Repeating and triggered slow slip events in the near-trench region of the Nankai Trough detected by borehole observatories

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Author(s): Saffer, D. M.; Araki, E.; Kopf, A.; Toczko, S.; Wallace, L. M.; Davis, E. E.; Roesner, A.
Author Affiliation(s): Primary:
Pennsylvania State University, Department of Geosciences, University Park, PA, United States
JAMSTEC Japan Agency for Marine-Earth Science and Technology, Kanagawa, Japan
MARUM - University of Bremen, Bremen, Germany
GNS Science, Lower Hutt, New Zealand
Pacific Geoscience Center, Sidney, BC, 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: Slow slip events (SSE), non-volcanic tremor, and very low-frequency earthquakes (VLFE) are well documented down-dip of the seismogenic zone of major faults, yet similar observations for the shallowest reaches of subduction megathrusts are rare. Here, we document a family of repeating strain transients in the outermost Nankai subduction zone, updip of the region that ruptures in great (M8-class) earthquakes. We report on data from two borehole observatories: IODP Site C0002, which penetrates the accretionary prism and monitors a zone 931-980 m below seafloor (mbsf) at a location 36 km landward of the trench; and Site C0010, 25 km landward, which monitors a zone spanning 389-407 mbsf. We focus on a time window from Dec. 2010 - Apr. 2016, for which we recovered records of formation pore pressure at both sites. After filtering oceanographic noise using a local hydrostatic reference at each site, the pressure records reveal seven transient signals that are synchronous at the two holes. Of these, five arise spontaneously, and occur at ≈1 yr intervals with durations of ≈7-21 days. All are positive in sign at C0010, with magnitudes of ≈0.3-0.9 kPa; at Site C0002 three are negative in sign and two are positive, with magnitudes of ≈0.3-0.7 kPa. The remaining two events are larger (1.7-2.7 kPa), exhibit a negative sign at both sites, and immediately follow: (1) the Mar. 2011 M9 Tohoku earthquake; and (2) a sequence including an Apr. 1 M6 thrust event on the plate interface nearby and the Apr. 16 M7 Kumamoto event. In most cases, the pressure transients are accompanied by swarms of VLFE on the shallow plate interface. We interpret the pressure signals to reflect volumetric strain in response to SSEs. Simple dislocation models illustrate that the data at both sites are well fit by slip of ≈1-2 cm on a patch at the plate interface that extends 20-40 km in the down-dip direction, and is centered beneath Site C0002 (spontaneous events) or slightly updip (triggered events). This coincides with a region of the megathrust characterized in previous studies by anomalously low Vp, and elevated pore fluid pressure. The repeating nature of the events, taken together with apparent triggering by regional earthquakes, indicates that the outermost reaches of the subduction megathrust are highly sensitive to perturbation and are perched near a state of failure.
Year of Publication: 2016
Research Program: IODP Integrated Ocean Drilling Program
Key Words: 07 Marine Geology and Oceanography; IODP Site C0002; Integrated Ocean Drilling Program; Kumano Basin; NanTroSEIZE; Nankai Trough; North Pacific; Northwest Pacific; Pacific Ocean; West Pacific
Coordinates: N331800 N331801 E1363801 E1363800
Record ID: 2017066451
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