No abstract

Summary Long-period (LP) seismic events at active volcanoes are thought to be generated by oscillations of fluid-filled resonators. The resonator geometry and fluid properties of LP sources have been estimated by comparing observed frequencies and quality factors (Q) with those calculated by numerical simulations with a crack model. A method to estimate all the parameters of crack geometry and fluid properties using an analytical formula for crack resonance frequencies has recently been proposed, but this method requires long computational times to compare observed and simulated Q values, especially for LP events with large Q. To resolve this problem, we used numerical simulations to systematically investigate the empirical relation between Q and crack model parameters. We found that Q can be calculated with an empirical formula expressed by the crack width-to-length ratio and the ratio of P-wave velocity in the solid medium to sound speed in the fluid. We applied this formula to LP events at Kusatsu-Shirane volcano, Japan, between August 1992 and January 1993 and at Galeras volcano, Colombia, in January 1993. Assuming misty gas as the fluid in the crack at Kusatsu-Shirane and dusty gas as the fluid at Galeras, the empirical formula provided more detailed estimates of the parameters than those obtained previously using the Q values estimated in numerical simulations. We then applied the empirical formula to LP events with large Q values observed at Galeras between December 2006 and January 2007. When we assumed dusty gas as the fluid in the crack, we found decreasing trends in both crack volume and the gas-weight fraction of water vapour in the crack. We also found that the dust volume was proportionally related to the product of crack aperture and crack length or width. These trends and relations were similar to those in January 1993, suggesting that the LP events at Galeras between 2006 and 2007 were triggered by the explosive fragmentation of intruded magma and the production of a dusty gas, as was previously inferred for the LP events in January 1993. Welding of ash in the dusty gas and dense magma remaining in the conduit after fragmentation led to a decrease in the source crack size prior to the next LP event. These results demonstrate that our empirical formula for Q can be used to estimate the source properties of LP events with large Q values without requiring long computational times. Use of the formula may thus contribute to improved monitoring of fluid states and understanding of LP triggering processes beneath many volcanoes.

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.

Summary Long-period (LP) seismic events have occurred repeatedly at Galeras volcano, Colombia, during the transition from effusive dome formation to explosive Vulcanian eruptions. Since 1989, two types of LP events have been observed there: one characterized by long-lasting, decaying harmonic oscillations (NLP events) and the other by non-harmonic oscillatory features (BLP events). NLP events are attributed to resonances of a dusty gas-filled crack in the magma plugging the eruptive conduit. Sixteen episodes of NLP events occurred at Galeras during 1992–2010, each characterized by systematic temporal variations in the frequencies and quality factors of NLP events. Our and previous estimates of crack model parameters during three of those NLP episodes indicate that the similar temporal variations in crack geometry and fluid properties can be explained by an increase in the ash content within the crack and a decrease in crack volume. We found that NLP events, associated with low SO2 fluxes, are anti-correlated with BLP events, which are accompanied by high SO2 emissions. From our observations and analytical results, we inferred that BLP events are generated by resonances of open cracks in the uppermost magma plug, corresponding to tuffisite veins, that efficiently transfer volcanic gases. After sufficient degassing and densification, the magma plug effectively seals the conduit. The growing overpressure in the deeper magma is then released through a shear fracture along the conduit margin. The intrusion of deeper, vesiculated magma into the shear fracture depressurizes and fragments the magma, producing a dusty gas and triggering the crack resonances that generate NLP events. Our results thus indicate that the evolution of the properties of the magma plug controls the occurrences of BLP and NLP events at Galeras. Although NLP events do not always precede explosive eruptions, they indicate that an important overpressure is building in the shallow conduit.