In a small area off the eastern coast of Osumi Peninsula, eight events have been observed from January 2001 to March 2019, in which M3 to M4 earthquakes occurred successively in a short time, from several seconds to several minutes. Based on the waveform correlation and cluster analysis, we found three seismic patches (asperities) which generate small repeating earthquakes in this area. The arrival time differences between P and S waves of the three patches’ earthquakes observed at NARU station suggested that these patches were close to each other. Furthermore, we confirmed that many earthquakes originated from these patches were included in the successive occurrences of earthquakes in this area. These results suggest that the earthquake from one of these patches affects another patch in the immediate vicinity and triggers successive occurrences of earthquakes.
It is observed that seismic waves generate electromagnetic variations. The excitation mechanisms have been analyzed and formulated by many researchers. Three mechanisms have been mainly considered; stress effects for physical properties of rocks such as the piezoelectricity and the piezomagnetic effects, the electromagnetic induction due to the oscillation in the geomagnetic field, and the electrokinetic phenomena in pore fluids. While the stress effects and the electrokinetic phenomena take place in the crust, the induction process functions in various regions of the earth as the crust, the oceans, and the ionosphere. The characteristics of the electromagnetic signals produced with these mechanisms are also reviewed. It is expected that the electromagnetic signals derived from seismic waves are used to investigate the dynamics and the structures of the earth and planets.
Large amplitude microseisms were recorded during the passage of typhoon Hagibis in 2019 at the Hirono Thermal Power Station (HRN) located in the middle of eastern Fukushima Prefecture. The amplitude of microseisms during the passage of typhoon Hagibis was 10 to 20 times the amplitude of microseisms when there was no typhoon. The dominant period of microseisms was about 5 seconds just before the typhoon passage and about 6 seconds after the typhoon passage. The amplitude of microseisms at HRN is much larger (10 times more) than those at F-net Hirono (HRO) station on bedrock, but the predominant period is almost the same except for the vertical component after the typhoon passage. The same predominant period cannot be considered from each underground structure model. The predominant period is determined by the source of microseisms. Coherence analysis using three components of HRO station’s record suggests that the Rayleigh wave is the main component of observed microseisms. On the other hand, the same tendency was not derived from the coherence of HRN, possibly due to the superposition of other wave components. The temporal change of amplitude of microseisms shows a different pattern from those of the significant wave height recorded off the coast of Fukushima, and the predominant period of microseisms did not correspond to 1/2 of the significant wave period. The source of microseisms after the passage of a typhoon needs to be considered not only in the sea area near the coast but also in the offshore sea area.