Crustal structure of central Japan and its petrological implications

The crustal structure of central Honshu, Japan, has been influenced by several tectonic events: a series of collision processes during the late Tertiary, extensive faulting, highly compressive deformation and Quaternary volcanism, The southern part of the study area is covered by various granitic rocks which intrude into old Palaeozoic formations. Quaternary volcanoes occupy the northern part and cover the basement, which is mainly composed of granitic rocks and Tertiary formations. A traveltime inversion for velocity and interface depth was applied to the seismic data, together with constraints from amplitude forward modelling to produce a seismic velocity model of the crust in central Honshu, In this experiment both refracted P and S waves are clearly recorded along a 110 km profile. These provide constraints on the 2-D P- and S-wave velocity structure down to a depth of about 10 km. Moreover, clear P-wave reflections from intracrustal interfaces provide constraints on deeper parts of the crust down to about 25 km. For the P-wave velocity model, the velocities of near-surface layers show large lateral variations from 3.7 to 4.4 km s(-1), increasing towards the south, Underlying layers have velocities ranging from 4.85 to 5.6 km s(-1). Velocities in the range 5.85-6.2 km s(-1) characterize the upper crust that extends to a depth of about 15 km, For the S-wave velocity model, the near-surface layer velocities change from 1.7 to 2.55 km s(-1), showing remarkable lateral heterogeneity, whilst the upper crust is characterized by velocities of 3.4-3.6 km s(-1). The deeper parts are modelled by two prominent reflectors in the mid-crust. The shallower one, which delineates the bottom of the upper crust, dips southwards from 14 to 16 km. The deeper reflector lies between 20 and 24 km depth and dips northwards. The velocity in this mid-crustal layer is 6.35 km s(-1) at the top and 6.7 km s(-1) at the bottom. The crustal thickness under the profile could not be determined because of a lack of P-n and/or PmP phases

Anomalous depth dependency of the stress field in the 2007 Noto Hanto, Japan, earthquake: Potential involvement of a deep fluid reservoir

We have elucidated depth variations in the stress field associated with the 2007 Noto Hanto, Japan, earthquake by stress tensor inversion using high-quality aftershock data obtained by a dense seismic network. Aftershocks that occurred above 4 km in depth indicated a strike-slip stress regime. By contrast, aftershocks in deeper parts indicated a thrust faulting stress regime. This depth variation in the stress regime correlates well with that in the slip direction derived from a finite source model using geodetic data. Furthermore, the maximum principal stress (σ1) axis was stably oriented approximately W20°N down to the depth of the mainshock hypocenter, largely in agreement with the regional stress field, but, below that depth, the σ1 axis had no definite orientation, indicating horizontally isotropic stress. One likely cause of these drastic changes in the stress regime with depth is the buoyant force of a fluid reservoir localized beneath the seismogenic zone