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.
Intense swarm-like seismicity associated with shallow normal faulting was induced in Ibaraki and Fukushima prefectures, Japan, following the 2011 Tohoku-Oki earthquake. This seismicity shows a ...systematic spatiotemporal evolution, but little is known of the heterogeneity in crustal structure in this region, or its influence on the evolution of the seismicity. Here, we elucidate a high-resolution model of crustal structure in this region and determine precise hypocenter locations. Hypocenters in Ibaraki Prefecture reveal a planar earthquake alignment dipping SW at ~45°, whereas those in Fukushima Prefecture show a more complex distribution, consisting of conjugate sets of aligned small earthquakes. On the north of the hypocenter of the largest earthquake in the sequence (the M7.0 Iwaki earthquake), we imaged a high-velocity body at shallow depths that lacks aftershock seismicity. Based on fault source models, the large-slip region of the Iwaki earthquake is situated along a zone that roughly coincides with this high-velocity body. We delineated a separate low-velocity anomaly directly beneath the hypocenter of the Iwaki earthquake, indicating crustal fluids in this region. We hypothesize that strong crust underwent structural failure due to the infiltration of crustal fluids into the seismogenic zone from deeper levels, causing the Iwaki earthquake.
To investigate microseismicity around the Nagamachi-Rifu fault, the B class active reverse fault with a northeast-southwest strike and a northwest dip, Miyagi Prefecture, northeastern Japan, a ...temporal seismic observation composed of sixteen high-sensitivity stations was carried out for the total period of one year between 1996 and 1998. In spite of high artificial noise in the urban area of Sendai City, we succeeded in obtaining high-quality seismic data by using a DAT continuous recording system. The total number of the earthquakes recorded is 119, of which the smallest one is magnitude -0.5. The depths of the events range between 3 and 13km. Most of hypocenters are concentrated in the northwestern region of the Nagamachi-Rifu fault, while few earthquakes in the southeastern region regardless of our high sensitivity observation. This indicates that these earthquakes occur only in the hanging wall side of this active fault. Focal mechanisms in this region are not necessarily coincident with that of the Nagamachi-Rifu fault. In particular, focal-plane solutions of strike-slip are predominant along the Kagitori-Okubushi line, which is a flexure with a conjugate strike to the Nagamachi-Rifu fault. The spatial pattern of the horizontal crustal strain also changes its characteristics across this line. These results strongly suggest that not only the Nagamachi-Rifu fault but also the Kagitori-Okubushi line may control the seismogenesis of this area.
PDF
