東海地震の想定震源域中に発生した中規模地震の前に現れた地震活動の静穏化とCHASEによるその検出 Quiescence before Moderate Earthquakes Occurred within the Focal Region of the Anticipated Tokai Earthquake and Its Detection by Using the CHASE Method
We quantitatively evaluated seismic quiescence before moderate earthquakes which occurred within the focal region of the anticipated Tokai earthquake by using the CHASE method. It is shown that the CHASE value decreased exceeding the standard deviation three times in the analyzed period, and for all of the three cases a moderate earthquake occurred afterwards. This result indicates the effectiveness of the CHASE method for the detection of seismic quiescence and the usefulness of the phenomenon as a precursor for the intermediateterm earthquake prediction. A noticeable feature of the seismic quiescence is that the area is much larger than the focal regions of the main shocks. The feature is favorable for the detection of a precursory seismic quiescence, however, it makes difficult to pinpoint the focus of the earthquake which is relevant to the quiescence. Further, the pre-stage activity of small earthquakes in the vicinity of the foci of the three moderate earthquakes were very different. These observational facts seem to suggest that the immediate prediction of earthquake occurrence on the basis of seismicity patterns is difficult.