11:00 〜 13:00
[MAG37-P03] Seismoacoustic signals accompany slow slip events at the front of the Izu-Bonin convergent margin
キーワード：水中音波、海底観測、比較沈み帯学
Earthquake studies have supported that there is a significant difference in the mode of plate motion at interplate boundaries between the two types of subduction zones. Extreme cases are Chile, where plate motion is seismic, and the Marianas arc, where it is aseismic (Uyeda and Kanamori, 1979). Discovery of slow slip events at the plate boundaries with source durations ranging from days to years motivated seismologists to think about slip mode in the transitional regime between the unstable seismic regime and the stable sliding regime. We recently reported detection of slip events amid the transitional regime by using an ocean bottom pressure gages array at the Izu-Bonin Trench (Fukao et al., 2021; Kubota et al., 2021). There have been no great earthquakes historically reported and the subduction is believed to occur largely by aseismic slip, which is classified as a typical case of the Mariana-type (Scholz and Campos, 1995). The detected events were four orders of magnitude faster (about 1 hour) than previously reported slow slip events but four orders of magnitude slower than ordinary earthquakes. The finding provides a missing link in understanding the whole spectrum of slip modes.
The aseismic slow slip events came from an array observation with 10 stations at depths of around 5,000 m close to the west of the steep slope of the northern Bonin Trench for one year from May 2015 (Fukao et al., 2021). The array consisted of equilateral triangles with the minimum and maximum side lengths of 10 and 30 km, respectively. Each station except for the center deployed an ocean bottom pressure gauge, equipped with an absolute pressure gauge (APG) (PARO-8B7000-I-005, Paroscientific Inc., USA), which nominally had an accuracy of 0.01% and a resolution of 1 × 10−8 for absolute pressure at water depths of 7,000 m (Fukao et al., 2018). The APG records were sampled at 4 Hz with a cutoff frequency of 0.7 Hz. At the center of the array, a broadband ocean bottom seismometer (Guralp 3T, Guralp Systems Ltd., UK) and a differential pressure gauge (DPG, Araki and Sugioka, 2009) with a sampling rate of 100 Hz were installed instead of APG.
The detected aseismic slip events were accompanied with the Mw 6.0 thrust earthquake on September 1 of 2015 just beneath the array. Numerous aftershocks of this earthquake were also detected by the array at least for weeks, most of which were too small to be reported by the US Geological Survey. We determined the hypocenters of these aftershocks based on an envelope correlation method for the pressure waveform (Ide et al., 2008). The migration of sources from east to west in almost parallel to the plate motion was significantly observed. Each event was formed by a characteristic wave train with periods of about 10 s. The characteristic wave train has an apparent propagation velocity of slightly less than 1.5 km/s through the triangle pressure gauge array, which is consistent with its interpretation as being the group velocity of the Airy phase of ocean-acoustic-mode dominant Rayleigh waves, suggesting it might be an indicator of such slow seismic activity at the vicinity of the trench.
The aseismic slow slip events came from an array observation with 10 stations at depths of around 5,000 m close to the west of the steep slope of the northern Bonin Trench for one year from May 2015 (Fukao et al., 2021). The array consisted of equilateral triangles with the minimum and maximum side lengths of 10 and 30 km, respectively. Each station except for the center deployed an ocean bottom pressure gauge, equipped with an absolute pressure gauge (APG) (PARO-8B7000-I-005, Paroscientific Inc., USA), which nominally had an accuracy of 0.01% and a resolution of 1 × 10−8 for absolute pressure at water depths of 7,000 m (Fukao et al., 2018). The APG records were sampled at 4 Hz with a cutoff frequency of 0.7 Hz. At the center of the array, a broadband ocean bottom seismometer (Guralp 3T, Guralp Systems Ltd., UK) and a differential pressure gauge (DPG, Araki and Sugioka, 2009) with a sampling rate of 100 Hz were installed instead of APG.
The detected aseismic slip events were accompanied with the Mw 6.0 thrust earthquake on September 1 of 2015 just beneath the array. Numerous aftershocks of this earthquake were also detected by the array at least for weeks, most of which were too small to be reported by the US Geological Survey. We determined the hypocenters of these aftershocks based on an envelope correlation method for the pressure waveform (Ide et al., 2008). The migration of sources from east to west in almost parallel to the plate motion was significantly observed. Each event was formed by a characteristic wave train with periods of about 10 s. The characteristic wave train has an apparent propagation velocity of slightly less than 1.5 km/s through the triangle pressure gauge array, which is consistent with its interpretation as being the group velocity of the Airy phase of ocean-acoustic-mode dominant Rayleigh waves, suggesting it might be an indicator of such slow seismic activity at the vicinity of the trench.