沈み込む海山と1995年奄美大島近海地震のテクトニクス的解釈

抄録

The 1995 Amami-Oshima-Kinkai Earthquake occurred near the Nansei-Shoto Trench where the upheaval area of the Philippines Sea plate subducts beneath the Nansei-Shoto islands. The main shock was M_JMA 6.6 and its largest aftershock was M_JMA 6.5. The aftershock distribution for these two events by Yamada et al. (1996) corresponds to two distinct and nearly vertical fault zones. The focal mechanisms obtained by Kikuchi (1996) are consistent to the aftershock distribution.
The authors propose that the seamount found beneath the trench-continental-slope indirectly triggered this earthquake activity. If a subducting oceanic plate is normal oceanic denser than an overriding island arc, the oceanic plate should be faulted near vertically priori to the plate subduction by horizontally tensional force due to plat bending. On the other hand, an oceanic plate with seamounts or an oceanic plateau lighter than a normal oceanic plate, might resist to plate subduction due to its small density and delaying normal faultings might occur in the subducting oceanic plate. The delaying normal faultings between a subducting seamount and a preceding normal portion of the oceanic plate can compensate the subduction process. The compressional convergence margin such as the Nankai Trough, however, may not generate such normal faultings due to the nature of stress field.
The low seismicity area existing across the trench axis is also seen both in this aftershock activity and ISC hypocenters. This is the same result as those in other regions. This might imply low earthquake potential for this portion of plate interface due to the existence of low density sediments and water contained in the sediments.