[S-CG40] Science of slow-to-fast earthquakes

[SCG40-03] A clearer view of seismicity offshore NE Japan through deep-learning-based analysis of S-net data: evidence for the role of forearc fluids in interplate coupling

*内田 直希^1,2、鈴木 琳大郎²、Zhu Weiqiang³、Beroza Gregory⁴、中山 貴史²、豊国 源知²、高木 涼太²、東 龍介²、吉田 圭佑²、長谷川 昭² (1.東京大学地震研究所、2.東北大学理学研究科、3.カリフォルニア大学　バークレー校、4.スタンフォード大学)

キーワード：機械学習、沈み込み帯、非地震性すべり、流体移動

The new cabled ocean bottom seismic network (S-net) offshore NE Japan provides an important opportunity to investigate the nature of shallow subduction processes based on locally recorded microearthquakes. We trained a deep neural network, PhaseNet with 435,566 ocean-bottom and 287,256 land arrival times to measure arrival times accurately and efficiently from 150 S-net and 444 land stations. We applied this PhaseNet model to the continuous waveforms from June 2016 to August 2020 when S-net was already deployed, but for which the data have not yet been used in the Japan Meteorological Agency (JMA)’s earthquake catalog. Our new catalog contains 602,255 earthquakes, which is 1.6 times as many as in the JMA catalog. The increase of the number of earthquakes was especially prominent in offshore area (6 times) thanks to S-net. The offshore interplate earthquakes (low-angle thrust and small repeating earthquakes) are aligned along the previously estimated plate boundary, demonstrating good depth control in the catalog. The catalog shows intense seismicity extending shallower from the plate boundary (supraslab events) at 160 - 180 km from the trench. We found that repeating earthquakes tend to concentrate at the base of the supra-slab event clusters and that intraslab earthquakes were active in the area below and downdip of the repeating earthquakes. The near-vertical alignment of seismicity can be explained by relatively high pore pressure and fluid transfer to the overlying plate from the plate boundary at 35-45 km depth, which is the downdip extension of the megathrust earthquake rupture. The fluid migration explains the spatial extent of major interplate earthquakes, lateral extent of the 1923 Kanto earthquake and Boso slow slip events, and shallow seismicity beneath the Tokyo metropolitan area.