15:30 〜 16:30
[J05-P-15] The gravity anomalies analysis over the active reverse fault zones in Japan
Our object is to examine subsurface features of the reverse fault zones and defines their faulting type, dipping direction, continuity and segmentation through gravity anomalies .In this study, we analyze 43 reverse fault zones in northeast Japan and the northern part of southwest Japan among major active fault zones selected by Headquarters for Earthquake Research Promotion.
We compiled the gravity data published by the Gravity Research Group in Southwest Japan (2001), the Geographical Survey Institute (2006), Yamamoto et al. (2011), Honda et al. (2012), and the Geological Survey of Japan, AIST (2013) in this study. We apply terrain corrections with 10 m DEM (Sawada et al. 2015) under the assumed density of 2670 kg/m3 and a band-pass filtering, then remove linear trend. We calculate the derivatives and structural parameters from a gravity gradient tensor, such as a first horizontal derivatives (HD), a first vertical derivatives (VD), a normalized total horizontal derivative (TDX), a dip angle (β), and a dimensionality index (Di),
At 21 fault zones, the subsurface structural boundaries are highlighted clearly along the faults with the derivatives. The structural boundaries are partly detected at 13 fault zones and not detected at 9 fault zones. At these 9 fault zones, gravity anomalies do not highlight the fault structures but they show low anomaly basins, which restrict surface traces of the active fault zones.
We divide the results into several groups according to their faulting types, such as reverse faults, reactivated reverse faults in which a reverse fault structure is dominant, reactivated reverse faults in which a normal fault structure is still dominant, and normal faults with gravity anomalies of a trough structure. The Nagano basin east margin fault zone is newly formed reverse faults in the sedimentary basin in the Quaternary. However, gravity anomalies detect a normal faulting. We suggest that gravity anomalies detect the east margin of the trough.
We compiled the gravity data published by the Gravity Research Group in Southwest Japan (2001), the Geographical Survey Institute (2006), Yamamoto et al. (2011), Honda et al. (2012), and the Geological Survey of Japan, AIST (2013) in this study. We apply terrain corrections with 10 m DEM (Sawada et al. 2015) under the assumed density of 2670 kg/m3 and a band-pass filtering, then remove linear trend. We calculate the derivatives and structural parameters from a gravity gradient tensor, such as a first horizontal derivatives (HD), a first vertical derivatives (VD), a normalized total horizontal derivative (TDX), a dip angle (β), and a dimensionality index (Di),
At 21 fault zones, the subsurface structural boundaries are highlighted clearly along the faults with the derivatives. The structural boundaries are partly detected at 13 fault zones and not detected at 9 fault zones. At these 9 fault zones, gravity anomalies do not highlight the fault structures but they show low anomaly basins, which restrict surface traces of the active fault zones.
We divide the results into several groups according to their faulting types, such as reverse faults, reactivated reverse faults in which a reverse fault structure is dominant, reactivated reverse faults in which a normal fault structure is still dominant, and normal faults with gravity anomalies of a trough structure. The Nagano basin east margin fault zone is newly formed reverse faults in the sedimentary basin in the Quaternary. However, gravity anomalies detect a normal faulting. We suggest that gravity anomalies detect the east margin of the trough.