetan Plateau, where has experienced intensely crustal thickening. The high P-wave velocity (~7.1 km/s) indicates that the Indian lower crust extends to the north of the Yarlung-Zangbo suture no more than 50 km. The homogeneous velocity (~6.4 km/s) structure of the upper crust in the Lhasa terrane, which corresponds with the Late Paleocene-Early Eocene flat-lying volcanic sequence, indicates that the upper crust experience insignificant shortening since the Cenozoic. Unlike the high S-wave velocity lower crust indicated by doublet in receiver function images, our results present an extremely low P-wave velocity in the lower crust of the Lhasa terrane. Comparison with the velocities of several typical crustal lithologies in different temperature regimes, the low P-wave velocity in the lower crust of the Lhasa terrane can be explained by the intermediate granulite, which prevented the lower crust from eclogitization and delamination.

Convergence between India and Eurasia continents resulted in the formation of the Tibetan plateau, which has a crust nearly twice as thick as the normal continent. Nevertheless, how the plateau crust thickens is still under debate. Elemental analysis implied the crust thickened at least 20 km in Cenozoic. Sedimentary stratigraphy suggested that there is not significant Cenozoic north-south shortening in the upper crust of Lhasa terrane. Thus, some studies suggested that the Lhasa crust was thickened by underthrusting of the Indian lower crust. Crustal average v_P/v_S ratio reflects the crustal composition, so it can be used to constrain the origin of the thickened crust. We processed the teleseismic waveform data recorded in central Tibet to calculate receiver functions. Then we analysed the north-south variations of crustal thickness and average v_P/v_S ratio from central-north Lhasa terrane to south Qiangtang terrane. Our results show that the crust is thicker in north Lhasa terrane (~74 km) than that in south Qiangtang terrane (~68 km), and the average crustal v_P/v_S ratio in Lhasa terrane (1.73) is lower than that in south Qiangtang terrane (1.78). Previous studies suggested that the northward increase of v_P/v_S ratio is due to increasing temperature and partial melting in the crust, but this interpretation is not consistent with the crustal electrical conductivity observations. When comparing with recent petrologic analysis results, the north-south variations of crustal thickness and crustal average v_P/v_S ratio are well correlated with the spatial difference of silica contents in Miocene volcanics. Thus, we inferred that the Lhasa crust was thickened by the intruded of the acid magma, and reduced the crustal average v_P/v_S ratio of Lhasa crust. The acid magma in the upper mantle should be related to the subduction of the Indian continental lithosphere slab in the Miocene.

We apply the improved H‐κ‐c method (Li et al., 2019) as well as updated joint inversion scheme on the dense Hi-CLIMB array in central-western Tibetan Plateau. The H‐κ‐c method corrects the azimuthal variations in arrival times of Ps and crustal multiples caused by crustal anisotropy and dipping interfaces before performing H‐κ stacking, which shows considerable improvements including greatly reduced errors, much less scattered H (crustal thickness) and κ (crustal Vp/Vs ratio) values, and clearer pattern in different Tibetan blocks, compared to traditional H‐κ method. The receiver functions after harmonic corrections are then stacked to form one isotropic receiver function for each station and used as input in the joint inversion. We introduce Pn station delay time into the joint inversion of surface wave dispersions and P receiver functions to reduce trade-offs of model parameters. The method is implemented using a global search-based algorithm and a flexible spline-based model parameterization. Extensive tests using synthetic and real data suggest that the method is suitable and robust for a variety of crustal structures, and can simultaneously provide crustal Vp/Vs ratio and better-constrained Moho depth. The inversion results show considerable improvements on crustal Vs structure with additional information of crustal Vp/Vs, of which the pattern is very similar to the crustal Vp/Vs from H‐κ‐c method. The high Vp/Vs regions correlating with low S velocities suggest possible partial melting in the mid-crust in parts of the plateau. Their correlation with relatively lower seismicity further suggests different deformation pattern (more ductile vs. more brittle) in different Tibetan blocks. The distinct Lhasa block (higher Vs, lower Vp/Vs, higher seismicity relative to other blocks) may be related to the hypothesis of proto-Tibetan Plateau.

