ARRAY MEASUREMENTS AND MODELING OF SOURCES OF SHALLOW VOLCANIC TREMOR AT KILAUEA VOLCANO, HAWAII

We show that resonances in near-surface layers (path effects), or a combination of near-surface resonances and source resonances (source effects), could be responsible for the sharp spectral peaks found in the records of gas-piston activity and volcanic tremor recorded near Puu Oo crater on Kilauea Volcano, Hawaii. Two types of sources are found to be compatible with observations and can not be distinguished using our data. In one model, impulsive, explosion point sources are embedded in the Puu Oo structure. In the other model, impulsive, point sources of pressure excite the resonances of a fluid-filled crack embedded in that structure. Both models require shallow sources (z < 100 m) and display a strong dependence of the radiated spectrum on source depth over a depth range comparable to the overall thickness of the surficial layers. Based on these results, it may be possible to track changes in source depth through temporal changes in observed spectra, a potentially useful tool for monitoring volcanic activity at this site. We estimate the depth and spatial extent of volcanic tremor and gas-piston activity using two dense arrays with respective apertures of 800 and 120 m located near Puu Oo. Measurements of slowness (ray parameter) and azimuth as a function of time clearly indicate that the sources of volcanic tremor and gas-piston activity are located beneath or in close proximity to the Puu Oo crater at depths shallower than approximately 1 km. Based on slowness and particle motion analyses we find that the records of volcanic tremor and gas-piston events at Puu Oo consist of a complex combination of body and surface waves.