Abstract
AbstractNew 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 × 10−6and 0.5 × 10−6, 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 along this seaward-most part of the subduction thrust interface.
Highlights
In September, 2004, a sequence of earthquakes occurred beneath the Nankai Trough offshore of the Kii peninsula of southwest Japan (Park and Mori, 2005), causing detectable crustal deformation onshore (Ashi et al, 2005; Hashimoto et al, 2005; Imakiire et al, 2006) and very low-frequency (VLF) earthquakes inferred to be in the accretionary prism (Obara and Ito, 2005)
In this paper we present data downloaded in August, 2006, and December, 2007, from two Advanced CORK (“Circulation Obviation Retrofit Kit”) hydrologic borehole
Comparison of the co-seismic strain estimated from the pressure change at Site 1173 with that predicted for a dislocation appropriate for the largest off-Kii earthquake (Fig. 1(b)) shows the observed strain to be consistent in sign with that predicted—a robust result that is insensitive to large changes in focal parameters used for the dislocation model—but the magnitude is much greater than expected from the dislocation model (Fig. 7)
Summary
In September, 2004, a sequence of earthquakes occurred beneath the Nankai Trough offshore of the Kii peninsula of southwest Japan (Park and Mori, 2005), causing detectable crustal deformation onshore (Ashi et al, 2005; Hashimoto et al, 2005; Imakiire et al, 2006) and very low-frequency (VLF) earthquakes inferred to be in the accretionary prism (Obara and Ito, 2005). With μ = 10−3 Pa s, and other properties given and determined below, ζ would be of the order of 10−9 Pa−1, and if k were 10−18 m2, a value typical of permeabilities measured on sediments sampled from the lower Shikoku facies sediments (Bourlange et al, 2005; Gamage and Screaton, αf = 1 − βs/βf, and where αm is the effective stress coefficient, βm, βs, and βf are the compressibilities of the formation matrix and the solid and fluid constituents, respectively, γ is the 3-D loading efficiency, ζ is the 3-D storage compressibility, and n is the porosity (Wang, 2004, and references therein) These relationships allow the sensitivity of pressure to strain to be expressed as a function of matrix compressibility, as shown in Fig. 6 for a range of porosities, assuming βs = 2.0 × 10−11 Pa−1 and βf = 4.0 × 10−10 Pa−1 (at the average temperature of the formation of roughly 50◦C). 808 (Fig. 3) suggests expansional strain of a similar magnitude, uncertainties associated with the incomplete seal and the poorly defined physical properties precludes a confident estimate
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