The excitation and characteristic frequency of the long‐period volcanic event: An approach based on an inhomogeneous autoregressive model of a linear dynamic system

Abstract

Abstract. We present a method to quantify the source excitation function and characteristic frequencies of long-period volcanic events. The method is based on an inhomogeneous autoregressive (AR) model of a linear dynamic system, in which the excitation is assumed to be a time-locMized function applied at the beginning of the event. The tail of an exponentially decaying harmonic waveform is used to determine the characteristic complex frequencies of the event by the $ompi method. The excitation function is then derived by operating an AR filter constr•ucted from the characteristic frequencies to the entire seismogram of the event, including the inhomogeneous part of the signal. We apply this method to three long-period events at Kusatsu-Shirane Volcano, central Japan, whose waveforms display simple decaying monochromatic oscillations except for the beginning of the events. We recover time-localized excitation functions lasting roughly i s at the start of each event and find that the estimated functions are very similar to e•ch other at all the stations of the seismic network for each event. The phases of the characteristic 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 both 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 choked-flow transport mechanism. The frequency and Q of the characteristic oscillation both display a temporal evolution from event to event. Assuming a crack filled with bubbly water as seismic source for these events, we apply the Van Wijngaarden-Papanicolaou model to estimate the acoustic properties of the bubbly liquid and find that the observed changes in the frequencies and Q are consistently explained by a temporal change in the radii of the bubbles characterizing the bubbly water in the crack.

Glacier microseismicity

“…They are typically rich in frequencies below 5 Hz and often have a dominant spectral peak and harmonics suggesting a resonant source. Several source mechanisms have been proposed (e.g., Julian, 1994;Chouet, 1996;Nakano et al, 1998;Neuberg and O'Gorman, 2002;Fujita and Ida, 2003), all based on the interaction of fl uid with the surrounding rock.…”

Volcano Seismology

“…During this activity, LP events were frequently observed with hypocenters located several hundred meters below the summit crater (NAKANO et al, 1998). High-quality records of this LP seismicity were obtained in September-November 1992 from a network of seismometers surrounding the source region (NAKANO et al, 1998;KUMAGAI et al, 2002;NAKANO et al, 2002).…”

Temporal evolution of a hydrothermal system in Kusatsu‐Shirane Volcano, Japan, inferred from the complex frequencies of long‐period events

“…Although several oscillation modes are apparent in the f ‐ g diagrams (Figure 3) and spectra (Figure 4), we use the complex frequencies of the dominant peaks in Figure 4 corresponding to the clusters of points shown within ellipses in Figure 3, to quantify the temporal variations of complex frequencies. The selected mode corresponds to the mode analyzed in a previous study by Nakano et al [1998]. We use available waveforms from the seven stations for each event and determine the complex frequency of the dominant mode from each individual waveform.…”

“…It is now recognized that the complex frequencies of LP events may show spatial as well as temporal variations [ Lesage and Surono , 1995; Gil Cruz and Chouet , 1997; Nakano et al , 1998; Kumagai and Chouet , 1999], in which Q can have values ranging from tens to several hundred. Kumagai and Chouet [2000, 2001] presented a comprehensive description of the acoustic properties of a crack containing various types of fluids whose compositions are compatible with those expected for magmatic and hydrothermal fluids.…”

“…Geochemical studies provide further support for the presence of a hydrothermal reservoir beneath the summit, with hydrothermal activity resulting from the interaction of hot volcanic gases with groundwater [ Hirabayashi , 1999; Ohba et al , 2000]. LP events characterized by nearly monochromatic oscillatory signatures have been frequently observed at this volcano [ Hamada et al , 1976; Oikawa et al , 1993; Fujita et al , 1995], and temporal variations in the complex frequencies of these LP events have also been documented [ Nakano et al , 1998]. The observed temporal variations in complex frequencies may be directly related to the evolution of the hydrothermal system in response to magmatic activity.…”