Excitation of Long-period (LP) Events at Galeras Volcano, Colombia, Inferred from Crack Model Simulations

Abstract:

Long-period (LP) seismic events observed at active volcanoes are thought to arise by the fluid pressure change in fluid-filled resonators. The frequencies and Q factors of observed LP waveforms vary depending on the resonator geometry and fluid properties, and the observed spectra change with the excitation defined by the area where the fluid pressure change occurred. Recently, a method based on analytical formulas for crack resonance frequencies and Q factors was developed to estimate the resonator geometry and fluid properties simultaneously. Furthermore, this method enables us to identify crack resonance modes for observed spectral peaks, which leads to constrain the excitation of LP events. We applied this method to LP events at Galeras volcano, Colombia, in 1993 and between 2006 and 2007, and investigated the excitations with numerical simulations of the crack model defined by length (L), width (W), and aperture (d). The vertical crack containing dusty gas was assumed for the sources of the LP events at Galeras. The resonance modes for the observed spectra showed the following two features. First, for the lowest peak frequencies, the modes with the wavelengths 2L/3 and 2L/5 were identified in 1993 and between 2006 and 2007, respectively. Second, the modes with wavelengths 2L/5 and 2W/3 and those with 2L/3 and 2W/3 were not identified in 1993 and between 2006 and 2007, respectively, for any observed spectral peaks. We found that the two features can be reproduced in the simulated spectra when we applied the pressure change extended across the entire crack width apart from the lower edge by 1/8 and 1/4 of the crack length for the events in 1993 and those between 2006 and 2007, respectively. At Galeras, LP events are thought to occur by fragmentation of magma intruded into the crack along the conduit in the lava dome. Estimated crack volume and gas-weight fraction of dusty gas between 2006 and 2007 showed temporal decreasing trends similar to those in 1993, even though the source crack was different due to the destruction of the lava dome by a Vulcanian explosion in 1993. Considering these similarities and the estimated excitations, we inferred that the LP events were triggered when the top part of the intruded magma was fragmented, in which the fragmentation heights from the lower crack edge were different in our two analysis periods.