Anatahan, Northern Mariana Islands: Reconnaissance geological observations during and after the volcanic crisis of spring 1990, and monitoring prior to the May 2003 eruption

Subsequent 1- to 2-day visits occurred in October 1990, May 1992, May 1994, and June 2001. Activity within the geothermal pits was relatively constant during every visit, although during this 11-year period the level of the water in each pit decreased. In June 2001, a ∼50-m-wide region of mud pots and steaming ground in the central part of the geothermal area had developed. No geologic evidence, however, suggested that an eruption would occur < 2 years afterward. Most of the EDM lines showed slight extension between late 1990 and 1992, followed by very gradual contraction from 1992 to 2001. A more extensive seismic-monitoring system was installed on the Northern Mariana Islands during these visits, and it recorded a small seismic swarm at Anatahan from May to July 1993. The telemetry component of the seismic equipment broke prior to 2001 and had not been repaired by the time of the May 2003 eruption, so no precursory seismic data were recorded to indicate pre-eruption unrest.