THE EXCITATION AND CHARACTERISTIC FREQUENCY OF THE LONG-PERIOD VOLCANIC EVENT - AN APPROACH BASED ON AN INHOMOGENEOUS AUTOREGRESSIVE MODEL OF A LINEAR DYNAMIC SYSTEM

We present a method to quantify the source excitation function and cha racteristic frequencies of long-period volcanic events. The method is based on an inhomogeneous autoregressive (AR) model of a Linear dynami c system, in which the excitation is assumed to be a time-localized fu nction applied at the beginning of the event. The tail of an exponenti ally decaying harmonic waveform is used to determine the characteristi c complex frequencies of the event by the Sompi method. The excitation function is then derived by operating an, AR filter constructed from the characteristic frequencies to the entire seismogram of the event, including the inhomogeneous part of the signal. We apply this method t o three long-period events at Kusatsu-Shirane Volcano, central Japan, whose waveforms display simple decaying monochromatic oscillations exc ept for the beginning of the events. We recover time-localized excitat ion functions lasting roughly 1 s at the start of each event and find that the estimated functions are very similar to each other at all the stations of the seismic network for each event. The phases of the cha racteristic oscillations referred to the estimated excitation function fall within a narrow range for almost all the stations. These results strongly, suggest that the excitation and mode of oscillation are bot h dominated by volumetric change components. Each excitation function starts with a pronounced dilatation consistent with a sudden deflation of the volumetric source which may be interpreted in terms of a choke d-how transport mechanism. The frequency and Q of the characteristic o scillation both display a temporal evolution from event to event. Assu ming a cra;ck filled with bubbly water as seismic source for these: ev ents, we apply the Van Wijngaarden-Papanicolaou model to estimate the acoustic properties of the bubbly liquid and find that the observed ch anges in the frequencies and Q are consistently explained by a tempora l change in the radii of the bubbles characterizing the bubbly water i n the crack.