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    Recurrence intervals for M > 7 Miyagi-ken-Oki earthquakes during an M ~ 9 earthquake cycle

    Ryoko Nakata, Naoki Uchida, Takane Hori, Ryota Hino

    Earthquake cycle, Recurrence interval, Slip deficit, Tohoku-Oki earthquake, Miyagi-ken-Oki earthquake, Repeating earthquake, Aseismic slip, Rate-and state-dependent friction law

    (Left) Temporal distribution of simulated slip deficits over a 3300-year period at a point within the M ~ 9 source area (black) and Miyagi-ken-Oki (MYG) source area (magenta). Cycles 1–4 (light gray lines) represent a period from one M ~ 9 earthquake to the next M ~ 9 earthquake. The slip deficit in the M ~ 9 area was set to zero when the first M ~ 9 earthquake of Cycle 3 occurred (T = 1919.3 years), and the vertical axis is on the left. The slip deficit at the MYG area was set to zero when the MYG earthquake occurred immediately after the first M ~ 9 earthquake of Cycle 3 (T = 1968.7 years), and the vertical axis is on the right. (Right) Time development of simulated slip velocity during Cycle 3. Horizontal axis: depth (km) through the approximate center of the Miyagi-ken-Oki (MYG) patch. Vertical axis: 0 years at the time of the first M ~ 9 earthquake during Cycle 3. Red horizontal lines: timing of M > 7 or M ~ 9 earthquakes. Black dashed line: center of the MYG patch; solid vertical lines: a radius of 23 km from the center of MYG patch.

    The 2011 Tohoku-Oki great earthquake increased the difficulty of evaluating the long-term probability of seismic activity along the Japan Trench because of the unknown impact of the unprecedentedly large slip. In this study, the long-term activity of “Miyagi-ken-Oki earthquakes”, an M > 7 earthquake sequence off Miyagi Prefecture, located at the edge of the source area of the Tohoku-Oki earthquake was simulated. We conducted numerical simulations of earthquake generation cycles based on the rate- and state-dependent friction law representing the stress accumulation and release process on the plate interface. We also validated the results based on analyses of observational data, including time intervals and afterslip distributions for the previous Miyagi-ken-Oki earthquakes. The simulation results were then compared with repeating-earthquake-derived interplate slip observations over 30¬†years. The results showed that the spatial and temporal characteristics of aseismic slips on the plate interface near the M > 7 Miyagi-ken-Oki source were qualitatively reproduced. The time interval between the M ~ 9 earthquake and the first M > 7 earthquake is shorter than the average recurrence interval of M > 7 earthquakes during the latter stage of each M ~ 9 earthquake cycle. In contrast, the interval between the first and the second M > 7 earthquakes is the longest in each M ~ 9 earthquake cycle. The time intervals of the M > 7 earthquakes fluctuated in the early stage compared to those in the latter stage of the M ~ 9 earthquake cycle. These characteristics were associated with the extent of the locked and afterslip areas in and around the source. Hence, monitoring the spatio-temporal distribution of the slip rate in and around the source area during the preparation process of earthquakes occurring in the downdip area off Miyagi Prefecture is very important to assess whether the next M > 7 earthquake is approaching. Furthermore, earthquake cycle simulations combined with earthquake and slow slip monitoring can provide meaningful insights for long-term assessments of great interplate earthquakes.