Features and genesis of micro-nanometer-sized grains on shear slip surface of the 2008 Wenchuan earthquake

Abstract

In coseismic surface rupture zones caused by the 2008 Mw 7.9 Wenchuan earthquake, some thin-layered fault gouges with strong deformation were observed in different locations. In this paper, fault gouge samples were taken as research objects from the Bajiaomiao village in the south-west segment of the principal rupture and the Heshangping village and the Shaba village in the north-east segment of the principal rupture where larger displacements were measured. Fabric characteristics of the fault gouge samples and the morphologies and structures of micro-nanometer grains on Y-shear surfaces were then analyzed by using a stereoscope and SEM. Observation results showed that obvious Y- and R-shears and obvious scratches were well developed in coseismic gouges caused by the 2008 Wenchuan earthquake. Micro-nanometer grains in the fault gouge of the Wenhcuan earthquake were formed mainly due to breaking, grinding, and powdering of fault slipping friction surface. Heat caused by fault slipping (maybe also including heat caused by thermal decomposition) played an important role in producing micro-nanometer sized grains. Existence occurrence state of micro-nanometer sized grains on fault slip surface includes singled grains and their complexes with shapes of ball, silkworm, pancake and mass. The structures mainly include dispersed and close-packed structures besides a few of striped and layered structures. All these structures were formed at the extreme unbalance conditions caused by rapid deforming during an earthquake. There always exist some voids between structures due to loosely contact. Only alienated grains are included in the stripped structure. But there are some singled grains with no deformation in dispersed and close-packed structures besides complexes of grains with morphologies of ball, silkworm, pancake and mass. The striped and close-packed structures are the results of plastic deformation, and the dispersed and layered structures are the results of brittle deformation whereas loose contact of different structures was caused mainly by discontinuous dynamic friction (fault stick-slipping). The structures of the micro-nanometer sized grains in coseismic fault gouge caused by the Wenchuan earthquake are the geological records of seismic fault slipping (it is not pseudotachylite), which could be used as an index of paleo-seismic events.