Unveiling a blind fault between Mts. Sinabung and Sibayak in North Sumatra (Indonesia): insight from integrated analysis of seismicity and satellite data

Abstract The earthquake swarms that occurred before Mt. Sinabung’s largest eruption on February 19, 2018, were concentrated between Mts. Sinabung and Sibayak at -1 to 5 km depth. We present an interdisciplinary analysis of travel time tomography, satellite-derived Global Gravity Model plus (GGMplus), Synthetic Aperture Radar (SAR), and optical satellite multispectral data to investigate the origin of the earthquake swarms. The earthquakes were categorized into five clusters based on residual anomaly. The origins of earthquake swarms in Z1, Z2, and Z4 were clearly defined, while this study focused on analyzing the origins of Z3 and Z5. The lineament derived from the mSTA method reveals northwest-southeast fault traces that intersect with the river in zones Z3 and Z5. We found a river with the largest lateral shift of 384 m, located within an earthquake cluster in Z3, suggesting the existence of a blind fault. The tomography in Z3 showed a high Vp (+ 7.76%) and low Vp/Vs (1.62) from a depth of 0 to 2 km, which possibly represents dense volcanic rock. Southeast of the Z3 anomalies, a low Vp (− 3.45%) and high Vp/Vs (1.71) that continues to Mt. Sinabung at depths of 0–1 km are revealed, suggesting a porous rock. Therefore, we thought the earthquake swarm at Z3 indicates fault reactivation influenced by tectonic stress and magma migration of Mt. Sinabung, which may have caused the rocks to slip.