Enhanced Ground Motions and Seismic Hazards due to Crustal Structures in Korea
김광희; 강수영; 유해수; 석봉출
Seismic waves, in general, attenuate with distances they travel, so does the potential seismic hazard. Although damages due to large earthquakes are limited by distances within a few tens of kilometers from the source region, some areas far from the epicenter may experience strong shakings and severe damages. A moderate size earthquake with ML 4.8 occurred in the central eastern Korean Peninsula in the evening of January 20, 2007. The earthquake was the largest inland earthquake ever recorded since the modern Seismic Network has been fully operational in the peninsula and provided unprecedented opportunity to observe strong ground shakings throughout the country. At the closest station from the epicenter, 6.5%g, 13.2%g, 15.6%g were registered on the vertical, east-west, and north-south components, respectively. While the observed PGAs during the earthquake follow the general trend of exponential decay with the distance, some strong motion stations experienced several times larger ground shakings than those expected from the empirically determined attenuation relation. The observed strong ground shakings may be caused by site responses which amplify the wave in the thick basin, directivity due to the rupture processes, dipping crustal structures, a simple crustal structure lacking mid-continental discontinuities, or the combination of above mentioned. After the examination of several potential factors, we propose Moho-reflected S wave, SmS, to be responsible for the enhanced amplitude observed in the strong motion stations at the distance range from 80 to 180 km. Empirical PGA predictions may have underestimated strong ground motions in this distance range. Thus, Moho-reflections should be taken into consideration in the prediction of the strong ground motion and the evaluation of seismic hazard mitigation efforts.