Abstract:

It is very important 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 have been made, the coseismic slip vector and net slip, however, have not been reported because most strike slips measured in the field are only apparent ones, some of which  are due to horizontal shortening (or extension) perpendicular to the surface break. The net slips estimated based only on these apparent strike offsets would include an appreciable amount of error. However, it is the apparent strike offset that makes us possible to estimate the horizontal slip vector. Based on apparent strike offset data measured on 7 sites from the central rupture zone, we calculated the horizontal slip vectors, and even strike slips, dip slips, faultnormal horizontal shortenings, fault dips, and netslip vectors combined with the measured vertical displacements. Synthesizing the net slip vector measured directly from the exposure fault, and comparing the results with those deduced from the moment tensor solutions, the following main conclusions can be drawn. First, on the southwestern section of the rupture, the slip azimuth at surface is larger than that in the depth, showing an anticlockwise rotation of slip vector on the rupture plane as the rupture propagated upward. Second, the slip azimuth increases northeastward, showing a clockwise rotation of slip vector on the surface and the decrease of strike slip component as the rupture propagated northeastward but with increase of dip slip components. Third, dip slip is larger than strike slip at almost all investigation sites, showing that the inverse faulting is the main rupture type everywhere in the rupture zone. Fourth, the dip angle ranges between 104° and 647°, average 41°, consistent with that in natural outcrop or revealed by trenches. Fifth, the strikeslip component decreases and the dipslip component increases from the middle to the two ends of the rupture.

Key words: Wenchuan earthquake of 2008, coseismic slip vector, Longmenshan fault zone