Relocation of earthquakes beneath the East Sea of Korea: Uncertainty of hypocentral parameters caused by refracted waves

摘要

In addition to direct waves, refracted (head) waves such as Pn and Sn have been used to determine hypocentral locations, especially when the number of direct waves is inadequate to determine the hypocentral parameters of local earthquakes. However, refracted waves tend to increase errors and the non-uniqueness problems of hypocentral parameters compared with direct waves, if the true velocity structure is not known. Beneath the East Sea of Korea (Japan Sea), conditions for determining hypocentral parameters of earthquakes are unfavorable. To estimate these parameters, this study applied the computational algorithm VELHYPO, which uses the best-fitting velocity model obtained from the results of grid searching methods. The accuracy of hypocentral parameters obtained from direct and different combinations of refracted waves was examined using a synthetic data set. The accuracy tests indicated that hypocentral parameters determined by direct waves alone tend to be more accurate and reliable than those determined by direct and refracted waves combined. The accuracy of hypocentral parameters increases as the grade of the weighting factor for refracted waves decreases when compared with that for direct waves. We relocated 72 earthquakes that occurred beneath the East Sea by applying VELHYPO and giving a lower weighting factor grade to refracted waves than to direct waves. The relocated epicenter distribution shows a close relationship with the regional fault distribution. We compared the hypocentral parameters determined by this study and the Korea Institute of Geoscience and Mineral Resources.