Source Properties and Triggering Processes of Long‐Period Events Beneath Volcanoes Inferred From an Analytical Formula for Crack Resonance Frequencies

Abstract

Long‐period (LP) seismic events are thought to arise from the oscillations of a fluid‐filled resonator. The fluid properties and resonator geometry of LP sources have been estimated by comparing observed frequencies to those calculated from a crack model. However, simultaneous determinations of crack geometry and fluid properties have been based on time‐consuming crack model simulations. We developed an alternative method to estimate these crack model parameters using a recently proposed analytical formula. We applied it to LP events at Kusatsu‐Shirane volcano, Japan and Galeras volcano, Colombia, which have been thought to be generated by resonances of cracks filled with misty and dusty gases, respectively. Our estimates of the crack model parameters displayed temporal changes in both crack geometry and fluid properties, which previous studies have not shown. Our estimated parameters had common features at the two volcanoes: (1) Crack volumes and gas‐weight fractions of water vapor in the cracks were positively correlated, and (2) mist and dust volumes were proportional to the product of crack aperture and crack length or width. From these features, we inferred that the LP events were triggered by volumetric changes associated with condensation of water vapor exsolved from magma at Kusatsu‐Shirane and by fragmentation of vesiculated magma intruded into the crack at Galeras. This study demonstrates a useful new method to investigate the state of fluids and triggering processes of LP events.

Pressure-Driven Opening and Filling of a Volcanic Hydrofracture Recorded by Tuffisite at Húsafell, Iceland: A Potential Seismic Source

“…This particle size broadly matches that of the Húsafell tuffisite, as well as other documented tuffisite veins for which more detailed particle size analysis has been conducted (e.g., Saubin et al, 2016;Heap et al, 2019). A similar model has been proposed at Galeras volcano, with LP events suggested to represent the propagation and increase in volume of a vertical crack injected by a gas-ash mixture (Taguchi et al, 2018). Both the crack volume and mass fraction of gas within the crack were inferred to have decreased as ash was deposited and welded before the next LP event (Taguchi et al, 2021).…”

Resonances of Fluid‐Filled Cracks With Complex Geometry and Application to Very Long Period (VLP) Seismic Signals at Mayotte Submarine Volcano

“…However, it is possible that the first two spectral peaks in the seismic data do not correspond to the fundamental mode and first higher mode if any of them are not properly excited. For instance, previous studies of VLP and LP events (Kumagai, 2006;Taguchi et al, 2018Taguchi et al, , 2021 indicated that the observed lowest frequencies were not the fundamental mode. To examine this hypothesis, we compute the frequency ratios between the first 5 resonant modes and the ratios between the first 4 longitudinal modes of a rectangular crack with varying C L and aspect ratios α (see Tables S7-S10 in the Supporting Information S1).…”