Crustal structure of the southernmost Ryukyu subduction zone: OBS, MCS and gravity modelling

In 1995, a combined ocean-bottom-seismometric (OBS) and multichannel seismic (MCS) survey with strong air-gun shots was carried out in the southernmost Ryukyu subduction zone. A crustal velocity structure constructed from the layer-stripping Monte Carlo inversion of three OBS/MCS profiles and the associated density models inverted from gravity data in the SW end of the Ryukyu arc-trench system are presented. Parallel to the arc in the southernmost Ryukyu subduction system, the OBS/MCS profiles show sedimentary layers of the Hoping, Nanao and East-Nanao forearc basins from west to east, warping of the arc basement and buckling of the subducted slab beneath the Hoping basement rise. The arc-parallel variation of the crustal structure may result from increasing lateral compression westward due to oblique subduction of the Philippine Sea plate and collision with the Luzon arc near the northwestern edge of the forearc region. Northward subduction and arc-parallel compression of the slab also have generated thrust faulting along the subduction interface and strike-slip faulting within the subducted slab, respectively. On 2002 March 31, an earthquake with a moment magnitude of 6.84 was induced by buckling of the subducted slab and strongly affected cities within an epicentral distance of 100 kin. The velocity-interface models, the density models and the focal mechanisms presented in this paper therefore suggest that earthquakes induced by slab buckling or arc-parallel compression have been stronger but less frequent than those generated by northward subduction in the Ryukyu seismogenic zone off Taiwan.