A post-Tohoku earthquake review of earthquake probabilities in the Southern Kanto District, Japan

Paul G Somerville

doi:10.1186/2196-4092-1-10

Abstract

The 2011 Mw 9.0 Tohoku earthquake generated an aftershock sequence that affected a large part of northern Honshu, and has given rise to widely divergent forecasts of changes in earthquake occurrence probabilities in northern Honshu. The objective of this review is to assess these forecasts as they relate to potential changes in the occurrence probabilities of damaging earthquakes in the Kanto Region. It is generally agreed that the 2011 Mw 9.0 Tohoku earthquake increased the stress on faults in the southern Kanto district. Toda and Stein (Geophys Res Lett 686, 40: doi:10.1002, 2013) further conclude that the probability of earthquakes in the Kanto Corridor has increased by a factor of 2.5 for the time period 11 March 2013 to 10 March 2018 in the Kanto Corridor. Estimates of earthquake probabilities in a wider region of the Southern Kanto District by Nanjo et al. (Geophys J Int, doi:10.1093, 2013) indicate that any increase in the probability of earthquakes is insignificant in this larger region. Uchida et al. (Earth Planet Sci Lett 374: 81–91, 2013) conclude that the Philippine Sea plate the extends well north of the northern margin of Tokyo Bay, inconsistent with the Kanto Fragment hypothesis of Toda et al. (Nat Geosci, 1:1–6,2008), which attributes deep earthquakes in this region, which they term the Kanto Corridor, to a broken fragment of the Pacific plate. The results of Uchida and Matsuzawa (J Geophys Res 115:B07309, 2013)support the conclusion that fault creep in southern Kanto may be slowly relaxing the stress increase caused by the Tohoku earthquake without causing more large earthquakes. Stress transfer calculations indicate a large stress transfer to the Off Boso Segment as a result of the 2011 Tohoku earthquake. However, Ozawa et al. (J Geophys Res 117:B07404, 2012) used onshore GPS measurements to infer large post-Tohoku creep on the plate interface in the Off-Boso region, and Uchida and Matsuzawa (ibid.) measured similar large creep off the Boso Peninsula. Thus some of the large stress transfer may be undergoing aseismic release, consistent with pre-Tohoku geodetic data, so a large earthquake on the Off Boso segment may have a low probability.

Highlights

The 2011 Mw 9.0 Tohoku earthquake generated an earthquake sequence that affected a large part of northern Honshu, including plate boundary regions of the Okhotsk, Pacific, Philippine Sea and Eurasian plates (e.g. [1])
Fault behaviour: stick–slip and creep Before investigating the impact of the 2011 Tohoku earthquake on the seismic potential of the Tokyo region, we review some aspects of fault behavior
This implies that slip beneath Kanto could be fully uncoupled, in which case the accelerated post-Tohoku slip on Kanto faults could be reducing, not increasing, the stress imparted by the Tohoku earthquake. This is consistent with the interpretation of geodetic data by Nishimura et al [4], who found that with the exception of the site of the 1938 Mw 7.3, 7.4, 7.5 earthquake swarm, which since ruptured in the 2011 Tohoku earthquake, the Pacific –Okhotsk subduction zone east of Tokyo including the Off Boso segment has low seismic potential, Global positioning system (GPS) data in the far offshore region are needed to confirm this conclusion

Summary

Introduction

The 2011 Mw 9.0 Tohoku earthquake generated an earthquake sequence that affected a large part of northern Honshu, including plate boundary regions of the Okhotsk, Pacific, Philippine Sea and Eurasian plates (e.g. [1]). It is generally agreed that the 2011 Mw 9.0 Tohoku earthquake has increased the stress on faults in the southern Kanto district through the mechanism of stress transfer. There currently exist widely divergent forecasts of changes in earthquake occurrence probabilities in the southern Kanto district. There are widely divergent models of the configuration of subducted plates beneath the southern Kanto District. Intense interest is focused on the evolution of the earthquake sequence that began with this earthquake. It generated seismic activity over a vast area of Japan (Figure 2), which is a manifestation of the widespread changes in the stress field that it generated.

Objectives

Findings

Conclusion