Permanent gravity changes recorded on superconducting gravimeters from earthquakes in central Japan—The Noto Hantou and Niigataken Chuetsu-oki events in 2007

Abstract Coseismic gravity changes are interesting signals to be studied with precise gravity observations by means of the superconducting gravimeter (SG). Using the data from two SG stations in Japan, Matsushiro and Kamioka, an attempt is made to detect permanent gravity changes caused by the two recent earthquakes, the 2007 Noto Hantou earthquake ( M w  = 6.9) and the 2007 Niigataken Chuetsu-oki earthquake ( M w  = 6.8). Although these earthquakes are much smaller in magnitude than the 2003 Tokachi-oki earthquake studied by Imanishi et al. (2004) , the expected magnitude of the coseismic signals reaches the observable level of the order of 0.1 μGal (=1 × 10 −9  m s −2 ), owing to the fact that the epicentral distances are relatively short (120–170 km). A total of four possible combinations of source and receiver are investigated. In two cases in which the epicentral distances are shorter, there are instrumental offsets in the gravimeter records inhibiting estimation of gravity changes of natural origin. In the remaining two cases, the observed gravity changes are in agreement with theoretical predictions at least in their sign, but the differences between them are unexplained. Our result indicates the difficulty in identification of the coseismic signals caused by typical inland earthquakes.