Interactions of magmatic intrusions with the multiyear flank instability at Anak Krakatau volcano, Indonesia: Insights from InSAR and analogue modeling

Abstract

Volcano flank collapses have been documented at ocean islands worldwide and are capable of triggering devastating tsunamis, but little is known about the precursory processes and deformation changes prior to flank failure. This makes the 22 December 2018 flank collapse at Anak Krakatau in Indonesia a key event in geosciences. Here, we provide direct insight into the precursory processes of the final collapse. We analyzed interferometric synthetic aperture radar (InSAR) data from 2014 to 2018 and studied the link between the deformation trend and intrusion occurrence through analogue modeling. We found that the flank was already moving at least 4 yr prior to collapse, consistent with slow décollement slip. Movement rates averaged ~27 cm/yr, but they underwent two accelerations coinciding with distinct intrusion events in January/February 2017 and in June 2018. Analogue models suggest that these accelerations occurred by (re)activation of a décollement fault linked to a short episode of magma intrusion. During intrusion, we observed a change in the internal faults, where the outward-directed décollement accelerated while inward faults became partially blocked. These observations suggest that unstable oceanic flanks do not disintegrate abruptly, but their collapse is preceded by observable deformations that can be accelerated by new intrusions.

Are eruptions reliable precursors to marine volcano collapses?

“…The most recent event occurred at Anak-Krakatau, a stratovolcano in the Sunda strait in Indonesia. Following an intense yet ordinary eruptive period of several months (Cutler et al, 2022), its southwestern flank -including the summit area-collapsed on 22 December 2018 (∼ 0.18-0.31 km 3 ) generating a maximum tsunami run-up of 13 m. While it was unclear whether the magmatic activity was directly leading to the collapse (Cutler et al, 2022), long-term deformation records indicated lateral motion for over a decade before the event (Zorn et al, 2023).…”

“…Giachetti et al (2012) studied volcanogenerated tsunamis triggered by a hypothetical collapse of Anak-Krakatau, highlighting the regional damage such an event would cause and emphasizing the need to include tsunami hazards in volcano monitoring. However, no warning was issued before the 2018 collapse of Anak-Krakatau (Zorn et al, 2023), which resulted in a tsunami and caused 437 deaths along nearby shores (Grilli et al, 2019;Walter et al, 2019). Despite a small earthquake with a local magnitude ML = 2-3 (Walter et al, 2019), there was no significant earthquake associated with the collapse that could trigger the warning system.…”

Potential Sunda Strait tsunami hazard due to the current deformation of Anak Krakatau

“…Following the 2018 tsunami, the topography in the AK vicinity has altered from 305 m to only 10 m[4], and the area has been decreased from 290.64 ha to 287 ha on December 2018. The western part of AK reduced as well from 339 m to 110 m [10].…”

“…The AK instability that produced the 2018 tsunami grew at the same rate as the AK from 1928 to 1981 [8] and had a similar form to the AK in 1950 [9]. The growth rate of AK is 8-33 cm/month, and some studies have shown that it has reached 40 cm/year over the last three years [10]. Following the 2018 tsunami, the topography in the AK vicinity has altered from 305 m to only 10 m[4], and the area has been decreased from 290.64 ha to 287 ha on December 2018.…”

Satellite Geodesy Unveils a Decade of Summit Subsidence at Ol Doinyo Lengai Volcano, Tanzania

“…Interferometric Synthetic Aperture Radar (InSAR) is a good alternative to obtain satellite geodetic measurements with a cm or even better accuracy (Hanssen, 2001). In particular, the processing of hundreds of SAR images in InSAR time series can uncover previously unknown deformation processes over several years to a decade, as shown recently in active volcanic areas in Guatemala (Gonzalez-Santana & Wauthier, 2021), the East African Rift (Albino & Biggs, 2021;Wauthier et al, 2018), and Indonesia (Zorn et al, 2023). However, the spatial resolution can be a limiting factor to obtain enough deformation measurements for highly localized deformation processes around or inside summit craters at volcanoes worldwide (Bemelmans et al, 2023;Pritchard et al, 2022;Richter et al, 2013).…”