Sequence of slow slip events and low frequency earthquakes in the shallow part of the Nankai Trough seismogenic zone observed by seafloor observation network

Author(s): Araki, E.; Saffer, D. M.; Kopf, A.; To, A.; Ide, S.; Nakano, M.; Kimura, T.; Machida, Y.
Author Affiliation(s): Primary:
Japan Agency for Marine-Earth Science and Technology, Kanagawa, Japan
Pennsylvania State University, Department of Geosciences, University Park, PA, United States
University of Bremen, Bremen, Germany
University of Tokyo, Bunkyo-ku, Japan
Volume Title: AGU 2016 fall meeting
Source: American Geophysical Union Fall Meeting, Vol.2016; American Geophysical Union 2016 fall meeting, San Francisco, CA, Dec. 12-16, 2016. Publisher: American Geophysical Union, Washington, DC, United States
Note: In English
Summary: Seismic behavior of the thrust zone in trench side of the seismically coupled plate interface in the Nankai Trough is poorly understood because shore based seismic and geodetic observation does not have enough sensitivity to detect slow activity in the area. In these years, we constructed dense seafloor observation network in combination with pore-fluid pressure, strain, and seismic sensing in IODP deep boreholes (C0002G and C0010A) and 20+ seafloor broadband seismometers cabled to the observation network called DONET for long-term continuous observation in the To-Nankai area of the Nankai Trough, south of Japan. Analysis of the seismic records from DONET seafloor seismometer and pore-fluid pressure records from the boreholes in the period from Jan. 2011 to Apr. 2016 revealed the activities of the slow slip events (SSE), low frequency tremor (LFT), and very low frequency earthquakes (VLFE) in the observation network, detecting seven sequence of pore-fluid pressure transients in these boreholes representing SSEs and many LFT and VLFEs from seismic records. Some of the SSE sequence accompanies active LFT swarms in the regions offshore of the locked seismogenic zone. Some of the pressure transient initiate precedent to the LFT swarms, as well as some does not accompany obvious LFT activity, as if the SSE occurs "silently", suggesting LFT does not express SSE but LFT seems activated by the SSE. This is also supported by change of SSE pressure transient rate in accordance with LFT activity, observed in sequences in Mar. 2011, Oct. 2015, and April 2016. In the Oct. 2015 sequence, observed pressure transient in two boreholes indicates the slip propagates updip in the shallow subduction zone. In many sequences including this sequence, we identify that the LFT swarm tends to migrate updip direction. The pressure transient in Apr. 2016 also followed this tendency, initiating from co-seismic compression by Apr. 1 earthquake occurred downdip side of the boreholes, followed by further compression due to the after slip, and slow release of the pressure suggesting SSE along with very active LFT and VLFE activities migrating offshore direction in the following two weeks period. The SSE seemed further activated by teleseismic events Kumamoto earthquake in Apr. 17.
Year of Publication: 2016
Research Program: IODP Integrated Ocean Drilling Program
Key Words: 19 Geophysics, Seismology; Earthquakes; IODP Site C0002; IODP Site C0010; Integrated Ocean Drilling Program; Kumano Basin; NanTroSEIZE; Nankai Trough; North Pacific; Northwest Pacific; Pacific Ocean; Seismicity; Seismotectonics; Slip rates; Tectonics; West Pacific
Coordinates: N331800 N331801 E1363801 E1363800
N331236 N331236 E1364112 E1364112
Record ID: 2017066638
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