THE OFFSHORE ULJIN, KOREA, EARTHQUAKE SEQUENCE OF APRIL 2006: SEISMOGENESIS IN THE WESTERN MARGIN OF THE ULLEUNG BASIN

Abstract

An unusual earthquake sequence comprising 11 events with magnitude range ofM L 2.0 to 3.2 occurred off the eastern coast of the southern Korean Peninsula in April 2006. Since there is no obvious mainshock in this sequence unlike a typical mainshock-aftershock sequence, the seismicity pattern shows the characteristics of swarm behavior. Focal mechanism of the largest event (M L 3.2) on the 29 of April is normal to strike-slip faulting. Hypocenter relocations of nine events improve the epicenter locations that fall within an area with a radius of about 0.7 km while depths are less well constrained with ranges of 1.6 km to 13.0 km. We propose that a swarm behavior of the sequence is closely related to the marginal geometry of the Ulleung Basin and the regional stress regime. The fact that epicenters of the April 2006 sequence are at the same transitional zone of continental to rifted continental crust as that of the 29 May 2004,M w 5.1, earthquake indicates that the Ulleung Fault is seismically active.