Abstract
Since the caldera-forming eruption of Miyakejima Volcano in 2000, low-frequency (LF) earthquakes have occurred frequently beneath the caldera. Some of these LF earthquakes are accompanied by emergent infrasonic pulses that start with dilatational phases and may be accompanied by the eruption of small amounts of ash. The estimated source locations of both the LF earthquakes and the infrasonic signals are within the vent at shallow depth. Moreover, the maximum seismic amplitude roughly correlates with the maximum amplitude of the infrasonic pulses. From these observations, we hypothesized that the infrasonic waves were excited by partial subsidence within the vent associated with the LF earthquakes. To verify our hypothesis, we used the infrasonic data to estimate the volumetric change due to the partial subsidence associated with each LF earthquake. The results showed that partial subsidence in the vent can well explain the generation of infrasonic waves.
Highlights
Miyakejima Island is one of the Izu Islands in the northern Izu-Bonin volcanic arc
At Miyakejima, two types of earthquakes are accompanied by infrasonic waves: eruptions, which are associated with continuous tremors, and low-frequency (LF) earthquakes, which are accompanied by waves such as those described in the present study
The LF earthquakes at Miyakejima can be characterized as follows: LF earthquakes occasionally occur after MF microearthquake swarms
Summary
Miyakejima Island is one of the Izu Islands in the northern Izu-Bonin volcanic arc. It is about 200 km SSW of Tokyo and is the site of one of the most active stratovolcanoes in Japan. The latest large eruption at Miyakejima occurred in August 2000. After three large eruptions in August 2000, large amounts of volcanic gas, especially SO2, began to be emitted from the vent, which is near its southern margin within the caldera. These emissions are ongoing (Kazahaya et al 2004)
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