Evidence for deformation associated with the 1998 eruption of Axial Volcano, Juan de Fuca Ridge, from acoustic extensometer measurements

Acoustic extensometer instruments capable of making precise daily measureme nts of horizontal distance were deployed across the north rift zone of Axia l Volcano in June 1996 and were in place when a submarine eruption began on Axial's south rift zone in January 1998. The instruments recorded a gradua l 9-cm extension over a 405-m baseline leading up to the eruption, and then an abrupt, 4-cm contraction at the time of the eruption. An elastic point- source deformation model shows that deflation of Axial's summit can explain both the 4-cm distance decrease at the extensometer: array and a 3.2-m sub sidence measured by another instrument in the caldera, if the pressure sour ce is located at a depth of 3.8 km below the center of the caldera. The 9-c m distance increase may represent pre-eruption spreading across the rift zo ne.