Aftershock Distribution of the M_S8.0 Wenchuan Earthquake and 3‐D P‐Wave Velocity Structure in and Around Source Region

Abstract

3‐D P‐wave velocity structure in crust and uppermost mantle in and around Wenchuan earthquake source region was studied using long‐term accumulated seismic travel time data in Sichuan–Yunnan region and aftershock data of Wenchuan earthquake. The result shows that shallow P‐wave velocity structure has good correlation with surface geology. Longmenshan fault zone is imaged as high P‐wave velocity region in 0~20 km depth. Pengguan complex and Baoxing complex are imaged as two local high velocity anomaly bodies. The upper to middle crustal high velocity anomaly bodies in Longmenshan fault zone control the distribution of aftershocks. At the southern part of the aftershock zone, aftershocks occurred only in the northeast of the high velocity body related with Baoxing complex; In the middle part, the distribution of aftershocks seems to be controlled by the high velocity anomaly body corresponding to Pengguan complex to some extent; In the northeast part, the high velocity body around Ningqiang–Mianxian may prevent the further extension of aftershocks to the northeast. The existence of upper crustal high P‐wave velocity zone in Longmenshan fault zone implies that the upper crust has relatively high strength, which may play an important role in obstructing the extrusion of Tibet Plateau material to east, and is prone to accumulate energy in deep depth. Yangzi Block is characterized by obvious high velocity region below 30 km depth, and its front edge extends to Tibetan Plateau with depth increasing and reaches to the west side of Longmenshan fault zone in the lower crust and upper mantle.

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

“…The shear strength and frictional strength of the Baoxing massif are great, so it is hard to break or slip. Body wave traveltime tomography results also show that there is high-velocity anomaly in the upper crust near Baoxing massif [15]. Baoxing massif controls the distribution of the aftershocks to a certain extent and prevents aftershocks spreading to the northwest.…”

“…Usually the aftershocks of a thrust earthquake are mainly distributed in the hanging wall, and are relatively fewer in the footwall, while the aftershocks of a strike-slip fault are evenly distributed on both sides [15,24,36]. The relocation results show that the focal depths of the aftershocks are deep in the west and shallow in the east, and the aftershock distribution characteristics is similar to the typical thrust earthquake, such as Wenchuan earthquake and Taiwan Chi-Chi earthquake [24,36].…”

“…We relocated the mainshock of Lushan earthquake using the waveform data provided by Data Backup Center of National Digital Seismic Network [14] with 'Sichuan-Yunnan traveltime table earthquake location software' [15,16]. The software takes into account the earth ellipticity, station elevation, topography and other factors, and is based on 3-D velocity discontinuities and 3-D velocity model.…”

Extensional Seismotectonic Motion and its Dynamics in the Eastern Margin of the Tibetan Plateau and its Surroundings

“…It might be relation with lateral differences in the lithospheric structure [46]. Based on the tomography results, a westward extension front of a high velocity zone beneath the South China block has crossed the Longmenshan fault by 20 km westward and arrived in the eastern margin of the plateau in the lower crust and the upper mantle at 50 km depth [13,47]. Aftershocks of the M8 Wenchuan earthquake were almost concentrated on the inner side of the high velocity zone front of the South China block [48].…”

Deep physical structure and geotectonic implications of the eastern margin of the Qinghai–Tibet Plateau

“…The QTP's eastern margin is the focus of studies on eastward lateral extrusion of the former's crustal material (Tapponnier et al, 1982). Over the years, researchers have produced tomographic imaging results for the QTP's crust-mantle structure (Wang, 2007;Wu et al, 2009;Wang et al, 2005;Zhang et al, 2012), as well as geophysical detection results on a semi-molten, lowvelocity, and high-conductivity layer in the region's middle and lower crust (Sun et al, 2003;Zhao et al, 2008;Wang et al, 2009Wang et al, , 2013Wang et al, , 2014. With these results as the basis, Royden et al (1997) proposed a lower-crustal channel flow model to explain the rapid uplift movements at the QTP's eastern margin.…”