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 c...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.展开更多
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 stud...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.展开更多
基金supported by the National Natural Science Foundation of China(41574045,41590862)State Key Laboratory of Earthquake Dynamics(LED2013A06)
文摘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.
文摘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.
