Surface reverse afterslip of the 2008 Wenchuan Earthquake at Yingxiu, Sichuan, China: A case of velocity-strengthening fault friction at shallow depths

The vertical component of surface afterslip of the 2008 Mw7.9 Wenchuan Earthquake, China, has been observed since 2011 using repeated levelings at the Yingxiu site, which spans the southwestern segment of the 2008 rupture. We present the geological, co-seismic rupture, and aftershock features around the Yingxiu site and employ the rate- and state-dependent friction theory to analyze the friction mechanism and depth of the afterslip. The logarithmically increasing afterslip series with a cumulative amount of -30 mm obeys a velocitystrengthening friction law, from which the afterslip depth is estimated to be 2.6-6 km. This depth is further constrained to -5 km that probably represents the thickness of a shallow velocity-strengthening layer of the fault, from information of sedimentary strata and aftershock distribution. The coseismic reverse vertical slip was <3 m around the Yingxiu site during the 2008 mainshock, whereas that on the adjacent rupture section to the northeast was >= 8 m above -15 km depth, including the largest slip of 12.5 m at -13 km depth (Zhang et al., 2012). These features suggest that an apparent deficit of coseismic slip existed on the rupture section at the Yingxiu site, compared with coseismic slip on the adjacent section. Furthermore, the surface-observed afterslip is uncorrelated with the aftershock slip on the fault in the underlying velocity-weakening layer. Therefore, the surface afterslip at the Yingxiu site is due to post-2008 relaxation that compensates for the coseismic slip deficit in the shallow layer of the fault and is probably governed by velocity-strengthening friction.

Automated summary

The surface afterslip of the 2008 Mw7.9 Wenchuan Earthquake in China has been observed through repeated levelings since 2011. The analysis of the geological features, co-seismic ruptures, and aftershocks suggest that this afterslip is due to post-2008 relaxation and is governed by velocity-strengthening friction.