Deep earthquakes beneath central Taiwan: Mantle shearing in an arc‐continent collision

Abstract

The island of Taiwan owes its existence to the collision of an island arc on the Philippine Sea Plate with the Eurasian continental shelf. Most of the earthquakes in the mantle beneath Taiwan clearly are related to one of these plates subducting beneath the other, but beneath the central part of the island there is a zone of subcrustal seismicity that is not obviously related to any subduction zone. We investigated the uncertainties in the locations of these anomalous earthquakes by rereading P and S wave arrival times and computing information density functions for several of these events. The seismic zone itself is then defined through a joint probability function incorporating the entire data set. Individual information density functions show that several events are likely to be located at depths exceeding 60 km, with two events exceeding 80 km depth. The joint probability function shows a narrow zone of seismicity, about 20 km wide along strike, plunging to the east. Combining these results with evidence from surface geology, focal mechanisms and subsurface tomography, we suggest that these events are caused by an eastward displacement of the Eurasian mantle north of 24°N which juxtaposes it against colder subducted crust to the south. Moreover, the heat produced by this type of shearing in the mantle must be insignificant for brittle failure to exist at these depths.