Evidence of an east-dipping slab beneath the southern end of the Philippine Trench (1°N–6°N) as revealed by ISC-EHB

Abstract

We examine recently released global seismic datasets in 3D visualization to study slab configurations in the Mindanao-Molucca region, where the present-day arc-arc collision of divergent double subduction propagates northward and attains completion on Mindanao Island, Philippines. The activity of inter-plate thrust earthquakes in the Philippine Trench is not significant until Mindanao (6°N) is reached, while shallow earthquakes on the island exhibit predominantly strike-slip movements on the Cotabato and Philippine Faults. The spatial distributions of GCMT thrust-type earthquakes shallower than 60 km reveal that current collisions mostly occur along the Central and Talaud-Miangas Ridges. ISC-EHB events deeper than 80 km, as well as Slab2 contours, show the eastward-dipping trench of the Halmahera slab flipping to the westward-dipping Philippine Trench northward. We identify a zone of ISC-EHB earthquakes steeply dipping to the east beneath the southern end of the Philippine Trench (1°N–6°N) that is not modeled by Slab2. This feature of a steeply east-dipping slab is explained by collision of the subduction arc and is consistent with steeply plunging T-axes of earthquakes. The identification of the east-dipping slab and estimation of its extent are crucial for understanding geodynamic and plate-boundary evolution in the Mindanao-Molucca region.