On July 23, 2020, an Ms6.6 earthquake occurred near Nima County, Xizang province. It is located in the Qiangtang active block in the central part of the Qinghai-Tibet Plateau and is close to the Riganpeicu fault. In order to improve the understanding of the seismogenic of the surrounding active faults associated with this earthquake, we used the Sentinel-1A data of the ESA ascending and descending orbit to perform differential interference processing on the single-look complex image of the SAR through ISCE software, to obtain the surface coseismic deformation field of the area. The preliminary results are as follows: It can be seen from the interference fringe patterns of the ascending and descending orbits that the impact range of the surface deformation caused by the Nima earthquake is about 30km×30km, and there is no obvious decoherence area around, indicating that there is no surface rupture in the near field of the earthquake. What’s more, the deformation field of the ascending orbit on the west side has an uplifting trend compared to descending orbit, which maximum value is 11.18cm, but the surface deformation is dominated by the east side subsidence, and respectively the maximum subsidence of the Los direction are about 29.01cm and 31.46cm, which is in line with the nature of normal faults in this area. The next stage is to perform a two-step inversion with deformation data as constraints. For the first step include determining the geometric parameters of the seismogenic fault, and inverting the slip distribution on the fault plane, and obtaining the focal mechanism solution of the earthquake and properties of seismogenic fault. The second step include calculating the Coulomb stress changes caused by the seismogenic faults on the surrounding receiving faults and the results could provide a reference for disaster risk assessment in this area.

The Longmenshan fault zone including the Back-range, the Central and the Front-range faults acts the boundary between the Sichuan Basin and the eastern Tibet. In this study, several seismic techniques are used to investigate the Longmenshan Central fault zone structure based on seismic data recorded by short-period dense seismic arrays.Based on ambient noise tomography, we obtained the S-wave velocity structure which shows that low shear wave velocities are mainly distributed around the surface rupture trace of the Wenchuan earthquake at least down to 2 km. By measuring the radial-to-vertical amplitude ratio of local initial P and applying horizontal-to-vertical spectral ratio analysis, our results showed that a low-velocity zone is clearly observed around the surface rupture trace and the low-velocity zone generally agrees well with the distribution of double-peak frequency analyzed from Horizontal-to-Vertical spectral ratio curves. Fault zone trapped waves analysis showed that low frequency trains with relatively large amplitude following the S wave are clearly observed at stations near the surface rupture. The slight time delay of direct P wave examined from local and teleseismic events indicates a relative shallow slow structure beneath the arrays. Through the comprehensive analysis of the central fault zone, our result suggests a shallow low-velocity zone with a width of ~150-160 m along the surface rupture trace. Moreover, our P wave receiver functions reveal that receiver functions at stations located in the surface rupture zone show much complicated waveforms than those off the surface rupture.

Pacific lithosphere subducted into the deep mantle and drove the opening of the back-arc basin in Japan Sea (Martin, 2011). The subduction rollback produced thermal heterogeneities in the mantle, which can be effectively constrained by seismic wave attenuation (Debayle et al., 2020; Zhao et al., 2013; Zhao et al., 2015). To understand the opening mechanism of the Japan Sea and the evolution of the back-arc basin, we conduct the 1°×1° high-resolution seismic Pn attenuation tomography to constrain the upper mantle thermal structure beneath the Japan Sea and its surrounding areas. The upper mantle under the intraplate volcanoes, parts of the back-arc basin and the Japan Arc are characterized by strong Pn attenuations. Our results, along with previous studies in this region (e.g., Lü et al., 2019), suggest that, beneath the Northeast China, the asthenosphere upwelling may have been separated into multiple magma branches to feed volcanoes in Changbaishan, Jingpohu and Chuga-Ryong regions. Two strong Pn attenuation anomalies are observed beneath the Japan Sea. Alongside with Pn velocity anisotropy (Lü et al., 2019), they suggest two possible regional mantle flows escaped eastward from Northeast China under the driving force of the divergent rollback of the subducting Philippine Sea and Pacific slabs. During Miocene and Pliocene, southwestern and northeastern parts of Japan Arc rotated clockwise and counterclockwise for about and around two vertical poles, respectively (Otofuji et al., 1994; Otofuji et al., 1985). The stretch of the divergent rollback and the push of the suggested mantle flows may drive the “double-door” opening of the Japan Sea with similar rotation angles. At the same time, the distribution of low beneath the back-arc basin suggests that the heating and intrusion of the mantle material thickened the back-arc crusts in the Tsushima Basin and the northwestern edge of the Japan Basin.

