Seismicity gaps and the shape of the seismic zone in the Banda Sea region from relocated hypocenters

Abstract

We relocate hypocenters for more than 800 earthquakes deeper than 50 km with mb ≳ 5.0, along the Banda arc, using several thousand handpicked direct, depth, and core‐reflected phases, in addition to phases reported by the International Seismological Centre. The seismicity distribution is found to be very nonuniform both along the arc and in depth. Gaps in the relocated hypocenters exist along depth in most places of the arc, with the upper edge of the gaps varying from 100 to 450 km depth and the lower edge varying from 350 to 670 km in different portions of the arc. The seismic zone between 129 and 131°E in the 100–200 km depth range is the widest along the arc both in strike and downdip. This region, near the highest arc curvature, has the highest seismic activity and is the only part of the arc with earthquakes continuously occurring from the surface down to below 600 km. The very deep earthquakes under Sulawesi are shown to be part of the west‐southwest dipping Seram slab. In the westernmost part of the Banda arc the slab is under downdip tension in the 50–250 km depth range, while the deepest portion of the slab in this region is under compression. From 128 to 131°E the slab between 100 and 200 km depth is under mainly horizontal compression. Our study supports the “two‐slab” model for the Banda arc. The depth of the Wadati‐Benioff zone below the volcanoes is ∼60–100 km for the five volcanoes between 128 and 130°E and ∼150 km for the 23 volcanoes between 118 and 124°E.