Sustained High Seismicity Rate Beneath Tokyo Associated with after-Slip of the 2011 M=9.0 Tohoku-Oki Earthquake

Abstract:

The 2011 Tohoku-oki earthquake brought a broad and unprecedented increase in seismicity in northern Honshu, Japan, including a high occurrence rate of moderate-size earthquakes in and around Tokyo, where five M>7 earthquakes struck in the past 400 years. Immediately after the Tohoku-oki earthquake, the seismicity rate in the Kanto seismic corridor beneath Tokyo jumped ten-fold and then decayed for 6-12 months, after which it steadied at three-to-two times the pre-Tohoku rate (continuous at least until the end of July, 2014). The time series of the seismicity does not fit by a simple Omori-decay formula and not observed in a typical off-fault aftershock sequence. To well explain the rate increase and its unique time dependency, we employ rate and state dependent Coulomb model incorporating not only the coseismic stress step but also stressing rate change. In the model, coseismic static stress changes are calculated onto the nodal planes of 338 pre-mianshock focal mechanisms as proxies for potential nucleation sites. We found ~80% were brought closer to failure by the M=9.0 mainshock and M=7.9 aftershock with the mean increase of 1 bar. We then use the histogram of the stress changes with plausible rate/state parameters to reproduce the seismicity time series following the method of Toda et al. (2012). Because the Kanto seismicity did not decay back to the pre-Tohoku rate, we included a 3-times higher stressing rate after the Tohoku mainshock. The origin of the higher post-Tohoku-oki stressing rate is likely explained by postseismic creep. Both on land GPS data (Ozawa et al., 2012) and repeating earthquakes (Uchida and Matsuzawa, 2013) inferred a significant postseismic after-slip on up-dip extension of the upper Pacific slab. We speculate that gradual stress transfer on deeper plate interface from such a transient shallow postseismic creep is responsible for the unique long-lasting off-fault seismicity beneath Tokyo.