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[J05-2-05] Characteristics of spatiotemporal variation of hypocenters and the diversity of waveforms of deep low-frequency earthquakes in northeastern Japan
We have investigated the characteristics of spatiotemporal variation of hypocenters of deep low-frequency (DLF) earthquakes in northeastern Japan using the earthquake catalog of the Japan Meteorological Agency, and relocated hypocenters by the hypoDD method. DLF earthquakes are believed to occur by another mechanism than the fault motion because they occur below the depth of brittle/ductile transition. However, the mechanism is still unknown. Thus the comparative study of spatiotemporal variation of hypocenters and waveform characteristics among many source areas is important to clarify the source process of DLF earthquakes.
We found that the thickness of the seismogenic layer of DLF earthquake varies in space at distances of 100–250 km. We also found that the areas with nearly constant seismicity have a spacing of 100–150 km. These values are several times larger than the spacing of hot fingers that correlate well with areas of low-velocity anomalies in the Tohoku district. No clear temporal change of seismicity of DLF earthquakes was found after the 2011 Tohoku-Oki earthquake, which suggests a small change in the stress at the source depths of DLF earthquakes.
We next examined the diversity of waveforms of DLF earthquakes by grouping the events using waveform cross-correlation. In a time window just after S-wave, closely located earthquakes tend to form a single group, which suggest the similarity of focal mechanisms among the grouped events. However, the values of cross-correlation are much lower than those of ordinary high-frequency events. This suggests that the focal mechanisms and/or source process of DLF earthquakes are different from event to event. In time windows of later phases, the correlation take lower values and the grouped events were not colocated. Thus the later phases have less information of source mechanism than S-wave, which should be considered for the source study by waveform modeling.
We found that the thickness of the seismogenic layer of DLF earthquake varies in space at distances of 100–250 km. We also found that the areas with nearly constant seismicity have a spacing of 100–150 km. These values are several times larger than the spacing of hot fingers that correlate well with areas of low-velocity anomalies in the Tohoku district. No clear temporal change of seismicity of DLF earthquakes was found after the 2011 Tohoku-Oki earthquake, which suggests a small change in the stress at the source depths of DLF earthquakes.
We next examined the diversity of waveforms of DLF earthquakes by grouping the events using waveform cross-correlation. In a time window just after S-wave, closely located earthquakes tend to form a single group, which suggest the similarity of focal mechanisms among the grouped events. However, the values of cross-correlation are much lower than those of ordinary high-frequency events. This suggests that the focal mechanisms and/or source process of DLF earthquakes are different from event to event. In time windows of later phases, the correlation take lower values and the grouped events were not colocated. Thus the later phases have less information of source mechanism than S-wave, which should be considered for the source study by waveform modeling.