MECHANISMS FOR GROUNDWATER LEVEL CHANGE DURING THE 2000 UNREST OF MIYAKE-JIMA

During the 2000 volcanic unrest at Miyake-jima groundwater levels in three wells tapping unconfined aquifers on the island changed. Changes in rainfall patterns, boiling away of recharge, vertical displacement of the ground surface relative to sea level, and volumetric strain could cause groundwater level variations. We compared tidally corrected water levels with average monthly rainfall values, seismic data, GPS data, and tilt data to determine which factors could have acted in 2000. Changes in rainfall patterns, boiling away of recharge, and changes in aquifer characteristics did not cause the detected water level changes. Groundwater at Miyake-jima shows response to both vertical displacement of the ground surface relative to sea level and volumetric strain. The groundwater levels respond to vertical displacement of the ground surface because the water table equilibrates to the new apparent sea level. Strain responses in unconfined aquifers are unusual, but in this case the volumetric strain changes are large enough to produce significant water level response. Groundwater level monitoring is not a replacement for traditional geodetic monitoring methods such as tiltmeters, strainmeters, and GPS stations. However, not all volcano observatories can afford dense geographic coverage of these costly instruments.