Tracking the evolution of the summit lava dome of Merapi volcano, between 2018-2019, using DEMs obtained from TanDEM-X and Pleiades data

Located about 30 km North of the city of Yogyakarta on Java island, Merapi is considered one of the most dangerous dome building stratovolcanoes, as about 2 million people live less than 30 km away from the crater. Its recent eruptive activity consists in cyclic effusive growth of viscous lava domes, followed by partial or total destruction of domes. Dome destruction favors gravitational collapses (VEI 2) every 4-5 years, or bigger explosive eruptions (VEI 3-4) every 50-100 years resulting in pyroclastic density currents (PDCs) driven downhill at high velocities that are a major risk for surrounding population. Therefore, it is crucial to assess precisely the location, the shape, the thickness, and the volume of emplaced lava in order to prevent populations from sudden PDCs.

The last major explosive eruption (VEI 3-4) occurred in November 2010, resulting in a horseshoe-shaped crater of 500 m wide and 250 m depth hosting a lava dome shaped like a plateau. Within the crater, a new dome appeared on 11 August 2018 and was partially destroyed as of late 2019. In this study, we take advantage of 2 high resolution remote-sensing datasets, Pléiades (optical acquisitions in tri-stereo mode, 1 m resolution) and TanDEM-X (radar acquisitions in StripMap mode, 2 m resolution), to produce 19 Digital Elevation Models (DEMs) between July 2018 and December 2019. We calculate the difference in elevation between each DEM and a reference DEM derived from Pléiades images acquired in 2013 in order to track the evolution of the dome in the crater between 2018 and 2019. Uncertainties are quantified for each dataset. We show that the DEMs derived from Pléiades (optical) and TanDEM-X (radar) data are consistent with each other and provide good spatio-temporal constraints on the evolution of the dome. Furthermore, the remote-sensing estimate of lava volume is consistent with local drone measurements carried on by BPPTKG at the time of dome growth.

The time period covered by the TanDEM-X data is larger than that covered by the Pléiades acquisitions, allowing coverage of the growth and destruction of the dome. However, the Pléiades data allow us to evidence an accumulation zone below the crater that is not well imaged by TanDEM-X. We show the dome reached 40 meters (+-5 m) high and 0.5 Mm³ (+- 0.1Mm³ ) between August 2018 and February 2019, corresponding to an effusion rate of 3000 m³/day. Its shape was initially radial,then extended asymmetrically to the northwest and southeast from October 2018. From February 2019 onwards, the dome elevation remained constant, but lava was continuously emitted, as evidenced by TanDEM-X amplitude maps. Lava supply was balanced by destabilization southwards downhill in an accumulation zone of 400 meters long and 15 meters (+-5m) high maximum. In late 2019, several minor explosions partially destroyed the center of the dome. This study highlights the strong potential of the combination of TanDEM-X and Pléiades DEMs to quantitatively monitor domes at andesitic stratovolcanoes.