A possible restart of an interplate slow slip adjacent to the Tokai seismic gap in Japan

Shinzaburo Ozawa,Mikio Tobita,Hiroshi Yarai

doi:10.1186/s40623-016-0430-4

Shinzaburo Ozawa, Mikio Tobita + Show 1 more

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https://doi.org/10.1186/s40623-016-0430-4

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Abstract

The Tokai region of Japan is known to be a seismic gap area and is expected to be the source region of the anticipated Tokai earthquake with a moment magnitude of over 8. Interplate slow slip occurred from approximately 2001 and subsided in 2005 in the area adjacent to the source region of the expected Tokai earthquake. Eight years later, the Tokai region again revealed signs of a slow slip from early 2013. This is the first evidence based on a dense Global Positioning System network that Tokai long-term slow slips repeatedly occur. Two datasets with different detrending produce similar transient crustal deformation and aseismic slip models, supporting the occurrence of the Tokai slow slip. The center of the current Tokai slow slip is near Lake Hamana, south of the center of the previous Tokai slow slip. The estimated moments, which increase at a roughly constant rate, amount to that of an earthquake with a moment magnitude of 6.6. If the ongoing Tokai slow slip subsides soon, it will suggest that there are at least two different types of slow slip events in the Tokai long-term slow slip area: that is, a large slow slip with a moment magnitude of over 7 with undulating time evolution and a small one with a moment magnitude of around 6.6 with a roughly linear time evolution. Because the Tokai slow slip changes the stress state to one more favorable for the expected Tokai earthquake, intense monitoring is going on.

Highlights

The Tokai region in Japan is located near the Suruga trough, where the Philippine Sea plate subducts in the northwest direction beneath the Amurian plate at an annual rate of 2–3 cm/year (Sella et al 2002) (Fig. 1)
The Tokai region is deemed a seismic gap because it did not rupture at the time of the 1944 Tonankai earthquake [moment magnitude (Mw) = 8.1] (Ishibashi 1981; Mogi 1981)
There are southward displacements of approximately 1 cm in the north–south position time series from early 2013 in Fig. 2a–c compared with approximately 2 cm eastward movements, which cannot be explained by either afterslip or viscoelastic relaxation due to the 2011 Tohoku earthquake, suggesting aseismic slip on the plate interface in the Tokai region

Summary

Introduction

The Tokai region in Japan is located near the Suruga trough, where the Philippine Sea plate subducts in the northwest direction beneath the Amurian plate at an annual rate of 2–3 cm/year (Sella et al 2002) (Fig. 1). Because of the subduction of the Philippine Sea plate, large interplate earthquakes have repeatedly occurred in the Tokai region at time intervals of around 150 years (e.g., Kumagai 1996). The Tokai region is deemed a seismic gap because it did not rupture at the time of the 1944 Tonankai earthquake [moment magnitude (Mw) = 8.1] (Ishibashi 1981; Mogi 1981). In this tectonic setting, the dense Global Positioning System (GPS) network (GEONET) in Japan detected

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