TY - JOUR
T1 - Co-seismic and post-seismic pore-fluid pressure changes in the Philippine Sea plate and Nankai decollement in response to a seismogenic strain event off Kii Peninsula, Japan
AU - Davis, Earl
AU - Becker, Keir
AU - Wang, Kelin
AU - Kinoshita, Masa
N1 - Funding Information:
Acknowledgments. We thank the Ocean Drilling Program for ongoing support for long-term borehole monitoring experiments, T. Pettigrew, R. Meldrum, and R. Macdonald for engineering assistance, and the pilots and crews of the submersible Shinkai 6500 and the remotely-operated vehicles Kaiko 12k and Kaiko 7k and the captains and crews of the JAMSTEC Research Vessels Kairei and Yokosuka for their capable support during site visits. Financial support has been provided by the U.S. National Science Foundation, the Geological Survey of Canada, and the Japanese Agency for Marine-Earth Science and Technology. GSC contribution no. 4711.
PY - 2009
Y1 - 2009
N2 - New pressure data from a pair of Ocean Drilling Program (ODP) hydrologic borehole observatories at ODP Sites 1173 and 808, located off Japan in the subducting Philippine Sea plate and in the nearby Nankai accretionary prism, respectively, show clear signals associated with an earthquake swarm off the Kii Peninsula that began on September 5, 2004, roughly 220 km away from the observatory sites. At Site 1173, formation pressures rose by 1.0-1.5 kPa at the time of the largest earthquake (Mw = 7.5), then continued to rise to a total anomaly of 4 kPa during the following 200-300 days. These transients are inferred to reflect co-seismic and slow continuing volumetric contraction of the plate by amounts of roughly 0.2 × 106 and 0.5 × 106, respectively. The sign of the estimated strain is consistent with that predicted with a seismic-moment-constrained elastic half-space dislocation model, but the amplitude is much larger, by roughly a factor of 6 at the time of the main earthquake, and by nearly a factor of 20 when the total pressure-estimated strain at the end of the post-seismic period is compared to that estimated from the total cumulative seismic moment including aftershocks. The simplest inference that can be drawn is that a large component of aseismic slip occurred in the epicentral area. At Site 808, pressure at the deepest monitoring zone just above the subduction decollement fell at the time of the largest earthquakes. This may reflect shear-induced dilatation which would be consistent with strain- or velocity- hardening behaviour alone this seaward-most part of the subduction thrust interface.
AB - New pressure data from a pair of Ocean Drilling Program (ODP) hydrologic borehole observatories at ODP Sites 1173 and 808, located off Japan in the subducting Philippine Sea plate and in the nearby Nankai accretionary prism, respectively, show clear signals associated with an earthquake swarm off the Kii Peninsula that began on September 5, 2004, roughly 220 km away from the observatory sites. At Site 1173, formation pressures rose by 1.0-1.5 kPa at the time of the largest earthquake (Mw = 7.5), then continued to rise to a total anomaly of 4 kPa during the following 200-300 days. These transients are inferred to reflect co-seismic and slow continuing volumetric contraction of the plate by amounts of roughly 0.2 × 106 and 0.5 × 106, respectively. The sign of the estimated strain is consistent with that predicted with a seismic-moment-constrained elastic half-space dislocation model, but the amplitude is much larger, by roughly a factor of 6 at the time of the main earthquake, and by nearly a factor of 20 when the total pressure-estimated strain at the end of the post-seismic period is compared to that estimated from the total cumulative seismic moment including aftershocks. The simplest inference that can be drawn is that a large component of aseismic slip occurred in the epicentral area. At Site 808, pressure at the deepest monitoring zone just above the subduction decollement fell at the time of the largest earthquakes. This may reflect shear-induced dilatation which would be consistent with strain- or velocity- hardening behaviour alone this seaward-most part of the subduction thrust interface.
KW - CORK borehole observatory
KW - Co-seismic strain
KW - Ocean Drilling Program
KW - Post-seismic strain
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U2 - 10.1186/BF03353174
DO - 10.1186/BF03353174
M3 - Article
AN - SCOPUS:70350158684
VL - 61
SP - 649
EP - 657
JO - Earth, Planets and Space
JF - Earth, Planets and Space
SN - 1343-8832
IS - 6
ER -