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Anatahan Island is located at the southern end of the Mariana volcanic chain. On May 10, 2003, the eastern crater of the island erupted for the first time in recorded history. The Plinian eruption column reached an altitude as high as 13 km on May 11, and a thick layer of ash covered the island. The eruptive activity continued to June 2003, but most of the erupted material was expelled during the first week. The volcanic activity declined in the second half of 2003, but resumed in April 2004. In order to determine crustal deformation associated with the eruption, we conducted GPS measurements in July 2003 at a benchmark (ANAT) located approximately 7 km west–northwest of the active crater, where GPS campaign measurements had been repeated four times since 1992. In the period from January to July 2003 during the eruption, significant subsidence – as much as 21 cm – was detected, but horizontal movement was negligible. We began taking continuous GPS measurements at the same site in July 2003 to monitor the transient deformation that was probably associated with magma migration. To assess the spatial extent of the deformation more accurately, we established another permanent GPS site (ANA2) at a site approximately 3 km from the active crater in the northeastern part of the island in January 2004. The coordinates of this time series at ANAT probably show a change in trends at the beginning of 2004. Another subsidence of 2.8 cm and a westward motion of 2.1 cm were estimated to have occurred in the period from July to December 2003. This was followed by an uplift of 5.2 cm and movement in an eastward direction of 1.0 cm in the period from January to June 2004. We developed three preliminarily models of inflation/deflation sources for three different time periods. During the period from January to July 2003, a deformation source was located beneath the ANAT site and acted as a deflation source. Considering the gap in the GPS time series and errors in data (especially after July 2003), we expected that the deformation sources were located beneath the western part of Anatahan Island and not below the active crater.

[1] Repeated GPS surveys in the Mariana Islands show that the Mariana block is moving apart from the Philippine Sea plate. The velocities of the islands relative to the Philippine Sea plate range from about 15 mm/yr in the northern islands to about 45 mm/yr near Guam. The data also suggest convergence rates for the Mariana forearc with respect to the Pacific plate of 35–45 mm/yr at 19°N increasing to 55–70 mm/yr at 13.5°N. In addition, the velocity vectors show a slight north-south expansion of the arc. The estimated location of the Euler pole of the Mariana forearc with respect to the Philippine Sea plate is well south of the geographical point where the back-arc basin narrows to zero width.