Spatial and temporal distribution of low-frequency earthquakes in Japan.

Abstract

Seismic waves from some shallow earthquakes in and near Japan are characterized by appreciable lack of high-frequency components. The size of felt areas for these earthquakes (low-frequency earthquakes) is relatively small for their magnitudes determined from amplitudes of medium to long period seismic waves. About 800 shallow earthquakes of M≥6.0 which occurred during the last 76 years have been classified into three groups, N (normal), L (low-frequency), and VL (very low-frequency) events by the use of the relation between the size of felt area and the magnitude. The proportion of L and VL events to N events varies spatially in a systematic manner. The pattern of epicenter distribution of N, L and VL events changes also with time. In the active seismic zone along the plate boundary, this pattern seems to be related to the occurrence of major earthquakes of magnitude about 71/2 or more. Few earthquakes of L and VL classes with M≥6.0 occur for several to more than 10 years just prior to the occurrence of major interplate earthquakes within or close to their future focal regions. The cause of low-frequency earthquakes is primarily attributed to the weak radiation of high-frequency waves from the source due to slow faulting or some unknown mechanisms. The attenuation of high-frequency waves during the transmission through a low-Q zone is also responsible for the low-frequency nature of crustal earthquakes on the marginal sea side.