On-site determination of slip vectors along the Longmenshan fault during the Mw 7.9 Wenchuan, China, earthquake of May 12, 2008

Abstract It is useful to estimate the co-seismic slip vector on the source fault in order to understand the rupture style and process. Many studies on co-seismic displacement distribution of Wenchuan great earthquake of Mw 7.9 have been made. The co-seismic slip vector and net slip, however, has not been reported because most strike slips measured in field are only apparent, of which some or whole is due to horizontal shortening (or extension) perpendicular to a surface break. The net slips estimated based only on these apparent strike slips would include an appreciable amount of error. Based on co-seismic apparent strike offset data measured in field, we calculated horizontal slip vectors, including strike slips, dip slips, fault-normal horizontal shortenings, fault dips, and net-slip vectors combined with the measured vertical displacements. Adding the net-slip vector measured directly from the exposure rupture, and comparing the results with those deduced from the moment tensor solutions in depth and GPS at surface, the following main conclusions have been drawn. First, on the southwestern segment of the rupture, the slip azimuth at surface, similar to that deduced by GPS data, is less than that in deep, showing an anti-clockwise rotation of slip vector on the rupture plane as the rupture propagated upward. Second, dip slip is larger than strike slip at all investigation sites, indicating that the dominant faulting of this earthquake is thrust. Third, the gradual increase of amount of horizontal slip northeastward is partly due to the continuous decrease of dip angles northeastward. Fourth, a little less calculated dip angles of suppositional fault plane than those on natural outcrops or demonstrated by trenching is partly due to the surface effect: as the rupture propagated upward and approach the surface, the frontal end of the oblique-thrust wall bends down and dip angle decreases.