The 2021 Semeru volcano eruption: An insight from visual, seismic, and deformation monitoring data

Abstract

Semeru is the highest volcano on the island of Java and one of Indonesia’s volcanoes that has a potential threat of pyroclastic flows. The eruption of Semeru often shows a transition pattern between explosive and effusive styles. An increased eruptive activity occurred in 2021 with series of explosions dominated the first two thirds of the year and then transition to series of pyroclastic flows in December 2021. On 4 December 2021, a dome collapse triggered significant pyroclastic flows that reached a maximum distance of 16 km from the crater and resulted in 51 casualties. In this paper, we aimed to study the dynamics of magma movement from depth to the surface and assessing the style transition of the eruptions between explosive and effusive activity from visual, seismic and deformation monitoring data. We analyzed Semeru volcano monitoring data throughout the year 2021. The volcanic activity monitoring system of Semeru volcano consists of 4 seismic stations, 5 deformation stations (2 tiltmeters and 3 GPS stations), and 2 Web Camera/CCTV. Our analysis on seismic data indicates that the series of large pyroclastic flows were triggered by excess pressure at shallow depths a few hours before the events. Deep volcanic earthquakes are relatively increased after the collapse of the lava dome or pyroclastic flows, possibly caused by the sudden decrease in hydrostatic pressure of the rock mass around the magma pocket, thus triggering gas expansion. Deformation monitoring using a tiltmeter at Argosuko and Jawar stations indicates inflation of a deep source since 15 August 2021. Since 7 November 2021, tiltmeter measurement has shown deep source deflation patterns which indicate magma movement from the deep reservoir to a shallower reservoir. After 21 November, inflation of the deep source was observed again, indicating an increase in magma supply from the deep pocket. The results of the pressure source modeling from GPS vector data in the period before the eruption, 1 November – 4 December 2021, showed an anomaly in the form of an increased volume at a depth of > 1.7 km of about 0.84 million/m3. Between 5 December and 31 December 2021, after the 4 December pyroclastic flow, the deformation modeling indicates the transition of the pressure source from a depth of 1.7 km to 8 km. In addition, we also observed a decrease in volume of 5.6 million/m3. The deformation pattern at shallow depths showed a deflationary pattern indicating a decrease in magma overpressure.