Seismic activity beneath the Nankai trough revealed by DONET ocean-bottom observations

Abstract

We conducted a detailed investigation of seismic activity from January 2011 to February 2013 along the Nankai trough off the Kii Peninsula, central Japan, by using data obtained from the DONET ocean-bottom observation network. The hypocenters are mostly within the subducting Philippine Sea (PHS) plate, although a few are along the plate boundary or in the sedimentary wedge below the Kumano forearc basin. The seismic activity can be separated into events above and below 20 km depth, which corresponds approximately to the Moho. The hypocenter distributions are distinctly different for these groups. The seismic activity in the oceanic crust can be further separated into three clusters. Most of the seismic activity recorded in our data represents aftershocks of the 2004 off the Kii Peninsula earthquakes (M JMA = 7.1, 7.4, and 6.5), which occurred in the PHS plate. The hypocenter distribution in the oceanic crust correlates well with the location of the Paleo-Zenisu ridge, which is formed by a chain of seamounts that is subducting beneath the forearc basin. The hypocenters in the uppermost mantle are aligned on a plane dipping to the southeast, consistent with the existence of a thrust fault cutting through the lithosphere of the oceanic plate. The focal mechanisms of the earthquakes show that the axis of compressive stress in the PHS plate is oriented N-S, almost perpendicular to the direction of plate convergence, indicating a complex tectonic regime in this region. These results suggest that intraplate shortening may be occurring in the subducting oceanic plate.

The 2016 Mw 5.9 earthquake off the southeastern coast of Mie Prefecture as an indicator of preparatory processes of the next Nankai Trough megathrust earthquake

“…1 and Additional file 1: Figure S1) has facilitated continuous monitoring of offshore seismic activities Kawaguchi et al 2015). These observations revealed that, in recent years, earthquake activities along the Nankai Trough have mostly occurred in the oceanic crust or upper mantle of the subducting PSP (Obana et al 2004Obana and Kodaira 2009;Mochizuki et al 2010;Akuhara et al 2013;Nakano et al 2013Nakano et al , 2014Nakano et al , 2015Akuhara and Mochizuki 2014). The most recent notable events during this time have been the 2004 M JMA 7.1, 7.4, and 6.5 earthquakes off the Kii Peninsula, which occurred in the subducting plate (e.g., Sakai et al 2005;Seno 2005).…”

Hypocenters in the Nankai Trough Determined by Using Data from Both Ocean‐Bottom and Land Seismic Networks and a 3D Velocity Structure Model: Implications for Seismotectonic Activity

“…Our initial model was the 1D velocity model used in our previous studies (Nakano et al, 2013(Nakano et al, , 2014, which represents the sedimentary layers and oceanic plate below DONET stations (Fig. 4a).…”

“…7a). We determined the focal mechanisms of earthquakes of local magnitude M L > 2 by the method of Nakano et al (2014), which is based on the method of , Imanishi, Cho, et al (2006), and . We fitted the ω 2 -model (Boatwright, 1978) to the amplitude spectrum of the direct P wave at each station and used the level extrapolated to 0 Hz for its amplitude.…”

“…In our previous studies, in which we used only DONET oceanbottom observations, we assumed a 1D model because that approximation is likely valid near the trough axis (Nakano et al, 2013(Nakano et al, , 2014. In a region where there are both land and ocean-bottom observation networks, and where an oceanic plate is subducting beneath continental lithosphere, there may be strong horizontal heterogeneities that invalidate the 1D approximation.…”

“…In previous studies of DONET data, we detected strong seismic activity below the trough axis, which we interpreted to be aftershocks of the 2004 Off the Kii Peninsula earthquakes (Nakano et al, 2013(Nakano et al, , 2014. For our hypocenter determinations in these studies, we used a 1D velocity structure for the oceanic plate and sedimentary wedge.…”