Based on a dense seismic array deployed to monitor the seismic gap between the Wenchuan and Lushan earthquakes, a joint inversion of dispersion curves and receiver functions yielded a fine velocity structure in this region with the highest resolution of approximately 20 km. The results show that the upper crust of the Songpan‐Ganzi terrane overthrusted onto the basement of the Sichuan Basin, indicating that the crustal shortening model may be the major mechanism that accounts for the growth of the Longmenshan. The results also show a low‐velocity zone (Vs < 3.5 km/s) and thickened crust (>67 km) beneath the seismic gap, which could be associated with partial melting. The heat transferred from the partial melting at the base of the seismic gap may increase the temperature of the faults to as high as 300–400 °C and cause the segment of the Longmenshan Fault in the seismic gap to become “aseismic.” In contrast, the Shuangshi‐Dachuan Fault, Dayi Fault, and other blind faults to the east‐southeast of the seismic gap are located above a high‐velocity body. The seismogenic environment of these faults is similar to the segments ruptured during the Wenchuan and Lushan earthquakes, both of which occurred above the westward extension of the strong Sichuan Basin. Higher seismic activity was also observed along these faults than that above the low‐velocity zone. Therefore, we deduce that a major earthquake on these faults is possible and that the magnitude could be larger than that of the Dayi earthquake (M 6.2).
Natural fractures are well developed in the Sangtamu carbonate formation which is the primary oil and gas production unit in the Tarim Basin, China. The analysis of converted waves via shear wave splitting (SWS) is an effective tool for predicting natural fractures in carbonate units. Compared with surface seismic data measurements, vertical seismic profiling (VSP) is more advantageous for acquiring and imaging converted waves. At present, there is a lack of robust methods on using 3D three-component (3C) VSP data for fracture prediction. We propose an innovative workflow for predicting the spatial fracture distribution with the first ever application of SWS analysis to a 3D3C VSP dataset. Instead of traditional Cartesian coordinates, we generated image bins based on a polar coordinate system to obtain accurate P-to-S converted waves of different azimuth angles. This was followed by a two-step SWS analysis: first, we conducted a multi-directional SWS analysis to estimate the fracture-induced anisotropy in the upper layers. Then, the wave field of the target layer was corrected using time compensation and coordinate rotation. Finally, we applied SWS analysis again to obtain the azimuth and spatial intensity of the fractures in the target layer. We found that there was an overall good agreement in the fracture densities derived from the VSP waveforms and well-log data. The areas with high fracture development, as indicated by the SWS and ant-tracking analyses, are also consistent. Our study shows that azimuth processing of walkaround 3D3C VSP combined with SWS analysis can serve as a quantitative diagnostic tool for fractures in a carbonate reservoir.
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