Fast Aseismic Growth of a Fault-related Fold Associated with the 2007 Chuetsu-Oki Earthquake (M6.8) in Japan: Space Geodetic Observation and Modeling

Niigata basin is located at a diffuse plate boundary between Eurasian plate and North American plate. Although there are no clear plate boundaries, GPS data indicate that a broad area from Niigata to Kobe is undergoing significant strain concentrations (Sagiya et al. 2000, PAGEOPH). Over the past 50 years, there have been a couple of large inland earthquakes, and the 2007 Chuetsu-Oki earthquake (Mj6.8) on July 16 is one of such events. Geologically, this area is known for active folding and thick sedimentary layer (Sato and Kato, 2005, EPS; Okamura et al. 2007, JGR). Using an L-band ALOS/PALSAR InSAR data, we detected crustal deformation signals associated the earthquake. Besides broad signals near the hypocenter, we found that significant deformation took place around Nishiyama Hill, which was ~15 km away from the epicenter and known as a local anticline axis, extending ~30 km. The aftershock data revealed very few events around the Nishiyama Hill, and thus the detected deformation indicates an aseismic growth of fault-related fold. Neither InSAR data nor nationwide GPS data shows significant postseismic deformation signals; the earliest post-earthquake SAR data was acquired 3 days after the earthquake. It turns out, therefore, that the aseismic slip generating the fault-related fold was rather fast, although the slip velocity was not fast enough to generate seismic waves. We constructed a fault source model, consisting one main shock fault and two other inland faults. Our modeling result indicates that the moment magnitude of the aseismic fault for the fold growth is Mw 6.0, and that the slipped area is almost comparable to that of main shock fault (Mw 6.6). The estimated size is overwhelmingly larger than the earlier study by Nishimura et al (2008, GRL), which we think is due to the difference in the derived area of aseismic deformation.