南海トラフ沿いのジシン活動

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Occurrences of large earthquakes with estimated magnitudes as large as 8 along the Nankai Trough have been well documented in historical materials, and the oldest documented earthquake dates back to A.D. 684. The recurrence periods are fairly constant, and are approximately 200 years for the earthquakes occurring before 1361, and about 100 years for those after 1361. However, the number of smaller earthquakes observed by the on-land seismic stations is very small. Both seismic and tectonic couplings along the interface between the overriding and subducting plates are estimated to be nearly 100% from seismic and geodetic data. Therefore, it has been considered that relatively simple physics govern the generation of the historical large earthquakes along the Nankai Trough. With the availability of abundant information on large earthquakes and modern data sets from dense seismic and geodetic networks, the Nankai Trough is one of the best-studied seismogenic zones. Although improvements have been seen in estimating the fault-plane parameters for the historical earthquakes, there still remain important unanswered questions, such as if there have been unidentified earthquakes that fill in the 200-year recurrence period. Estimates of precise fault-plane parameters are being demanded to have a better understanding of earthquake generation. One of the most important topics is where the updip limit of the seismogenic zone is located. The location had not been resolved by on-land seismic observations. Seismic observations using ocean bottom seismometers have recently been conducted in both western and eastern regions along the Nankai Trough. The observations in the western region confirm that the updip limit coincides well with both the updip limit of the estimated fault plane for the 1946 Nankaido earthquake and the estimated 150℃ isotherm along the interplate interface. Earthquake activitiy has been proved to be very low in both regions.

南海トラフ沿いのジシン活動

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