Changes in physical properties of the Nankai Trough megasplay fault induced by earthquakes, detected by continuous pressure monitoring

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Author(s): Kinoshita, Chihiro; Saffer, Demian M.; Kopf, Achim; Roesner, Alex; Wallace, Laura; Araki, Eiichiro; Kimura, Toshinori; Machida, Yuda; Kobayashi, Reiji; Davis, Earl; Toczko, Sean
Author Affiliation(s): Primary:
Kyoto University, Kyoto, Japan
Pennsylvania State University, United States
University of Bremen, Germany
University of Texas at Austin, United States
Japan Agency for Marine-Earth Science and Technology, Japan
Kagoshima University, Japan
Pacific Geoscience Center, Canada
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: The Nankai Trough Seismogenic Zone Experiment (NanTroSEIZE) is a multi-expedition Integrated Ocean Drilling Program (IODP) project aimed at understanding seismogenic processes at subduction zones. The most recent NanTroSEIZE expedition was conducted in March-April 2016, with the main objectives of recovering a temporary observatory instrument and installing a permanent Long-Term Borehole Monitoring System at IODP Site C0010. The recovered instrument package, termed the "GeniusPlug" was installed in 2010 and produced a 5.3 year record of pressure and temperature within a screened interval spanning 387-409 mbsf, sampled at 30 sec intervals from November 2010 through March 2016. In this study, we evaluate changes in the properties of the fault zone induced by several earthquakes, using the pressure timeseries. The GeniusPlug has two pressure sensors to measure the formation pressure and a hydrostatic reference. Here we focused on two phenomena: step changes in pressure induced by regional earthquakes, and associated changes in formation properties. For the former, we filter the data to remove ocean tides and other oceanographic signals. For the latter, we use the amplitude of the response to ocean tide loading to probe changes in formation compressibility. To define step change in pressure, we compare a 10 minute average of pressure before and after each earthquake. We identify 30 events that exhibit a pressure response, and find that pressure changes strongly correlate with earthquake magnitude and hypocentral distance. To identify the details of the underlying cause of these step changes, we calculated the theoretical static volumetric strain for each event based on Okada (1992). Ongoing work will compare the observed pressure changes with peak ground acceleration (PGA) and peak ground velocity (PGV) determined using ocean bottom seismometers in the DONET (Dense Oceanfloor Network system for Earthquakes and Tsunamis) cabled offshore observatory. To evaluate changes in loading efficiency during the deployment time, we estimated the tidal loading efficiency at one-day intervals. We find that (1) there is a correlation between tidal loading efficiency and hypocentral distance, (2) when large pressure changes are detected, the tidal loading efficiency also deviates.
Year of Publication: 2016
Research Program: IODP Integrated Ocean Drilling Program
Key Words: 19 Geophysics, Seismology; Earthquakes; Faults; IODP Site C0001; Integrated Ocean Drilling Program; NanTroSEIZE; Nankai Trough; North Pacific; Northwest Pacific; Pacific Ocean; Seismicity; Seismotectonics; Tectonics; West Pacific
Coordinates: N331400 N331400 E1364300 E1364300
Record ID: 2018005613
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