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Although many geophysical models have been proposed in the Longmenshan fault zone (LFZ) and its surrounding areas, the deep structure of the seismic gap and its constraint of the Wenchuan and Lushan earthquakes remain uncertain. Based on the compiled aeromagnetic data and Bouguer gravity data, we have tried to create a more detailed and reasonable magnetic and density model using 2D forward modeling and 3D inversion and made the deep structure of the LFZ visible. The research shows that structure is heterogenous across the LFZ. The earthquake epicenters are located in regions with high-magnetic anomalies and gravity gradients that are associated with rigid blocks that were likely to accumulate stress. However, the seismic gap shows low-magnetic anomalies and transition of gravity anomalies related to a weak zone. The Sichuan Basin has two NE-trending banded high-magnetic blocks extending beneath the LFZ that firmly support the idea that the crust of the Sichuan Basin has subducted downward the LFZ. More importantly, the basement subducts to approximately 33 km west of the Wenchuan–Maoxian fault, with a low dip angle beneath the middle segment of the LFZ, whereas the distance decreases to approximately 17 and 19 km under the southern segment. Thus, the crust of the Sichuan Basin beneath the middle segment extends farther than that beneath the southern segment, with the seismic gap as the transition zone. Therefore, we propose that the structural heterogeneity of the basement on the western margin of the Sichuan Basin may be the main reason for the different focal mechanisms and geodynamics of the Wenchuan and Lushan earthquakes. [ABSTRACT FROM AUTHOR]

Although many geophysical models have been proposed in the Longmenshan fault zone (LFZ) and its surrounding areas, the deep structure of the seismic gap and its constraint of the Wenchuan and Lushan earthquakes remain uncertain. Based on the compiled aeromagnetic data and Bouguer gravity data, we have tried to create a more detailed and visible magnetic and density model beneath the LFZ using 2D forward modeling and 3D inversion. The research shows that structure heterogeneities are widely distributed beneath the LFZ. The earthquake epicenters show high magnetic anomalies and the edge of high Bouguer gravity anomalies that consist of rigid blocks where apt to accumulate stress. However, the seismic gap shows low magnetic anomalies and transition of Bouguer gravity anomalies related to a weak zone. The Sichuan Basin has two NE-trending banded high magnetic blocks extending beneath the LFZ that firmly support the crust of the Sichuan Basin was downward subduction toward the LFZ. More importantly, the basement subducts to approximately 33 km west of the Wenchuan-Maoxian fault with a low dip angle beneath the middle segment of the LFZ, whereas the distance decreases to approximately 17 and 19 km under the southern segment. Thus, the crust of the Sichuan Basin beneath the middle segment extends farther than that beneath the southern segment with the seismic gap as the transition zone. Therefore, we propose that the structural heterogeneity of the basement on the western margin of the Sichuan Basin may be the main reason for the different focal mechanisms and geodynamics of the Wenchuan and Lushan earthquakes. [ABSTRACT FROM AUTHOR]

On April 20, 2013, a strong earthquake of Ms 7.0 occurred in Lushan County, Sichuan Province, causing large casualties and property losses. It has drawn a lot of attention from seismologists and brought extensive researches. The velocity model and slip distribution of the aftershock zone shows low velocity, high Vp/Vs and nearly no slip in the shallow parts, which may reveal highly development of cracks. By doing shear wave splitting analysis and tomography, we found that the anisotropy in this area is complicated (not unique distribution of fast shear wave polarizations) and higher (anisotropy percentage) in upper crust than lower crust. In order to obtain high-resolution image of 3D anisotropic structure, here we developed a local-scale joint P and S wave travel time azimuthal anisotropy tomography method and applied it to the aftershock zone of Lushan earthquake.Body wave travel time anisotropy tomography can simultaneously characterize the spatial distribution of isotropic velocity, anisotropic velocity magnitude and fast direction, but mostly uses P wave travel times only. Liu and Zhao (2016) have developed regional-scale joint P and S wave travel time azimuthal anisotropy tomography method following Eberhart-Phillips and Henderson (2004). In order to obtain fine anisotropic structure of Lushan aftershock zone, we have developed local-scale joint P and S wave travel time azimuthal anisotropy tomography method following Eberhart-Phillips and Henderson (2004) and Liu and Zhao (2016). We constructed a checkerboard model to perform synthetic test and the results show that both anisotropy magnitude and fast direction can be better resolved by using both P and S wave travel times than only P wave travel times.We used almost 4200 aftershocks recorded over a continuous period between 24/4/2013 and 19/5/2013. The distribution of anisotropic magnitude shows that anisotropy is higher in upper crust and is highest near the faults within each layer. This strong anisotropy implies that there was some deformation in the diagenetic layer although there was little evidence of a clear surface rupture of the Lushan earthquake. The high anisotropy area is separated into two stripes, which are associated with two conjugated faults, respectively. We think that the conjugated faults extended to subsurface and the edge of high anisotropic stripes at shallow depth may reveal their shape in the subsurface. The orientation of the anisotropy is primarily parallel to the trend of the Longmenshan fault system (LMF) in high anisotropy area, which is nearly perpendicular to the maximum horizontal compressive stress direction. It is likely that there is a damage zone of fractured rock along the fault, which could be very effective in generating seismic anisotropy with a fast direction parallel to the plane of the fractures, especially if they are water rich. [ABSTRACT FROM AUTHOR]

The southern Sichuan-Yunnan region (SSY), located in the southeastern margin of the Tibetan Plateau, has been an important region to study the mechanism of continental collisions and the deformation model and geodynamic process of the Tibetan Plateau. Many researchers have investigated the SSY region with geological and geophysical approaches in the past 30 years. Compared with the previous tomographic studies, we have used a much more data set and updated tomographic method to determine a small scale three dimensional P wave velocity structure with spatial resolution less than 0.4 degree in regional scale. The regional double-difference seismic tomography (tomoFDD) applied to 291871 P-wave absolute arrival times and 2430967 high quality relative P arrival times of 44754 earthquakes recorded by 90 seismic stations of Yunnan, Sichuan and Guizhou Seismic Network to simultaneously determine the detailed crustal 3D P wave velocity structure and the hypocenter parameters in SSY area. Our high-resolution tomographic model not only displays the general features contained in the previous models, but also reveals some new features. An obvious low-velocity anomaly is visible in the middle and lower crust beneath SSY area, which may reflect the weak material flow in the middle and lower crust in the southeastern margin of Tibetan Plateau. The results shows that the Honghe fault may be the southwestern boundary of Sichuan-Yunnan rhombic block existing obvious velocity anomalies among the segments of Honghe fault, which indicated the potential danger of strong earthquakes.The low-velocity anomalies in the west of the Tengchong volcanic area may represent the intra-crustal magmatic activity since the Pleistocene. The high-velocity anomaly in the east of the volcanic area reflects the condensed magmatic intrusions and volatile high-density residues in the Pliocene volcanic channel. [ABSTRACT FROM AUTHOR]