Preliminary Results of Modeling of Strong Ground Motion due to the 2004 Parkfield Earthquake

An earthquake of M 6.0 struck the central coast of California at 10:15:24 a.m. Pacific Standard Time (17:15:24 UTC) on 28 September 2004. The epicenter was 11 km southeast of Parkfield, at a depth of approximately 8 km, with strike-slip mechanism. Analysis of the aftershocks and rupture models indicate that it ruptured along the same section of the fault as those of the similar magnitude Parkfield earthquake series, i.e. along the San Andreas fault in the NW-SE direction. Liu et al. (2006) performed a slip inversion, which suggested predominant NW rupture direction with two main asperities located NW and SE off the epicenter. The aim of the presented study is to investigate mainly recordings at stations located on a particular position "above" the fault, i.e. lying very close to the intersection of the Earth surface and the up-dip prolongation of the fault. Theoretical modeling using a 1D medium and assuming a perfectly planar fault shows that fault-parallel (FP) and vertical (UP) ground motions should be zero exactly on this intersection unlike the fault-normal (FN) ones, or at least very low for stations close to the this intersection. Note that it is a consequence of the properties of the S-wave radiation pattern. However, the observed seismograms show relatively strong signals with even the same maximum amplitudes at these "zero" (FP and UP) components as the FN one. We suggest two possible explanations for such a controversy: 1) The medium surrounding the fault, which is in reality 3D heterogeneous, can generate such a strong signal at the "zero" components. 2) The fault is not perfectly planar, which results in variability of the mechanism along the fault and hence allows to generate signal at the "zero" components. In this contribution we test and quantify these two working hypotheses. To this end, synthetic seismograms are computed by the Discrete Wavenumber and the ADER-DG methods using 1D and 3D velocity structures, respectively, and assuming a perfectly planar and non- planar rupture surfaces with kinematic properties retrieved by Liu et al. (2006). Moreover, we compare our numerical results with observed data.