Seismological evidence for a seamount subduction in eastern Shikoku, Japan

Abstract:

Based on the harmonic decomposition analysis of the receiver functions (RFs), we detected the unusual depression geometry of the oceanic Moho beneath eastern Shikoku, southwest Japan. We assume this geometry represents the basement of the subducting seamount.

We calculated the RFs using the tele-seismograms observed at 153 stations in Shikoku and estimated the plunge azimuth of a clear velocity interface and its approximate depth beneath each station. In the central and western Shikoku, the interface was dipping to the NW. As this direction corresponds well to the slab model proposed by the previous researches, we concluded that this interface was the oceanic Moho. On the other hand, we detected southern-dipping discontinuities, becoming deeper towards the Nankai trough, at several stations in eastern Shikoku. The land-side Moho may deepen toward the trough at the edge where the slab contacts with the land-side Moho. In this case, the oceanic Moho exists as another clear discontinuity below the land-side Moho. According to our RFs, however, no distinct Ps phases were detected following the target phases. Sudden conversion depth change related to the change of the target from the oceanic Moho to the land-side Moho was not detected. Thus, we assumed that the southern-dipping discontinuity was not the land-side Moho. Another possibility of this geometry is the basement of the seamount. This anomaly exists the northern extension of Kinan Seamount Chain, fossil ridge of Shikoku Basin. On this line, the subducted seamount was already discovered at off Shikoku (Kodaira et al., 2002). Deep low-frequency earthquake (LFE) activity, which is related to the slab subduction, are disturbed at eastern Shikoku. The southern edge of LFEs in eastern Shikoku corresponds well to the station distribution where the southern-dipping interface was detected. This means the disturbed LFE activity may be controlled by the existence of the subducting seamount.

This study was supported by JSPS KAKENHI Grant Number JP16H06475 and JP16H06473.