Downdip tensional earthquakes beneath the Tonga Arc: A double seismic zone?

Abstract

Characteristics of seismicity at intermediate depth beneath the Tonga arc are studied. Special attention is paid to newly identified downdip tensional events because the dominance of downdip compression has been known in this area. We reexamine the relatively shallow (<70 km) downdip tensional events, reported in the Harvard centroid moment tensor solutions, by modeling bodywave waveforms of World‐Wide Standard Seismograph Network seismograms and conclude that they are in fact intraplate downdip tensional events. Furthermore, these downdip tensional events are consistently located below and seaward of downdip compressional events and appear to constitute a double seismic zone. The result of a relative relocation technique applied to these earthquakes also supports this observation, which was originally based on routinely determined epicenters. The subducting slab beneath the Tonga arc has been considered as a prototype of a compressionally loaded slab. The existence of a double seismic zone in Tonga, therefore, would suggest that double seismic zones, located at intermediate depth in subducting slabs, are more common than previously thought.