3:45 PM - 4:00 PM
[SCG62-07] The Apr 2013 earthquake swarm and dyke intrusion in the Okinawa trough
Keywords:earthquake swarm, normal fault, Okinawa trough, back-arc rifting, dyke intrusion, Ryukyu trench
A kinematic GPS analysis of theses data reviles that this deformation started at the time of the earthquake swarm and slowly continued over two days. During the period no step-like movements exceeding a threshold level occurred, suggesting the absence of sudden slips at the earthquake source area. We propose two source models to interpret these GPS data, 1) a normal fault of Mw 7.0 and 2) a magma intrusion with the thickness of 3 m. These models cannot be distinguished from the GPS data alone.
Prior to the earthquake swarm, the GPS velocity vector at Yonaguni is 6.5 cm/yr in the SSE direction but that increases at 8.4 cm/yr after the earthquake swarm and furthermore 9.5 cm/yr throughout 2014. The long-lasting and accelerating GPS displacements suggest a strong preference for the dyke intrusion model.
A question may arise whether such magma intrusion causes the rifting of the Okinawa trough and hence the southward migration of the Ryukyu arc. To solve the question, the data from the baseline of Iriomote and hateruma islands is critical. This baseline is aligned perpendicular to the general trend of the western Ryukyu trench. The baseline of 40 km between the two islands shows a constant extension of 1 mm/yr from 2001 to 2015. The long-term and steady extension of the baseline suggests that rifting of the Okinawa trough is caused by the retreat of the Ryukyu trench due to a rollback of the Philippine Sea plate in the western Ryukyu trench.