Abstract
Abstract Two M=7 interplate earthquakes recently occurred off Miyagi prefecture, northeastern Japan. These belong to a new sequence of earthquake series. The occurrence of this sequence was forewarned by the government, based primarily on a statistical approach utilizing historical records. Such a process, however, does not account for the present on-going status in the assessment. During the stage when stress approaches a critical level, there is a possibility of stress redistribution caused by a quasi-static slip, which will be reflected in a temporal change in seismic activity. Delineating a spatial map of the seismic-activity rate changes in the seismogenic zone off Miyagi prefecture, we found that there was an activation of microseismicity in the rupture zones several months prior to both M=7 earthquakes. This was interpreted as evidence of a preparatory process preceding each M=7 earthquake, during which a quasi-static slip progressed, driving stress redistribution and resulting in stress concentration on asperities of these earthquakes.
Highlights
A major earthquake of M=7.2 occurred off Miyagi Prefecture in northeastern Japan on 16 August 2005
Recent developments in observations have made it possible to examine detailed features of microearthquake occurrences throughout the whole of Japan, and this paper focuses on whether or not a preparatory process appears in the microseismicity preceding a major event
Dieterich (1994) proposed a theoretical formalism based on the rate-and-state constitutive friction law (Ruina, 1983) to prove that changes in the seismicity rate are in proportion to the stress rate
Summary
There is a quasi-periodicity in the time sequence, with a recurrence interval of about 21 years for all of the sequences, and about 36 years for the larger earthquakes of M=7.2 and greater (marked with large solid circles). The earthquakes in the latter sequence are located almost at the same place, as shown in the figure. In the assessment of occurrence probability, none of the observed information is taken into account, with the exception of statistical evidence of the periodic occurrences This approach is based on a consideration that in a probabilistic process, an individual occurrence within a sequence is occasional rather than deterministic. What we notice here is characteristic activation that could be detected preceding the two recent M=7 earthquakes
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