Gravity Changes in the Izu Peninsula, Japan

Precise gravity surveys have been repeated at 36 stations in the Izu Peninsula since December 1974. We have found a gravity decrease occurred concurrently with a dome-like uplift centering at Hiekawa Pass. The ratio of the gravity change to the elevation change is approximately equal to a free-air rate. A possible cause of the uplift can be presumed as, say, an earthquake-triggered volcanic gas pressure increase. At a benchmark located nearby a Sacks-Evertson borehole volume-strainmeter station, we found an interesting fact that a gravity change is proportional to a volumetirc strain change with a rate of 1.4 × 10−6/μgal. This fact may suggest that the gravity increase is probably induced by a density increase due to a postseismic crustal compression started immediately after the 1978 Izu-Oshima Kinkai earthquake of M7.0.