Earth Planets Space, Vol. 52 (No. 8), pp. xix-xxvi, 2000

The 2000 Miyakejima eruption: Crustal deformation and earthquakes observed by the NIED Miyakejima observation network

Motoo Ukawa¹, Eisuke Fujita¹, Eiji Yamamoto¹, Yoshimitsu Okada¹, and Masae Kikuchi²

¹National Research Institute for Earth Science and Disaster Prevention, Tsukuba, Ibaraki 305-0006, Japan
²Association for the Development of Earthquake Prediction, Tokyo 101-0064, Japan

(Received September 7 , 2000; Revised September 11, 2000; Accepted September 11, 2000)

Abstract: The Miyakejima observation network had been constructed by the National Research Institute for Earth Science and Disaster Prevention mainly until early 1999. This observation network has provided the crustal deformation data by tiltmeters and GPS and the seismic data by short-period and broadband seismometers in association with the 2000 Miyakejima eruption. The subsurface magma movement at the first stage of the present activity, during the period from June 26 to 27, was successfully detected mainly by the tilt measurements. The tilt change observed at five stations indicates the migration of magmas from the eastern part of Miyakejima to the western part. The most distinctive phenomenon appearing after the first stage is tilt steps, which started on July 8 with the first eruption from the summit crater. Each tilt step indicates an abrupt uplift of the summit area. These tilt steps continued until the eruption of August 18, which is the largest eruption up to early September, 2000. 45 tilt steps in total were observed in this period. The seismic data show a variety of seismograms including VT (volcano-tectonic) earthquakes, LF (low frequency) earthquakes and volcanic tremor. At the time of the tilt steps, very long period events with predominant periods of about 100 s were detected by the broadband seismometers. As the activity has still continued, this report summarizes the observation during June, July, and August, 2000.