Crustal structure beneath the Qilian Orogen Zone from multiscale seismic tomography

The Qilian Orogen Zone(QOZ), located in the north margin of the Tibetan Plateau, is the key area for understanding the deformation and dynamics process of Tibet. Numerous geological and geophysical studies have been carried out on the mechanics of the Tibetan Plateau deformation and uplift; however, the detailed structure and deformation style of the Qilian Orogen Zone have remained uncertain due to poor geophysical data coverage and limited resolution power of inversion algorithms. In this study, we analyze the P-wave velocity structure beneath the Qilian Orogen Zone, obtained by applying multi-scale seismic tomography technique to P-wave arrival time data recorded by regional seismic networks. The seismic tomography algorithm used in this study employs sparsity constraints on the wavelet representation of the velocity model via L1-norm regularization. This algorithm can deal efficiently with uneven-sampled volumes, and can obtain multi-scale images of the velocity model. Our results can be summarized as follows:(1) The crustal velocity structure is strongly inhomogeneous and consistent with the surface geological setting. Significant low-velocity anomalies exist in the crust of northeastern Tibet, and slight high-velocity anomalies exist beneath the Qaidam Basin and Alxa terrane.(2)The Qilian Orogen Zone can be divided into two main parts by the Laji Shan Faults: the northwestern part with a low-velocity feature, and the southeastern part with a high-velocity feature at the upper and middle crust.(3) Our tomographic images suggest that northwestern and southeastern Qilian Orogen Zones have undergone different tectonic processes. In the northwest Qilian Orogen Zone, the deformation and growth of the Northern Tibetan Plateau has extended to the Heli Shan and Beida Shan region by northward overthrusting at the upper crust and thickening in the lower crust. We speculate that in the southeast Qilian Orogen Zone the deformation and growth of the Northern Tibet Plateau were of strike-slip style at the upper crust; in the lower crust, the evidence suggests ductile shear extrusion style and active frontage extension to the Alxa terrane.(4) The multi-scale seismic tomography technique provides multiscale analysis and sparse constraints, which has allowed to us obtain stable, high-resolution results.

汶川地震之震源破裂过程(英文)

Source rupture of the 2008 Wenchuan earthquake were estimated based on backward projection of seismic waves to its source plane. Observations from regional seismic arrays and near source stations were employed to study the rupture behavior in its different spatial and temporal stages. Regional broadband arrays from Taiwan and northern Vietnam were used to determine the overall rupture processes of this earthquake. With high density of station distribution of both seismic arrays, the rupture processes of this earthquake were determined. Seismic energy determined from array waveforms have been back-projected to the rupture plane of earthquake to determine its slip distributions in fault plane. The rupture processes have been reconstructed based on time-dependent imaged seismic energy radiated from earthquake fault plane. Analyzed results of this study indicated that the 2008 Wenchuan earthquake has rupture duration more than 80 seconds with major asperities radiated its energy on several seconds after the earthquake rupture initiation. The radiated seismic energy showed complex spatial distribution on the fault plane. The rupture initiated at its epicenter and extended to its northeast direction about 250 km. The averaged rupture velocity of this earthquake is determined to be near 3.3 km/s. According to near source records, the first large amplitude arrivals were delayed by seconds with respect to the origin time and were preceded by a small-scale slip. Based on a beamforming analysis, some near source recordings were used in this study to investigate the initial rupture process of this event. We will discuss the rupture behavior in multiple scales using both near fault and regional arrays for source images in its different stages.

强震孕育发生的大陆活动地块理论未来发展与强震预测探索

1地震预测预报研究的学术高地——强震孕育发生的大陆活动地块理论框架建立地震作为一种重大自然灾害,直接影响着人类生活和自然环境.20世纪中叶板块构造学说的建立,对发生在板块边缘约占全球80%地震的分布特征、形成机制、活动规律、动力过程等给出了合理解释,但是却很难解释发生在远离板块边缘,特别是大陆板块内部成灾性最大的大型、特大型地震[1,2].

Differences in both the structure and interaction of the crust and mantle on the eastern and western sides of the Ordos Block

The Ordos Block,the western part of the North China Craton(NCC),has preserved most of its cratonic lithospheric roots during the destruction and modification of the NCC,and nowconnects two distinctly different tectonic units,i.e.,the Tibetan Plateau(TP)to the west and the North China Plain to the east.It is important to studythe detailed crust-mantle structure of the Ordos Block and surroundings to understand the interactions between the Ordos and adjacent regions and the dynamics of the NCC evolution.In this study,the S-wave velocity structure at depths of 0–100 km along an E-W profile at around 36.5°N from the Qilian orogen(QLO)to the Trans-North China Orogen(TNCO)within the NCC was investigated by joint inversion of the receiver functions(RFs)and surface wave dispersion data from 104 broadband seismic stations deployed in the region under the ChinArray project(PhaseⅡand PhaseⅢ).The image of discontinuity structures along the profile was further constructed by common conversion point stacking of RFs.The main results are as follows:(1)The depth of the Moho in the study region gradually decreases from west to east,being the deepest in the QLO,the second in the Ordos Block,and the shallowest in the TNCO.Local depression and uplift of the Moho are also identified beneath the Haiyuan fault zone and the Shanxi-Shaanxi rift system(SSR),respectively.(2)The crust-mantle structure displays obvious lateral heterogeneities among tectonic regions.A pronounced low-velocity anomaly is observed at depths of 20–40 km beneath the QLO and becomes weakened and thinned to the east,indicating that thickening of the mid-to-lower crust probably occurred in the QLO during the growth and lateral extrusion of the northeastern TP under the resistance of the NCC.The lithosphere-asthenosphere boundary beneath the TNCO is imaged at~90 km depth,which is significantly shallower than that beneath the Ordos Block.This observation suggests that the lithosphere of the TNCO may have experienced a notable thinning,possibly by thermal erosion and modification due to the mantle upwelling associated with the western Pacific subduction in the Mesozoic.(3)The uplifted Moho under the SSR and its neighboring areas is spatially coincident with the thinned lithosphere in the TNCO,indicating a close relationship between the development of the SSR and the lithospheric thinning and modification in this region.