Anelastic properties beneath the Niigata–Kobe Tectonic Zone, Japan

Abstract

We estimate the three-dimensional (3D) P-wave attenuation structure beneath the Niigata-Kobe Tectonic Zone (NKTZ), central Japan, using high-quality waveform data from a large number of stations. The obtained results confirm the segmentation of the NKTZ into three regions, as suggested by 3D seismic velocity models, and reveal characteristic structures related to surface deformation, shallow subduction of the Philippine Sea slab, and magmatism. The lower crust beneath the NKTZ west of the Itoigawa-Shizuoka Tectonic Line (ISTL) is overall characterized by distinct high attenuation, whereas the upper crust shows marked high attenuation to the east of the ISTL. Differences in the depths of anelastically weakened parts of the crust probably result in a first-order spatial variation in surface deformation, forming wide (width of ~100 km) and narrow (width of 25-40 km) deformation zones on the western and eastern sides of the ISTL, respectively. Many M ≥ 6.5 earthquakes occur in the upper crust where seismic attenuation in the underlying lower crust varies sharply, suggesting that spatial variations in rates of anelastic deformation in the lower crust result in stress concentration in the overlying brittle crust. We interpret a moderate-to low-attenuation zone located in the lower crust at the northeast of Biwa Lake to reflect low-temperature conditions that are developed locally as a result of shallow subduction of the cold Philippine Sea slab.

Spatial variation in coda Q in the northeastern part of Niigata–Kobe Tectonic Zone, central Japan: implication of the cause of a high strain rate zone

“…They revealed, together with three-dimensional P-wave velocity structure, that the lower crust west of the ISTL and the upper crust east of the ISTL are characterized by high attenuation and low velocity. From these facts, Nakajima and Matsuzawa (2017) concluded that the differences in the depths of elastically weakened parts of crust probably result in a first-order spatial variation in surface deformation, which is in agreement with our discussion through the correlation between the coda Q, the strain rate, and the S-wave velocity.…”

High-Resolution Three-dimensional Azimuthal Velocity Anisotropy of S-waves in southwest Japan, based on Ambient Noise Tomography

“…Therefore, we posit that the observed azimuthal anisotropy beneath a depth of 3 km may be linked to the localized, high-strain-rate crustal deformation, manuscript submitted to Earth, Planets and Space extensive faulting and its associated fault fabrics, shearing in these fault zones or earthquakerelated fractures/cracks that are oriented in the same direction as the fast axes. Regarding the likely existence of fluid-filled cracks/fractures, our interpretation follows that of Hiramatsu et al (1998), who suggested that the cause of the observed anisotropy in the central part of the Kinki region is related to the heterogeneous structures created by fluid-filled cracks, and the interpretation of Nakajima and Matsuzawa (2017), who attributed the apparent low-velocity and high-attenuation anomaly in this area to the presence of aqueous fluids.…”

Temporal stability of coda Q in the northeastern part of an inland high strain rate zone, central Japan: implication of a persistent ductile deformation in the crust

“…It is important to note the difference in the location of the persistent ductile deformation in the NKTZ as the cause of the high strain rate zone suggested by coda Q analyses; in the upper crust in the NE-NKTZ and in the lower crust in the central NKTZ. The difference in depth of the elastically weakened part of the NKTZ revealed by Nakajima and Matsuzawa (2017) supports the heterogeneous depth distribution of the persistent ductile deformation in the NKTZ.…”