Rupture behavior of a moderate earthquake (M_w 5.9, April 2006) and its close relation with the 2003 Chengkung earthquake (M_w 6.8) at the southern termination of the plate boundary, southeast Taiwan

摘要

The Taiwanese orogen is the result of the active and vigorous collision between the Eurasian and the Philippine Sea plates. In this strong convergence context, a one month-long earthquake sequence composed of two mainshocks of magnitude 6 occurred in April 2006 in the Taitung area. The first main-shock (Tl) mobilized a structure just west to the plate boundary, while the second one (T2) is located on the other side. In order to retrieve the exact fault geometry of T2, we inverted waveforms from stations located at local and teleseismic distances. We performed a grid search on strike and dip (each strike-dip couple defining a geometry) considering the source as a point source then as an extended source. In the last case, a simple average over stations misfit failed to isolate a precise geometry. To overcome this problem, we used a more statistical approach on the distribution of misfits to define the best geometry. Comparing the best model to local structures, it appears the generative fault was the plate boundary that rotates from a strike pointing at N20°E north of the event to N0°E in T2 area with an identical eastward dip of 35°. For this model, the fault slip inversion provides a critical slip of 3.5 cm above which slip, rake and rupture time are constrained with uncertainties of 29%, 14° and 0.47 s respectively. The average slip along the rupture was 20 cm with a maximum of 46 ± 13 cm. The movement was inverse with a minor left-lateral component similar to the faulting behavior of the plate boundary. In addition, the slip pattern of T2 is contained within the southern portion of the deepest segment of the plate boundary and at the edge of the rupture area of the 2003 Chengkung earthquake (M_w 6.8), a large event also generated by the plate boundary but 2.5 years earlier. After 1 s of aseismic spreading, the rupture propagated seismically and circularly outward before being stopped by the fault bending.