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Pet S.-P.,CAS Institute of Tibetan Plateau Research | Liu J.,China Earthquake Administration | Ma H.-S.,China Earthquake Administration | Gao X.,CAS Institute of Tibetan Plateau Research | And 2 more authors.
Chinese Journal of Geophysics (Acta Geophysica Sinica) | Year: 2010

Variation of Q value can provide the information of crustal stress, rock cracks, and fluid variation, and evidence to understand preparation and generation of earthquake. In this study, the amplitude data from the Annual Bulletin of Chinese Earthquake and the provincial earthquake bulletins of Sichuan and Yunnan were tentatively divided into 6 time segments. Tomographic ML amplitude method has been applied for each time segment to obtain temporal Q value variations. Firstly, a representative time segment with more than 14000 ray paths was selected to estimate the resolution and error of tomographic attenuation structure by adding random noise to original data and bootstrap method. Then, based on the detailed analysis of reliability, the same tomographic method with the same parameters was applied for all time segments. The research result shows that (1) the imaging error is small from the observation error of original data and ignoring radiation pattern of earthquake source. The error is less than 6% if 40% random noise is added to the original data. (2) The bootstrap method shows that the maximal error is less than 8% of average Q value, that means the tomographic results are stable and reliable because of huge data set in Sichuan and Yunnan region. (3) In checkerboard test, the resolution can reach 20′ for the region with more than 50 ray paths in each cell. (4) The Q value variations were obtained after imaging for each time segment. A clear positive relation was found between the average Q value for each time segment and the number of middle and large earthquakes, in other words, the more earthquakes, the larger average Q, that may be resulted from the changes of regional stress. The relation between earthquake distribution and Q value variation was analyzed and we found that most of the large earthquakes did not occur in the maxima' variation region of Q, but in the transition zone between maxima' increase and drop of Q value. Maybe the differential stress has a maximal change in transition zone and the earthquake is easier to trigger. Source

Pei S.,CAS Institute of Tibetan Plateau Research | Chen Y.J.,Peking University | Feng B.,CAS Institute of Tibetan Plateau Research | Gao X.,CAS Institute of Geographical Sciences and Natural Resources Research | Su J.,Seismological Bureau of Sichuan Province
Tectonophysics | Year: 2013

To investigate upper crust anisotropy, a tomography algorithm was reformulated to include lateral variations in both velocity and azimuthal anisotropy, and it was applied to Pg travel time data around the epicenter of the 14 April 2010 Ms=7.1 Yushu earthquake. A high-resolution two-dimensional (horizontal) seismic velocity and azimuthal anisotropy model was obtained using this simple tomography method including both station and event depth corrections. The results show clear anisotropy in the upper crust and it is as important as the velocity variation in explaining the travel time residuals. The most striking result is a high velocity anomaly with large anisotropy at the epicenter on the Yushu-Garze fault. The main rupture originated within this high velocity anomaly and propagated southeastward into a low velocity anomaly with small anisotropy at Yushu. The azimuthal anisotropy shows fast velocity direction along Yushu-Garze fault and larger anisotropy exists in the epicenter than surrounding regions. These results demonstrate a clear example that lateral variation and anisotropy in seismic 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 and possible fracture distribution around seismogenic zone. © 2012 Elsevier B.V. Source

Jianga L.,Chinese University of Hong Kong | Lin H.,Chinese University of Hong Kong | Liu F.,Seismological Bureau of Sichuan Province
Annals of GIS | Year: 2010

The Xianshuihe fault of Sichuan province, southwest China, is a highly active strike-slip fault approximately 350 km long. Previous studies have described left-lateral slip of up to 10-17 mm/year on the Xianshuihe fault during recent decades, as indicated by geological criteria and GPS observations. Satellite InSAR observations provide an alternative effective technique used to measure the crustal deformation on the active fault. In this article, we present ScanSAR interferometry applied to ENVISAT WSM ASAR data to produce a deformation map of about 400 × 400 km over the Xianshuihe fault zone and the Garzê-Yushu fault zone. The preliminary stacked deformation results of six WS-WS interferograms indicated a northeast-southwest deformation trend nearly perpendicular to the two fault zones and demonstrated the potential to monitor such wideranging deformation using ENVISAT WSS data. © 2010 Taylor & Francis. Source

