Pore pressure distribution in the focal region of the 2008 M7.2 Iwate-Miyagi Nairiku earthquake

Abstract

The pore fluid pressure distribution in the focal region of the 2008 Iwate-Miyagi Nairiku earthquake was investigated through an analysis of the diversity of focal mechanisms. We inverted stress orientations and focal mechanisms directly from P-wave polarity data obtained from a dense aftershock observation network and other temporarily and routinely operated stations. The estimated stress orientation is consistent with that typical of NE Japan. Specifically, the σ1 axis is oriented WNW-ESE nearly parallel to plate convergence, and the σ3 axis is nearly vertical, consistent with a reverse-faulting stress regime, with some exceptions in the central part of the aftershock area. We obtained 2,497 well-determined focal mechanisms whose average number of P-wave polarity data is more than 70. The spatial distribution of pore fluid pressure was estimated by using the obtained orientations of the principal stresses and earthquake faults. The pore pressure ratio for each earthquake fault was calculated under the assumption that reduced frictional strength was caused by pore fluid pressure. The results showed that the diversity of the focal mechanisms here obtained requires high pore fluid pressures at many of the earthquake faults in the focal region. The spatial pattern of pore pressure ratio shows that areas of higher pore pressure ratio are distributed around the large coseismic slip area near Kurikoma volcano. Immediately beneath these high pore pressure areas, there exists a distinct seismic low-velocity zone that continues down to the mantle wedge below. These observations suggest that crustal fluids supplied from the mantle wedge have contributed to producing high pore pressures and to lowering the frictional strengths of those faults. Crustal fluids may also have contributed to generating the mainshock rupture.

Mid-crustal fluid related to the Matsushiro earthquake swarm (1965–1967) in northern Central Japan: Geochemical reproduction

“…The fluid pools as forms of velocity and electromagnetic conductivity anomalies are found in the root zones of plate boundaries (e.g., the San Andreas fault system; Becken et al, 2008Becken et al, , 2011Kirby et al, 2014), in the middle crust beneath active faults (Hobbs et al, 2004;Matsubara et al, 2004) and beneath volcanic fields (Parsons et al, 1992;Hasegawa and Yamamoto, 1994;Kanda and Ogawa, 2014). The observations support the positive involvement of crustal fluid not only in the clustering earthquakes in active geothermal fields (Todesco et al, 2004;Giammanco et al, 2008) but also in some of the subduction-unrelated inland earthquakes (Spicak and Horalek, 2001;Hainzl, 2004;Okada et al, 2012;Yoshida et al, 2014). Despite the overall acceptance of the influence of deep seated mid-crustal fluid on triggering earthquakes, the actual form of the fluid is not well understood because, by definition, it rarely breaches the earth's surface.…”

Anelastic properties beneath the Niigata–Kobe Tectonic Zone, Japan

“…4a), may be one of the manifestations of high stress concentration in the brittle upper crust. While it is considered that reduction in the effective normal stress on a fault as a result of the supply of overpressurized fluids plays key roles in facilitating crustal earthquakes (e.g., Sibson 1992;Hasegawa et al 2005;Yoshida et al 2014), our observations add growing body of evidence that anelastic deformation in the lower crust is equally important in seismogenesis in the upper crust. (Nakajima and Hasegawa 2007) at a depth of 25 km for each grid node located beneath the NKTZ (purple shading in Fig.…”

Heterogeneous stress state of island arc crust in northeastern Japan affected by hot mantle fingers

“…On the basis of the observed reactivation of poorly oriented reverse faults, Sibson [, ] suggested that overpressurization of aqueous fluids to near‐lithostatic values is widespread in the lower part of the seismogenic zone. Yoshida et al [] also suggested the presence of high pore fluid pressures along many of the seismogenic faults in the source region of the 2008 Iwate‐Miyagi inland earthquake.…”