To study the structure beneath the Kaga hot spring area of southern Ishikawa prefecture a gravity survey was conducted in and around the area. Combining these data with those obtained previously we obtained a high‑resolution Bouguer gravity anomaly map of the study
We investigated a detailed spatial distribution of coda Q around the Atotsugawa fault zone in a high strain rate zone, central Japan, using waveform data from dense seismic observations. Low coda Q at lower frequencies is localized along the fault zone, showing a good spatial correlation with a low velocity zone in the lower crust. On the other hand, we find no characteristic spatial pattern of coda Q at higher frequencies. The spatial correlation between the low coda Q at the lower frequencies, and the low velocity zone, suggests that ductile deformations below the brittle-ductile transition zone in the crust contribute to the variation in coda Q at lower frequencies. We estimated a spatial variation in the stressing rate of 15–18 kPa/year in the crust from that of coda Q in the analyzed region. This value is greater than that estimated from GPS data. We conclude, therefore, that a high deformation rate below the brittle-ductile transition zone causes the high stressing rate, which results in the high strain rate along the fault zone observed by GPS.
Abstract Gravity gradient tensor analysis has been a powerful tool for investigating subsurface structures and recently its application to a two-dimensional fault structure has been developed. To elucidate the faulting type and spatial extent, specifically the continuity and the size, of the subsurface fault structure of an active fault through gravity gradient tensor analysis, we analyzed Bouguer anomalies, which were composed of dense gravity measurement data over the land and seafloor, and indices calculated from a gravity gradient tensor around the Togi-gawa Nangan fault (TNF), Noto Peninsula, central Japan. The features of Bouguer anomalies and their first horizontal and vertical derivatives demonstrate clearly that the TNF is a reverse fault dipping to the southeast. Furthermore, the combination of those derivatives and the dimensionality index revealed that the spatial extent of the subsurface fault structure is coincident with that of the surface fault trace and that it shows no evidence of connecting the TNF with surrounding active faults. Furthermore, the dip angle of the subsurface fault structure was estimated as 45°–60° from the minimum eigenvectors of the gravity gradient tensor. We confirmed that this result is coincident with the dip angle estimated using the two-dimensional Talwani’s method. This high dip angle as a reverse fault suggests that the TNF has experienced inversion tectonics.
The Eastern Boundary Fault Zone of the Shonai Plain (EBFZSP), Northeast Japan is an active reverse fault zone, and its southern part is considered to have a high probability of earthquake occurrence. Also, the Aosawa Fault Zone (AFZ), categorised as a geological reverse fault, lies to the east of the EBFZSP by 3–10 km along strike, almost parallel to the EBFZSP. Therefore, the EBFZSP and its surrounding area are important in terms of their long-term earthquake occurrence potential and fault development history. After performing gravity measurements to compensate for the lack of coverage in the existing gravity database, a gravity gradient tensor analysis was applied around the EBFZSP to evaluate subsurface structural indices (dimensionality, dip angle, strike direction, and boundary indices). As a result, we conclude that the southern part of the EBFZSP appears to be at a relatively mature stage, which means that the part has both high two-dimensionality and boundary properties. On the other hand, the northern part of the EBFZSP appears to be relatively immature, which means that the northern part has both low two-dimensionality and boundary properties. A similar relationship was also found between the EBFZSP and the AFZ; that is, the former was immature, and the latter was mature. These features suggest that the EBFZSP developed from the south to the north, and was derived from the AFZ. We also found an area with high structural boundary indices and low two-dimensionality near some reported active folds beneath the Shonai Plain. A dip-angle index for a reverse-fault-like density structure showed results inconsistent with the dip angles observed on outcrop. To circumvent this contradiction, we propose the hypothesis that the EBFZSP, currently acting as a reverse fault, has a normal-fault-like density structure and we then attempt a phenomenological interpretation. The dip-angle index may be helpful only in simple situations.
