S34A-06
Repeating Earthquakes Confirm and Constrain Long-Term Acceleration of Aseismic Slip Preceding the M9 Tohoku-Oki Earthquake

Wednesday, 16 December 2015: 17:15
305 (Moscone South)
Andreas P Mavrommatis, Stanford University, Geophysics, Stanford, CA, United States, Paul Segall, Stanford University, Stanford, CA, United States, Naoki Uchida, Tohoku University, Sendai, Japan and Kaj M Johnson, Indiana University, Bloomington, IN, United States
Abstract:
Changes in the recurrence intervals of repeating earthquakes offshore northern Japan in the period 1996 to 2011 imply long-term acceleration of aseismic slip preceding the 2011 M9 Tohoku-oki earthquake, confirming a previous inference from completely independent GPS data (Mavrommatis et al., 2014, GRL). We test whether sequences of repeating earthquakes exhibit a statistically significant monotonic trend in recurrence interval by applying the nonparametric Mann-Kendall test. Offshore northern Tohoku, all sequences that pass the test exhibit decelerating recurrence, consistent with decaying afterslip following the 1994 M7.7 Sanriku earthquake. On the other hand, offshore south-central Tohoku, all sequences that pass the test exhibit accelerating recurrence, consistent with long-term accelerating creep prior to the 2011 Μ9 earthquake.

Using a physical model of repeating earthquake recurrence, we produce time histories of cumulative slip on the plate interface. After correcting for afterslip following several M~7 earthquakes in the period 2003-2011, we find that all but one sequence exhibit statistically significant slip accelerations. Offshore south-central Tohoku, the estimated slip acceleration is on average 2.9 mm/yr2, consistent with the range of 2.6–4.0 mm/yr2 estimated from independent GPS data (Mavrommatis et al., 2014).

From a joint inversion of GPS and seismicity data, we infer that a substantial portion of the plate interface experienced accelerating creep in the 15 years prior to the M9 Tohoku-oki earthquake. The large slip area of the Tohoku-oki earthquake appears to be partly bounded by accelerating creep, implying that most of the rupture area of the M9 earthquake was either locked or creeping at a constant rate during this time period. Accelerating creep would result in increasing stressing rate on locked parts of the interface, thereby promoting nucleation of moderate to large earthquakes.

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