In situ ground deformation measurements from the summit of axial volcano during the 1998 volcanic episode

Since September 1987, seafloor instrumentation monitoring ground deformatio n has been maintained on the summit of Axial Volcano on the central Juan de Fuca Ridge. During a January 1998 volcanic episode, pressure records from the central caldera showed a significant increase in pressure, indicating c aldera subsidence of over 3 m. This observed subsidence is interpreted to r epresent volcanic deflation as magma is erupted and/or injected into the vo lcanic flanks. Temperature anomalies of nearly 0.45 degrees C were also obs erved. A comparison of the subsidence record with hydroacoustic recordings indicates a continuous volcanic tremor signal coincident with the subsidenc e event. The time-coincidence of subsidence of the summit caldera with migr ation of epicenters into the volcanic rift zones confirms that during the 1 998 episode, the volcanic flanks were supplied by lateral dike injection fr om a central magma body, rather than locally derived magma sources.