Pei S.,CAS Institute of Tibetan Plateau Research | Pei S.,Massachusetts Institute of Technology | Su J.,Seismological Bureau of Sichuan Province | Zhang H.,Massachusetts Institute of Technology | And 6 more authors.
Tectonophysics | Year: 2010

We present three-dimensional (3D) seismic compressional wave velocity (Vp), shear wave velocity (Vs) and Vp/Vs models around the Longmen Shan fault, Sichuan, China region, using aftershocks associated with the 2008 Wenchuan Ms 8.0 earthquake. The velocity and ratio models are obtained using a new version of the double-difference seismic tomography code tomoDDPS (Zhang, 2003) to simultaneously solve for Vp, Vs, Vp/Vs and event locations. The data used in inversion include 73,013 P arrival times, 62,287 S arrival times and 61,823 S-P travel times recorded by 63 stations from both permanent and temporary networks in a region 400km northeast-southwest by 200km northwest-southeast. The velocity model shows structure heterogeneity both along and across the fault zone. Generally, the velocity model is consistent with the local geology, with older rocks having high velocity and younger rocks having low velocity. The Longmen Shan fault zone is a clear boundary in Vp, Vs and Vp/Vs ratio. Down to the depth around 15km, higher Vp and Vs and lower Vp/Vs ratio exist to the west of the fault, corresponding to the Songpan-Ganze Fold System, while lower Vp and Vs and higher Vp/Vs ratio exist to the east, corresponding to the Sichuan Basin. Along the fault zone, the velocity structure is generally consistent with various rupture slip models, with two high velocity bodies corresponding well to the two large slip patches. This shows the structure control on the slip distribution along the fault plane. © 2009 Elsevier B.V. Source

Wang Z.,CAS South China Sea Institute of Oceanology | Wang Z.,Chengdu University of Technology | Su J.,Seismological Bureau of Sichuan Province | Liu C.,Chengdu University of Technology | Cai X.,Chengdu University of Technology
Journal of Geophysical Research B: Solid Earth | Year: 2015

Global seismic waveform inversion can provide information on where the 2013 Ms 7.0 Lushan earthquake occurred in Sichuan Province, China, and how its aftershock sequence expanded. To investigate the generation mechanism of the Lushan earthquake and its relation to the 2008 Wenchuan earthquake (Ms 8.0), 50 temporary seismic stations were installed in the source area following the Lushan earthquake. Crustal stress data were also collected along the Longmen-Shan Fault zone (LMFZ) to reveal its influence on the generation of the Lushan earthquake. Seismic imaging and crustal stress analysis have revealed that the Lushan earthquake occurred in a distinct area with high velocity (Vp, Vs), low Poisson's ratio (σ), and high crustal stress. The high-velocity zone at the Lushan source may reflect Precambrian metasedimentary or igneous rock in the seismogenic layer, which enables the accumulation of high crustal stress to generate large earthquakes. However, a sharply contrasting gap zone with low-velocity, high-σ anomalies is clearly imaged in the upper crust under the concatenated area between the Lushan and Wenchuan earthquakes. Seismic images indicate that the low-velocity gap zone is associated with fluid-bearing ductile flow from the lower crustal materials of Tibet being pushed into a weakened segment of the LMFZ. This study suggests that the 2013 Lushan earthquake may have been triggered by high crustal stress accumulation together with high coseismic stress further increased by the Wenchuan earthquake in the metamafic seismogenic layer. Contrasting rheological variations in the crust and crustal stress changes along the LMFZ control the rupture processes that generated the Lushan and Wenchuan earthquakes, as well as the generation of new earthquakes in the future ©2015. The Authors. Source

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