Quaternary deposits affect coseismic offset on thrust faults; evidence from the 2008 M (sub w) 7.9 Wenchuan, China, earthquake

Coseismic offset is an important parameter to determine the characteristics of surface ruptures produced by large earthquakes and has significant implications for understanding fault-zone mechanics. To date, most studies have focused on broad-wavelength variations in coseismic offset and their related mechanisms. However, high-frequency variations in coseismic offset have been less commonly reported due to difficulty in field identification. Here, we show that three sites have typical abrupt changes in coseismic offset within short distances along surface ruptures produced by the 2008 M (sub w) 7.9 Wenchuan earthquake. The Bailu, Qingping, and Xiaoyudong sites on different segments of the seismogenic faults show that coseismic vertical offsets can vary from approximately 0.6 to approximately 1.7 m at neighboring locations. Moreover, the offset gradients at the three sites are estimated from approximately 2.5 to approximately 30.9 m/km. Based on geologic and geophysical investigations at the three sites, we suggest that Quaternary deposits are the primary factor affecting coseismic offset. Specifically, thick and loosely packed deposits are more likely to yield smaller coseismic offsets than thin and densely packed deposits. Finally, through a compilation of recent thrust-type earthquakes, we suggest that the coseismic vertical offset gradient for thrust faults can vary greatly, which requires caution in seismic hazard assessments when designing linear infrastructure projects and constraining slip rates at specific sites.