5:45 PM - 6:00 PM
[S13-4-06] Rupture on the megasplay fault along the Nankai trough during the off-Mie earthquake (Mw=6.0) on 1 April 2016
On 1 April 2016, a moderate-sized off-Mie earthquake (Mw=6.0) occurred off the Kii Peninsula, southwest of Japan. The epicenter is located updip of hypocenter of the 1944 Tonankai earthquake (Mw=8.2). Wallace et al. (2016) obtained that this earthquake occurred along the plate boundary of the Nankai trough, but their hypocenter determination was based on a 1D velocity structure. Horizontal heterogeneity along the dip direction is not negligible in subduction zones.
We determined the hypocenters of the 2016 earthquake by using a 2D velocity structure considering the horizontal heterogeneity in the study area. We used P-wave arrival time at each DONET station deployed immediately above the source region (Kaneda et al., 2015; Kawaguchi et al., 2015). The mainshock was located at 9.7 km depth shallower than 11.4 km of Wallace et al. (2016). By comparing with a reflection profile obtained from a multichannel seismic survey (MCS), the mainshock is attributed to a slip along the megasplay fault rather than the plate boundary.
In the transition zone of the accretionary wedge between the megasplay fault and the plate boundary is characterized by a zone of low seismic-wave velocity consisting of fluid-rich sediments, which could not support strong shear stress to cause large earthquakes. Accordingly, it is difficult to cause large earthquakes along the megasplay fault or the plate boundary in the source region of the 2016 earthquake. Wallace et al. (2016) attributed this earthquake to a slip along an unstable patch in conditionally stable zone of the plate boundary, but its geological meaning has not been clarified yet.
In MCS profiles, we can recognize locally very weak reflections along the megasplay fault or the plate boundary around the mainshock source, which we consider as an unstable patch because of the low impedance contrast. We hypothesize that fragments of seamounts, of which the main body has been subducted to deeper part, form the strong patch in the sediments.
We determined the hypocenters of the 2016 earthquake by using a 2D velocity structure considering the horizontal heterogeneity in the study area. We used P-wave arrival time at each DONET station deployed immediately above the source region (Kaneda et al., 2015; Kawaguchi et al., 2015). The mainshock was located at 9.7 km depth shallower than 11.4 km of Wallace et al. (2016). By comparing with a reflection profile obtained from a multichannel seismic survey (MCS), the mainshock is attributed to a slip along the megasplay fault rather than the plate boundary.
In the transition zone of the accretionary wedge between the megasplay fault and the plate boundary is characterized by a zone of low seismic-wave velocity consisting of fluid-rich sediments, which could not support strong shear stress to cause large earthquakes. Accordingly, it is difficult to cause large earthquakes along the megasplay fault or the plate boundary in the source region of the 2016 earthquake. Wallace et al. (2016) attributed this earthquake to a slip along an unstable patch in conditionally stable zone of the plate boundary, but its geological meaning has not been clarified yet.
In MCS profiles, we can recognize locally very weak reflections along the megasplay fault or the plate boundary around the mainshock source, which we consider as an unstable patch because of the low impedance contrast. We hypothesize that fragments of seamounts, of which the main body has been subducted to deeper part, form the strong patch in the sediments.