Clay mineral anomalies in the Yingxiu–Beichuan fault zone from the WFSD-1 drilling core and its implication for the faulting mechanism during the 2008 Wenchuan earthquake (Mw 7.9)

Abstract

The first hole of the Wenchuan earthquake Fault Scientific Drilling (WFSD-1) was started on Nov. 6, 2008 as a rapid response to the May 12, 2008 Wenchuan earthquake (Mw 7.9), which slipped along the Longmen Shan fault on the eastern margin of the Tibetan Plateau. WFSD-1 was drilled to a depth of 1201.15 m and intersected the presumed activefault zoneata depthof 590 m (FZ590), whichcontainsthe maximum value of fractureden- sity and fresh fault gouge. Here we characterized fault rocks of FZ590 by conducting XRD analyses with cohesive and non-cohesive rock samples collected from WFSD-1 borehole cores. The results indicate that a clay anomaly zone is located in the FZ590 fault zone. The slight enrichment of smectite distributed in fresh fault gouge implies that there is a fault-related authigenic clay formation. In addition, the location of the slight enrichment of smec- titeisconsistentwiththeplausibleactiveslipzonedeterminedbypreviousresults,addingconfidencetothesup- position that theprincipalslipzone(PSZ)ofthe2008Wenchuan earthquake islocatedata depthof589.2 mand situated at the lithological boundary of the Neoproterozoic Pengguan Complex and Triassic Xujiahe Formation. The tiny clay anomaly signal captured from borehole cores implies that low frictional heat was generated by coseismic slip, which drives only slight authigenesis processes. Thus, other dynamic weakening mechanisms, such as thermal pressurization may be involved in the fault zone of the Yingxiu–Beichuan fault during 2008 Wenchuan earthquake.