Structural weakening of the Merapi dome identified by drone photogrammetry after the 2010 eruption

In: Natural hazards and earth system sciences: NHESS, Band 18, Heft 12, S. 3267-3281

Open Access

Abstract

Abstract. Lava domes are subjected to structural weakening that can lead to
gravitational collapse and produce pyroclastic flows that may travel up to
several kilometers from a volcano's summit. At Merapi volcano, Indonesia,
pyroclastic flows are a major hazard, frequently causing high numbers of
casualties. After the Volcanic Explosivity Index 4 eruption in 2010, a new lava dome developed on
Merapi volcano and was structurally destabilized by six steam-driven
explosions between 2012 and 2014. Previous studies revealed that the
explosions produced elongated open fissures and a delineated block in the
southern dome sector. Here, we investigated the geomorphology, structures,
thermal fingerprint, alteration mapping and hazard potential of the Merapi
lava dome by using drone-based geomorphologic data and forward-looking
thermal infrared images. The block on the southern dome of Merapi is
delineated by a horseshoe-shaped structure with a maximum depth of 8 m and it
is located on the unbuttressed southern steep flank. We identify intense
thermal, fumarole and hydrothermal alteration activities along this
horseshoe-shaped structure. We conjecture that hydrothermal alteration may
weaken the horseshoe-shaped structure, which then may develop into a failure
plane that can lead to gravitational collapse. To test this instability
hypothesis, we calculated the factor of safety and ran a numerical model of
block-and-ash flow using Titan2D. Results of the factor of safety analysis
confirm that intense rainfall events may reduce the internal friction and
thus
gradually destabilize the dome. The titan2D model suggests that a
hypothetical gravitational collapse of the delineated unstable dome sector
may travel southward for up to 4 km. This study highlights the
relevance of gradual structural weakening of lava domes, which can influence
the development fumaroles and hydrothermal alteration activities of cooling
lava domes for years after initial emplacement.