The 2006 Pingtung intraslab earthquake doublet offshore southwestern Taiwan: Complex normal and strike-slip faulting in the northern Manila subduction zone where the continent-oceanic transitional slab subducts

While intraslab events in subducting oceanic slabs have been widely studied, intraslab earthquakes in slabs of continent-oceanic transition zones, where lithospheric rheology differs, remain little understood. Here, we investigate the 2006 Pingtung (southwestern Taiwan) offshore intraslab earthquake doublet (Mw 7.0 and Mw 6.9), striking around the northern Manila subduction zone, where the highly thinned continental crust subducts. The two main shocks were at the depth of ~40 – 60 km, below the local MOHO, and ~8 minutes apart. The several source models that have been proposed vary, and do not consider all available observations. In this study, we incorporate comprehensive datasets, including teleseismic body waves, regional broadband, near-field strong motion waveforms, and high-rate GNSS, to propose a new source model, and further discuss source characteristics in the regional tectonic context. We first determine a reliable near-field velocity model and the frequency ranges for waveform inversions by path calibration based on inverting a nearby Mw 5.6 aftershock. We then constrain the multiple point sources (MPS) solutions for both events. The location and fault planes from MPS are used to resolve slip distributions by finite fault inversions. We finally validate this slip model by the static coseismic displacement observed by the dense, near-field campaign GNSS and precise leveling. Our results show that the Mw 7.0 normal event ruptured a west-dipping fault at the depth of ~40 km, characterized by at least two major asperities. This rupture was followed by the deeper Mw 6.9 strike-slip event, located ~40 km to the north. The earthquake sequence was located around a failed rift indicated by seismic tomography and likely represented a reactivation of the faults formed in the mantle lithosphere during the continental rifting in the northern South China Sea margin. The doublet’s complex mechanisms could be explained by stress fields imposed by the subducting transitional crust.