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

Abstract

AbstractLong‐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.