Gravity survey in the Northern Hokkaido region, Northern Japan

The northern part of Hokkaido (Dohoku region) is recognized as the boundary region between the Okhotsk and Amurian plates. In this region, strain concentration is reported based on triangulation and GPS observations. However, there had not been observed any inland earthquakes larger than MJMA 6 in Dohoku region, until the 2004 Rumoi-Nanbu earthquake (MJMA 6.1) occurred. There may be a possibility of occurrence of intraplate large earthquake in the future. The mechanism of the earthquakes observed in the region is strikeslip and thrust type, owing to the east-west compression. In case the thrust type earthquake had been rupturing recurrently, it should be resulted as positive Bouguer gravity anomalies. There exists plenty amounts of gravity observation data in this region, for the purpose of oil and mineral exploration. However, it exists as a hand written paper document. Additionally, these data are basically unreleased. So we performed gravity measurement in the Dohoku region to draw more detailed Bouguer anomaly map. The observed gravity data is listed in this document