On August 4, 2020, a massive explosion occurred in the city of Beirut, Lebanon. This accident reported to be caused by the deflagration of 2,750 tons of ammonium nitrate (Guglielmi, 2020). We used regional seismic records to investigate seismic characteristics of this event, including magnitude calculation, yield estimation and discrimination between the explosion and natural earthquakes. Regional waveform data were collected for the explosion and 10 nearby earthquakes from both on- and off-shore stations. The dataset includes 171 vertical-component seismograms recorded by 67 broadband digital stations. Following Zhao et al. (2017); Zhao et al. (2008; 2012), we used both the third-peak amplitude method (Nuttli, 1973; 1986) and the rms-amplitude method (Patton et al., 2005) to calculate the Lg-wave and surface-wave magnitudes, which are mb(Lg)=3.05±0.18 and Ms=3.6±0.28, respectively. Considering the explosion was caused by chemicals stored above the ground surface, its seismic efficiency is expected much lower than a buried source. If the conventional magnitude-yield empirical relation for buried source, e.g., Bowers et al. (2001), is used, the resulted yield of the Beirut explosion could be only 30 - 40 ton trinitrotoluene (TNT), apparently lower than the actual explosives involved. Therefore, we chose another fitting curve based on the size of the crater produced by explosive charges above the ground surface (Ambrosini et al., 2002). The estimated yield is 3,040 ton TNT, close to the reported yield. The energy conversion coefficient from explosion to seismic waves can be obtained to be approximately 0.012. The P/S spectral ratios, including Pg/Lg, Pn/Lg and Pn/Sn, as the discriminants, can effectively discriminate the Beirut explosion from nearby natural earthquakes. This research was supported by the National Key Research and Development Program of China (2017YFC0601206) and the National Natural Science Foundation of China (41974061, 41974054, 41630210, and 41674060).

The ongoing NW–SE convergence between the African and Iberian (Eurasian) plates mainly attributes the tectonic activity and complexity, and seismicity of the western Mediterranean Region (Grevemeyer et al., 2015; Spakman et al., 2018). Whereas, not only the evolution of converging and subduction process, but the strength heterogeneities of the subducting continental lithosphere as well play an important role in shaping the rough terrain and volcanism of the Iberian Peninsula and northern Morocco (Bezada, 2017). As the most prominent phase in regional seismograms, Lg wave propagates within the relatively thick continental crust, and its attenuation which depicted by Q values is sensitive to thermal related activities (Zhao et al., 2010, 2013) Lg wave Q tomography enables us to better explore the rheological properties of crustal materials, particularly those related to high-temperature and partial melting. In this study, we construct a () crustal Lg wave attenuation model for the western Mediterranean Region. Lg-wave attenuation maps show strong lateral variations which correspond well with regional tectonics. Our results show rather high Lg Q-values in the Iberian Peninsula, France mainland, northern Morocco Algeria, which means no significate Lg-wave attenuation in the stable interior Eurasian plate. However, the regions that have a notable shift from high Lg Q-values to low Lg Q-values distribute in the margin of western Mediterranean. This transition in the extent of seismic attenuation may be link with the subducting process and the crustal rheological properties. This research was supported by the National Natural Science Foundation of China (41630210, 41674060) and the Special Fund of China Seismic Experimental Site (2019CSES0103).