Double seismic zone and seismicity in the mantle wedge beneath the Ogasawara Islands identified by an ocean bottom seismometer observation

Abstract

Around the Ogasawara Islands, only a few seismic stations in the area can be used to determine the hypocenters of regional earthquakes; thus, hypocenter location precision tends to be low. To more precisely determine hypocenter locations, we deployed a temporary seismic observation network of pop-up ocean bottom seismometers around the Ogasawara Islands from July to October 2015. We identified a double seismic zone in the 70-200 km depth range associated with the subducting Pacific slab. The slab-normal distance between the two planes of the double seismic zone is about 30-35 km, similar to such distances observed along the Japan and Mariana trenches. Furthermore, we found unusual seismicity in the mantle wedge at 20-50 km depth beneath the Ogasawara trough that might be related to structure formed at the onset of the oceanic slab subduction. The hypocenters determined from the ocean bottom seismometer observation were horizontally separated by a few tens of kilometers from hypocenters published by the Seismological Bulletin of Japan. USGS locations (Preliminary Determination of Epicenters) seem to be offset westward about 30 km compared with the locations determined in this study.

What Controls Effective Elastic Thickness of the Lithosphere in the Pacific Ocean?

“…However, the anomalously and uniformly high e T is observed at subduction zones characterized by oceanic crustal ages ranging between 20 and 180 Ma (Figure 4a), indicating that effects other than temperature control e T variations. We further note that deep earthquakes are found in the mantle wedge at various subduction zones (e.g., Chang et al, 2019;Nakata et al, 2019), suggesting strength at upper mantle depth landward of the trench. We conclude that large…”

Toward Waveform‐Based Characterization of Slab & Mantle Wedge (SAM) Earthquakes

“…Generally, the controversy stems from the fact that mantle wedge seismicity is rare. Only a small, but growing set of targeted high‐resolution studies in the last decade have been able to distinguish mantle wedge earthquakes from those in the slab (Bie et al., 2019; Chang et al., 2017, 2019; Davey & Ristau, 2011; Halpaap et al., 2019; Laigle et al., 2013; Malusà et al., 2016; Nakajima & Uchida, 2018; Nakata et al., 2019; Paulatto et al., 2017; Uchida et al., 2010; White et al., 2019). In the specific case of western Greece, the putative mantle wedge earthquakes overlie an aseismic portion of the slab, raising questions as to whether these are, in fact, mislocated slab earthquakes (i.e., upward shifted).…”

“…On the deep portion of the interface, cold systems such as the Hellenic and Honshu subduction zones exhibit repeating earthquakes (Halpaap et al, 2019;Shimamura et al, 2011;Uchida et al, 2005Uchida et al, , 2007Uchida et al, , 2012, while warm systems such as Cascadia and Nankai exhibit low-frequency earthquakes and episodic tremor and slip (e.g., Rogers & Dragert, 2003;Schwartz & Rokosky, 2007). In the forearc mantle wedge, a handful of cold subduction zones exhibit seismicity clusters (Bie et al, 2019;Chang et al, 2017Chang et al, , 2019Davey & Ristau, 2011;Halpaap et al, 2019;Laigle et al, 2013;Malusà et al, 2016;Nakajima & Uchida, 2018;Nakata et al, 2019;Paulatto et al, 2017;Uchida et al, 2010;White et al, 2019), while warm subduction zones do not appear to exhibit this type of seismicity -except perhaps for one observation of deep low-frequency earthquakes in Cascadia (Vidale et al, 2014).…”