17:15 〜 18:30
[SSS07-P06] 近地深発地震を用いた紀伊半島直下の地震学的構造解析
キーワード：レシーバ関数、スロー地震、紀伊半島、流体
Elucidating seismological structure around subducting interfaces is essential to understand physical processes on slow earthquakes. Shibutani & Hirahara (2018, SSJ Fall Meeting) showed lateral variations on receiver function (RF) images beneath the Kii peninsula, southwestern Japan. We, here, calculate high-frequency RFs from seismic data recorded at temporary high-dense observation in the whole Kii peninsula from 2004 to 2013, especially using regional deep-focus earthquakes to image the Philippine Sea plate (PHS) around bimodal tremor sources: episodic tremor at the updip portion and continual tremor activity at the downdip portions.
Deep slow earthquakes such as tectonic tremor (tremor; Obara, 2002), low-frequency earthquakes (LFE; e.g., Katsumata & Kamaya, 2003), very-low-frequency earthquakes (VLFE; Ito et al., 2009) have occurred at the transition zone from the shallow brittle region to the deep stable slip region (e.g., Hirose & Obara, 2006) in southwestern Japan. Seismological structures from RFs have configured the shapes of the oceanic plate (e.g., Shiomi et al., 2008; Ueno et al., 2008), and constrained source regions of slow earthquakes; LFEs were located at the mantle wedge corner in the Tokai (Kato et al., 2010) and the western Shikoku (Shiomi et al, 2020). RF phases from the subducting interface become weak at downdip portions due to the entry to hydrous mantle wedge (Akuhara & Mochizuki, 2015; Kato et al., 2014).
Some clusters of continual tremors with minor bursts of shorter durations were also found at downdip portions of the tremor zone (Obara et al., 2010). Sawaki et al. (under review) estimated seismological structures of the subducting slab beneath the northeastern Kii peninsula from high-frequency RFs, and showed that the bimodal tremor source areas are under different seismological structures; the updip episodic tremor occurs beneath the forearc crust on near-lithostatic pore-fluid pressures and the downdip continual tremor occurs at the mantle wedge corner. However, the relationship between the lateral variation of seismological structures and the bimodal distribution of tremor has not been well examined. In this study, we investigate detailed seismic structures by multi-band RF analysis, using records of regional deep-focus earthquakes and teleseisms from the dense seismic observations in the Kii Peninsula from March 2004 to March 2013 (Nishimura et al., 2005, 2006).
For the calculation of RFs, we use the extended-time multitaper method (Shibutani et al., 2008) and the iterative deconvolution method (e.g., Ligorria & Ammon, 1999). Following Sawaki et al. (under review), we select records of the deep earthquakes within about 10° of epicentral distance, and remove traces that include direct S waves or converted sP waves. As result, cross-section RFs along the PHS subduction show structures of the PHS slab and the mantle wedge. The amplitudes of the PHS Moho decrease around the mantle wedge corner on the western survey lines. This may support the eclogitation of the basaltic oceanic crust, which increases the speed of the oceanic crust (e.g., Bostock et al., 2002), or indicate a fluid pathway of dehydrated fluid from the further deep Pacific slab (Kato et al., 2014; Umeda et al., 2012).
Deep slow earthquakes such as tectonic tremor (tremor; Obara, 2002), low-frequency earthquakes (LFE; e.g., Katsumata & Kamaya, 2003), very-low-frequency earthquakes (VLFE; Ito et al., 2009) have occurred at the transition zone from the shallow brittle region to the deep stable slip region (e.g., Hirose & Obara, 2006) in southwestern Japan. Seismological structures from RFs have configured the shapes of the oceanic plate (e.g., Shiomi et al., 2008; Ueno et al., 2008), and constrained source regions of slow earthquakes; LFEs were located at the mantle wedge corner in the Tokai (Kato et al., 2010) and the western Shikoku (Shiomi et al, 2020). RF phases from the subducting interface become weak at downdip portions due to the entry to hydrous mantle wedge (Akuhara & Mochizuki, 2015; Kato et al., 2014).
Some clusters of continual tremors with minor bursts of shorter durations were also found at downdip portions of the tremor zone (Obara et al., 2010). Sawaki et al. (under review) estimated seismological structures of the subducting slab beneath the northeastern Kii peninsula from high-frequency RFs, and showed that the bimodal tremor source areas are under different seismological structures; the updip episodic tremor occurs beneath the forearc crust on near-lithostatic pore-fluid pressures and the downdip continual tremor occurs at the mantle wedge corner. However, the relationship between the lateral variation of seismological structures and the bimodal distribution of tremor has not been well examined. In this study, we investigate detailed seismic structures by multi-band RF analysis, using records of regional deep-focus earthquakes and teleseisms from the dense seismic observations in the Kii Peninsula from March 2004 to March 2013 (Nishimura et al., 2005, 2006).
For the calculation of RFs, we use the extended-time multitaper method (Shibutani et al., 2008) and the iterative deconvolution method (e.g., Ligorria & Ammon, 1999). Following Sawaki et al. (under review), we select records of the deep earthquakes within about 10° of epicentral distance, and remove traces that include direct S waves or converted sP waves. As result, cross-section RFs along the PHS subduction show structures of the PHS slab and the mantle wedge. The amplitudes of the PHS Moho decrease around the mantle wedge corner on the western survey lines. This may support the eclogitation of the basaltic oceanic crust, which increases the speed of the oceanic crust (e.g., Bostock et al., 2002), or indicate a fluid pathway of dehydrated fluid from the further deep Pacific slab (Kato et al., 2014; Umeda et al., 2012).