High-Precision Gravity Surveys Before and After the 1980 Izu-Hanto-Toho-Oki Earthquake

The repetition of high-precision gravity measurements has intensified on the eastern coast of the Izu Peninsula since the 1978 Izu-Oshima-Kinkai earthquake. The gravity decrease found near the Kawanazaki earthquake swarm epicenters became a clue to the discovery of the related crustal uplift. The gravity surveys conducted immediately before and after the 1980 Izu-Hanto-Oki earthquake of M 6.7 clarified the fact that the site of the gravity decrease had migrated about 10 km southwards. A similar phenomenon had been experienced at the time of the 1930 North-Izu earthquake of M 7.0