Wavefield properties of a shallow long-period event and tremor at Kilauea Volcano, Hawaii

The wavefields of tremor and a long-period (LP) event associated with the o ngoing eruptive activity at Kilauea Volcano, Hawaii, are investigated using a combination of dense small-aperture (300 m) and sparse large-aperture (5 km) arrays deployed in the vicinity of the summit caldera. Measurements of azimuth and slowness for tremor recorded on the small-aperture array indic ate a bimodal nature of the observed wavefield. At frequencies below 2 Hz, the wavefield is dominated by body waves impinging the array with steep inc idence. These arrivals are attributed to the oceanic microseismic noise. In the 2-6 Hz band, the wavefield is dominated by waves propagating from sour ces located at shallow depths (<1 km) beneath the eastern edge of the Halem aumau pit crater. The hypocenter of the LP event, determined from frequency -slowness analyses combined with phase picks, appears to be located close t o the source of tremor but at a shallower depth (<0.1 km). The wavefields o f tremor and LP event are characterized by a complex composition of body an d surface waves, whose propagation and polarization properties are strongly affected by topographic and structural features in the summit caldera regi on. Analyses of the directional properties of the wavefield in the 2-6 Hz b and point to the directions of main scattering sources, which are consisten t with pronounced velocity contrasts imaged in a high-resolution three-dime nsional velocity model of the caldera region. The frequency and Q of the do minant peak observed in the spectra of the LP event may be explained as the dominant oscillation mode of a crack with scale length 20-100 m and apertu re of a few centimeters filled with bubbly water. The mechanism driving the shallow tremor appears to be consistent with a sustained excitation origin ating in the oscillations of a bubble cloud resulting from vesiculation and degassing in the magma. (C) 2001 Elsevier Science B.V. All rights reserved .