Geophysics and Geophysical Exploration ISSN:1229-1064(Print) 2384-051X(Online) 지구물리와 물리탐사

Two moderate earthquakes with local magnitude 4.9 and 5.0 at the Yellow Sea area occurred successively around Hong island on March 22, 2003 and Baengnyeong island on March 30, 2003, respectively, close to the Korean Peninsula. Focal mechanisms by the waveform inversion analysis are strike slip faulting with a thrust component for the March 22 event, and normal faulting for the March 30 event. The direction of P-axes of two events were ENE-WSW which were similar to previous studies on that of P-axes in and around the Korean Peninsula. Moment magnitudes determined by the waveform inversion analysis were 4.7 and 4.5, respectively, whereas those determined by spectral analysis were 4.8 and 4.6, respectively. As regards the March 22 event, regional stress by combined tectonic forces from compressions of plates colliding to the Eurasian plate, rather than mere local stress, was indicated. However, it was estimated that the March 30 event took place when the weak zone generated from the existing collision zone was reactivated when subjected to local stress in the tensile direction. This seismological observation indirectly supports the idea that the collision zone may extend to the Korean Peninsula.