Upper boundaries of the Pacific and Philippine Sea plates near the triple junction off the Boso Peninsula deduced from ocean-bottom seismic observations

Abstract

We determined hypocenter distributions and focal mechanisms of earthquakes off the Boso Peninsula, Japan, based on seafloor observations using ocean-bottom seismometers (OBSs). From OBS data, we detected and located earthquakes that were closer to the trenches than had previously been reported. The determined focal depths of relocated hypocenters were systematically shallower than those in the Japan Meteorological Agency catalog determined from land-based data. The hypocenter distribution was separated into two groups: one associated with the Pacific plate (PAC) and the other with the North American and/or Philippine Sea plates (PHS). For the former group, both the focal depths and mechanisms of low-angle thrust-faulting earthquakes were consistent with the geometry of the PAC determined by previous studies. Among the latter group, we selected low-angle thrust-faulting earthquakes with a slip direction parallel to the direction of PHS subduction or with a dip direction parallel to the PHS to delineate the upper boundary of the PHS. The depth of the plate upper boundary off the Boso Peninsula was found to be approximately 6 km shallower than previously reported estimates.

Tomographic image of crust and upper mantle off the Boso Peninsula using data from an ocean-bottom seismograph array

“…We used data from temporary seismic observations performed using OBSs by the Japan Agency of Marine-Earth Science and Technology (JAMSTEC) and the Earthquake Research Institute (ERI) of the University of Tokyo. Observations using a total of 23 OBSs were performed by JAMSTEC for a year starting in March 2012 (Ito et al 2017). The ERI also conducted seismic observations from October 2011 to November 2012 across a broad area of the Japan Trench forearc region (Shinohara et al 2012).…”

“…2a), to relocate the initial hypocenters for use in a tomographic inversion. Because the focal depths determined by JMA are generally deeper than the hypocenters relocated with OBS data (Ito et al 2017), we conducted relocation to obtain more appropriate initial hypocenters for tomography. When we investigate seismic structure by using OBS data, it is necessary to take effect of slow sedimentary layers in an oceanic area into account.…”

“…Several OBS studies were performed in the aftershock area of the 2011 Tohoku-oki earthquake (Shinohara et al 2012;Ito et al 2017). Ito et al (2017) analyzed the location and focal mechanism of the earthquakes in the region, in order to determine the boundaries of the PHS and the NA.…”

“…Several OBS studies were performed in the aftershock area of the 2011 Tohoku-oki earthquake (Shinohara et al 2012;Ito et al 2017). Ito et al (2017) analyzed the location and focal mechanism of the earthquakes in the region, in order to determine the boundaries of the PHS and the NA. They found that the depth of the plate upper boundary of the PHS off the Boso Peninsula is approximately 6 km shallower than that previously estimated by Uchida et al (2010) at approximately 141.0°E.…”

Two long-term slow slip events around Tokyo Bay found by GNSS observation during 1996–2011

“…The directions of subduction of the PAC and the PHS relative to the continental plate are ~N70°W and ~N20°W, respectively (e.g., Nishimura et al 2007). The seismotectonics and the structure in the Kanto region is studied in detail in Nakajima et al (2009), Uchida et al (2010) and Ito et al (2017), for example. A short-term SSE with a duration of ~1 day occurred in 1989 in the deeper portion of the asperity associated with the 1923 Kanto Earthquake (Hirose et al 2000) (Fig.…”