Initial Rupture Processes of the 2006 Pingtung Earthquake from near Source Strong-Motion Records

The magnitude 7.1 Pingtung offshore earthquake occurred on 26 December 2006 at a depth of 40 - 60 km within the subduction zone of the Eurasian plate. This event exhibits an emergent onset. The first large amplitude arrivals were delayed by several seconds with respect to the origin time and were preceded by small-scale slip. This event is one of a few large intermediate depth earthquakes well recorded in a near source region. Based on beamforming analysis, some near source recordings were used in this study to investigate the initial rupture process of this event. Analyzed results indicate that initial fault rupture of this event at the hypocenter was of short duration with initial resultant fault slip extending southward toward the shallower portion of the fault plane. Thereafter, a detected rupture front propagated both northward and downward. Following small-scale initial slip, a major rupture arose. A detailed rupture process has been reconstructed using backward projection of array seismograms from the fault plane to image fault slip. The spatial and temporal evolution of initial onset presents the same rupture behavior determined by beamforming analysis. Our analysis appears to indicate the distinct possibility of an initial seismic nucleation phase; however, uncertainties remain and a cascade process cannot be ruled out completely.