Inflations prior to Vulcanian eruptions and gas bursts detected by tilt observations at Semeru Volcano, Indonesia

We examine the basic characteristics of inflations at Semeru Volcano, Indonesia, to clarify the pressurization process prior to two different styles of explosive eruptions: Vulcanian eruptions and gas bursts. Analysis of data obtained from tilt meters installed close to the active crater allows clarification of the common features and the differences between the two styles of eruptions. To improve the signal-to-noise ratio and to determine the mean characteristics of the inflations, we stack tilt signals obtained from eruptions of different magnitudes and evaluate the maximum amplitude of the seismic signal associated with these eruptions. Vulcanian eruptions, which explosively release large amounts of ash, are preceded by accelerating inflation about 200-300 s before the eruption, which suggests volume expansion of the gas phase. In contrast, gas bursts, which rapidly effuse water steam accompanied by explosive sounds, follow non-accelerating changes of inflation starting 20 s before each emission. Tilt amplitudes increase with the magnitude of eruptions for both eruption styles. This suggests that the volume and/or pressure of magma or gas stored in the conduit before eruptions controls the magnitude of volcanic eruptions. These results further suggest that the magnitude of eruptions can be predicted from geodetic measurements of volcano inflation.