The slow earthquake spectrum in the Japan Trench illuminated by the S-net seafloor observatories

Abstract

Slow earthquake segmentation The Japan Trench is responsible for disastrous megathrust earthquakes like the 2011 Tohoku-Oki quake. Nishikawa et al. used new observations from the S-net ocean-bottom seismic network to map slow earthquakes—disturbances that do not cause ground shaking—along the Japan Trench (see the Perspective by Houston). They found that the area that ruptured during the 2011 quake was bounded by areas that have large numbers of slow earthquakes. A segmentation likely caused the 2011 rupture to cease, an observation that is important for assessing risk from future major earthquakes. Science, this issue p. 808; see also p. 750 The S-net seismic network helped identify the slow earthquake behavior of three regions of the hazardous Japan Trench. Investigating slow earthquake activity in subduction zones provides insight into the slip behavior of megathrusts, which can provide important clues about the rupture extent of future great earthquakes. Using the S-net ocean-bottom seismograph network along the Japan Trench, we mapped a detailed distribution of tectonic tremors, which coincided with very-low-frequency earthquakes and a slow slip event. Compiling these and other related observations, including repeating earthquakes and earthquake swarms, we found that the slow earthquake distribution is complementary to the Tohoku-Oki earthquake rupture. We used our observations to divide the megathrust in the Japan Trench into three along-strike segments characterized by different slip behaviors. We found that the rupture of the Tohoku-Oki earthquake, which nucleated in the central segment, was terminated by the two adjacent segments.