Co-seismic slip history and early afterslip of the 2004 Parkfield earthquake

We studied the slip history of 2004 (Mw 5.9) Parkfield earthquake using waveforms of three strong motion and 13 1-Hz GPS stations. GPS vectors averaged over 1 day after the mainshock relative to 1 day before were investigated as well. We tested two possible fault orientations. The first one dips 83° to the northeast, constructed based upon the moment tensor solution and surface trace of the San Andreas fault. Another is a vertical fault plane based upon a recent double-difference relocation study of Waldhauser et al. Considering about 20 % velocity contrast across the San Andreas fault, we used two velocity structures to calculate the earth response. The slip amplitude, rake angle, rupture initiation time, rupture duration of each subfault are inverted simultaneously. Our preliminary results showed that the vertical plane fits the data better. The total seismic moment of this event is 9x{10}²⁴ dyne.cm with a peak slip of about 40 cm. This earthquake initiated at a depth of 8 km and rupture northwest for about 22 km with a rupture velocity of 3.0 km/sec. Most slip concentrates in a depth range from 6 to 11 km, roughly filling the gap of two streaks of seismicity from 1969-2002. This preliminary slip model will be further improved by including more strong motion data. The after-slip during the first day of earthquake will also be studied.