Relocation of Small EarthquakesiIn Western Sichuan, China and Its Implications for Active Tectonics

Abstract

We relocated 13367 small earthquakes that occurred in western Sichuan between 1992 and 2002 using a double‐difference (DD) earthquake location algorithm to improve the relative location precision. The relocated microseismicity forms highly organized structures that correlate with the surface faulting well: showing the flower structure across the simple strike‐slip fault, and the organized but dispersed structure across the pull‐apart basin and thrust fault. Beneath thrust fault, there exists an aseismic layer. The seismicity shows segmentation characteristics on active fault zones. Other significant tectonic features that were previously obscured by routine location errors are also revealed by the relocated seismicity. Some blind faults are delineated by lineages of seismicity that are suggestive of faulting structures. A series of large voids in seismicity appear with dimensions of tens of kilometers on the Xianshuihe‐Anninghe‐Zemuhe fault zone that have been aseismic over the 10‐year time interval, suggesting that these segments may be locked and storing strain energy for release in future large earthquakes. A 5km thick aseismic layer appears in most places in western Sichuan plateau at depths of 15~20km. The crustal strength envelopes are calculated for the western Sichuan plateau based on the results from hightemperature and high‐pressure experiments. The result shows that the granite at depths of 14 to 19km appears to be ductile, which is in good agreement with the thickness and depth range of the aseismic layer, suggesting that the aseismic layer may be the result of the ductile deformation from the granite in the upper crust.

Relocation of the mainshock and aftershock sequences of M S7.0 Sichuan Lushan earthquake

“…The location accuracy is on an order of hundred meters in a small area. Recently, the method has been widely applied to earthquake relocation at home and abroad [21][22][23][24][25][26][27]. The temporal stations installed after the earthquake are quite intensive, and may cause systematic change to the location results after the temporal stations were set up compared with the results before.…”

Evidence of crustal ‘channel flow’ in the eastern margin of Tibetan Plateau from MT measurements

“…The seismic energy produced by crustal deformation can be built up in the brittle upper crust, while continuing southeastward flow of the material in the low-resistivity layer could reduce stress and heterogeneity of motion, leading to active small earthquakes and little large events. Therefore the study area has become a "nest" [20,21,24,75] of small shocks primarily confined in the upper crust [75,76] .…”

Mapping the <i>b</i>-values along the Longmenshan fault zone before and after the 12 May 2008, Wenchuan, China, <i>M<sub>S</sub></i> 8.0 earthquake

“…In the local catalogues, due to the limitation of location accuracy, depth cannot be determined accurately with acceptable reliability, although there have been some results of accurate location of part of the earthquakes using double difference methods (e.g. Zhu et al, 2005). Also considering that the Longmenshan fault zone is a thrust fault zone with at least three parallel faults striking along the SW-NE direction, in this study, we do not consider the depth distribution and just take all the shallow earthquakes for the analysis.…”