Near‐field observations of an offshore M_w 6.0 earthquake from an integrated seafloor and subseafloor monitoring network at the Nankai Trough, southwest Japan

Abstract

An Mw 6.0 earthquake struck ~50 km offshore the Kii Peninsula of southwest Honshu, Japan on 1 April 2016. This earthquake occurred directly beneath a cabled offshore monitoring network at the Nankai Trough subduction zone and within 25–35 km of two borehole observatories installed as part of the International Ocean Discovery Program's NanTroSEIZE project. The earthquake's location close to the seafloor and subseafloor network offers a unique opportunity to evaluate dense seafloor geodetic and seismological data in the near field of a moderate‐sized offshore earthquake. We use the offshore seismic network to locate the main shock and aftershocks, seafloor pressure sensors, and borehole observatory data to determine the detailed distribution of seafloor and subseafloor deformation, and seafloor pressure observations to model the resulting tsunami. Contractional strain estimated from formation pore pressure records in the borehole observatories (equivalent to 0.37 to 0.15 μstrain) provides a key to narrowing the possible range of fault plane solutions. Together, these data show that the rupture occurred on a landward dipping thrust fault at 9–10 km below the seafloor, most likely on the plate interface. Pore pressure changes recorded in one of the observatories also provide evidence for significant afterslip for at least a few days following the main shock. The earthquake and its aftershocks are located within the coseismic slip region of the 1944 Tonankai earthquake (Mw ~8.0), and immediately downdip of swarms of very low frequency earthquakes in this region, illustrating the complex distribution of megathrust slip behavior at a dominantly locked seismogenic zone.

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

“…Aftershocks were distributed in a very limited area, about 10 km downdip of the mainshock, at depths of between 10 and 14 km, with source location errors of 1.9 km in both horizontal and depth directions. There is a seismic gap distinct from the mainshock hypocenter, which agrees with the results obtained by Wallace et al (2016). The aftershock activity continued for 2 weeks, and a burst of reactivation occurred on April 19 (Fig.…”

“…3). The mainshock depth was 9.2 ± 0.6 km below sea level, about 4 km shallower than that obtained by the routine analysis, and about 2 km shallower than that obtained by Wallace et al (2016). The differences can be attributed to the horizontal heterogeneities of the velocity structure considered in our analysis.…”

“…We used a velocity structure obtained from a wide-angle OBS survey along a line passing through the source region shown by Wallace et al (2016). We compared the hypocenter distribution with the MCS reflection profiles to confirm whether this earthquake occurred on the plate boundary.…”

“…Precise determination of the source location, especially for the depth, is crucial to clarify whether this earthquake occurred along the plate boundary. Wallace et al (2016) determined the hypocenter distribution using a 1D velocity structure, which showed that the sources were distributed very close to the plate interface. However, strong horizontal heterogeneities of seismic wave velocities due to plate subduction could affect the precision of determining the hypocenter locations.…”

“…Hence, large interplate earthquakes have not been known to occur until the middle of a megathrust earthquake cycle. Wallace et al (2016) consider that the 2016 off-Mie earthquake most likely occurred on the plate interface, based on integrated analyses of hypocenter distributions, seafloor crustal deformations, and tsunami modeling using seafloor geodetic and seismological data from DONET. After this earthquake, SSEs along the plate boundary, very-low-frequency earthquakes (VLFEs), and tectonic tremors have been observed updip of the hypocentral region ( Fig.…”