Source characteristics of the 2006 Pingtung earthquake doublet off southern Taiwan and the possible contribution of submarine landslides to the Tsunami

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On 26th December 2006, two successive large earthquakes of Mw ∼7.0, occurred offshore southwest Taiwan, being 8 min apart, known as the 2006 Pingtung earthquake doublet. The earthquakes triggered a hazard chain consisting of regional-scale tsunamis, submarine landslides, and turbidity currents. The tsunami waves with moderate height (10–60 cm) were recorded by 12 tide gauges along southern Taiwan, Guangdong, and Fujian provinces. Submarine landslide-induced turbidity currents inflicted widespread damage to telecommunication cables across the Kaoping Canyon and Manila Trench. Although the mechanisms of the individual hazard in the chain have been investigated previously, the interaction mechanisms and the causal relationship among them remain unclear due to the complexity of the dynamic process. Here, with the best available instrumental records of these events, we first reexamined the earthquake doublet's source characteristics and obtained the earthquake source models using regional seismic waves, high-rate GPS displacements, and strong motion waveforms. We then conducted forward tsunami modeling, along with spectral analysis of the tide gauge records and backward tsunami ray tracing technique, to gain insight of the hydrodynamic features of tsunamis and the contribution of landslides to tsunami generation. The main findings can be summarized as follows. The centroid depth of the first earthquake was found to be ∼28 km, which is shallower than the local CWB catalog suggested. The slip distribution was characterized by two close slip patches, ranging over depths of 15–35 km and occupying 40 km along the strike. The shallower focal depth and normal-faulting mechanism can well explain the recorded tsunami waves. Notably, wave analysis suggests that tsunami waves induced by landslides may have occurred in two episodes. The first round was likely produced by the landslides triggered simultaneously by the initial earthquake doublet. The largest aftershock, which occurred 14 h after the doublet with a strike-slip mechanism, may have caused the landslides near the head of Kaoping Canyon and possibly the second-round tsunami.