Since variations of slip on plate boundaries depend on frictional properties, it is essential to know frictional parameters on the fault, as well as initial values of simulation variables for earthquake generation prediction. In this study, an adjoint data assimilation method is introduced to a simpli.ed fault model with a rate-and state-dependent friction law as a .rst step toward the goal of estimating the frictional parameters and initial values of simulation variables in a realistic situation. The method is applied to the simpli.ed model which mimics the 2003 Tokachi-oki afterslip. We make synthetic data set, slip velocities on the fault surface by assigning the “true” frictional parameters and initial values artificially. We investigate the feasibility of estimating frictional parameters and initial values through the adjoint data assimilation method on the assumption of knowing “background” values and observations. It is confirmed that the adjoint data assimilation method is computationally efficient to estimate control variables such as frictional parameters and initial values by time-trajectory fitting of observation data, compared to the grid search method. Also, we examine the sensitivity of each frictional parameter and initial value to the observed afterslip velocity data. In the range of our search using afterslip data, we .nd that 1) the initial value of velocity can be constrained, 2) the initial value of state variable cannot be constrained, 3) the frictional parameter value of a-b is well retrieved in the region where aftreslip velocity is observed, and 4) the value of characteristic length L can be retrieved only from the early portion of afterslip velocity data, where the velocity is rapidly changing.

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Along the Nanakai-Suruga trough, where the Philippine Sea plate is being subducted beneath southwest Japan, nine series of great Tokai and Nankai earthquakes have recurred every 100-200 years during historical times since 684 A.D., and brought about signi.cant tsunamis. Recently, geologists investigated tsunami deposits in the Ryujin-ike lagoon in Oita Prefecture and found 40 sand layers during the past 3,300 years including eight remarkable thick layers. They inferred that the uppermost three thick layers had been deposited by large tsunamis due to the “giant Nankai earthquakes” in 684, 1361, and 1707 A.D. which ruptured the Tokai and Nankai source regions simultaneously, and that “giant Nankai earthquakes” had recurred every about 450 years. In order to examine these inferences, we carried out numerical tsunami simulation at the Ryujin-ike lagoon assuming various static fault models based on existing models of the 1707 Ho’ei, 1854 Ansei-Nankai, and 1946 Showa-Nankai earthquakes. As the results, it has been revealed that the tsunami waveform and its maximum height near the Ryujin-ike lagoon depend strongly on the southwesternmost fault slip of the “giant Nankai earthquake” and insensitive to faulting in the Tokai region. Therefore, we conclude that the large tsunamis near the Ryujin-ike lagoon suggested by thick tsunami deposits cannot show the occurrence of “giant Nankai earthquakes” which ruptured the Tokai and Nankai sources simultaneously. Moreover, for a large tsunami near the Ryujin-ike lagoon, the location and the slip amount of the southwesternmost fault plane of the Nankai earthquake have a trade-off relationship between each other, which makes it impossible to infer correctly the causal fault of the large tsunami by means of the Ryujin-ike data alone. In addition, there is disagreement between the “giant Nankai earthquakes” inferred by the thick sand layers in the Ryujin-ike lagoon and those inferred by the study of historiographical seismology. In conclusion, the eight remarkable sand layers in the Ryujin-ike lagoon cannot be regarded as the traces of the “giant Nankai earthquakes.”

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Changes of the seismicity pattern were examined in the Tokai and the Tonankai seismogenic regions and discussed with respect to possible crustal movements. Temporal transitions of the seismicity pattern were followed by calculating correlation coefficients between two spatial distribu.tions of earthquakes for the period of interest. I sampled M 3.5 and greater earthquakes from the JMA catalog. The catalog contains a problem of non-uniform observations, however the method utilized here is insensitive to this. Following the changes represented in the correlation coefficients, I found that the recent seismicity pattern is approaching the reference, which is precisely the pattern preceding the 1944 Tonankai earthquake. The tendency is common to both the Tokai and Tonankai regions. This implies that a similar crustal movement reappeared after a long hiatus of about sixty years in both regions. One possible cause of common changes in the Tokai and Tonankai regions is the long-term slow slip beneath Lake Hamana that has progressed during the last decade. If the cause of the recent changes in the seismicity pattern is attributed to this slow slip, the same slow slip can be interpreted as having progressed beneath Lake Hamana before the Tonankai event, which created a characteristic seismicity pattern in the past.

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