Abstract
Magnitude–frequency distributions of volcanic explosion earthquakes that are associated with occurrences of vulcanian and strombolian eruptions, or gas burst activity, are examined at six active volcanoes. The magnitude–frequency distribution at Suwanosejima volcano, Japan, shows a power-law distribution, which implies self-similarity in the system, as is often observed in statistical characteristics of tectonic and volcanic earthquakes. On the other hand, the magnitude–frequency distributions at five other volcanoes, Sakurajima and Tokachi-dake in Japan, Semeru and Lokon in Indonesia, and Stromboli in Italy, are well explained by exponential distributions. The statistical features are considered to reflect source size, as characterized by a volcanic conduit or chamber. Earthquake generation processes associated with vulcanian, strombolian and gas burst events are different from those of eruptions ejecting large amounts of pyroclasts, since the magnitude–frequency distribution of the volcanic explosivity index is generally explained by the power law.
Highlights
Volcanic explosion earthquakes are earthquakes that are observed in vulcanian and strombolian eruptions, or gas bursts, which often accompany strong air shocks or infrasonic signals
A pioneer study by Minakami (1960) first defined EXs, classifying volcanic earthquakes into four types: A, B-type, explosion earthquake, and tremor. He indicated that EXs at Sakurajima, Japan, follow Ishimoto–Iida’s formula of n(A) = kA−m, where n(A) represents the number of earthquakes with a maximum amplitude of A, m is the coefficient called Ishimoto–Iida’s m value, and k is a proportional coefficient (Ishimoto and Iida 1939). This relationship is linked to the Gutenberg–Richter equation (G–R eq.) that explains the magnitude–frequency distribution of earthquakes in general, and the m value is related to the b value in the G–R eq as m = b + 1 (Asada et al 1950; Suzuki 1953)
Magnitude–frequency distributions of EXs associated with strombolian eruptions at AkitaKomagatake, Japan, were explained by combinations of two power laws using two m values (Tanaka et al 1972)
Summary
Volcanic explosion earthquakes (hereafter termed EXs) are earthquakes that are observed in vulcanian and strombolian eruptions, or gas bursts, which often accompany strong air shocks or infrasonic signals. The EXs we analyze are those associated with vulcanian and strombolian eruption styles and gas bursts at Sakurajima, Suwanosejima, and Tokachi-dake in Japan, Semeru and Lokon in Indonesia, and Stromboli in Italy.
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