Seismic activity and gravity field interpretation for subsurface fault detection on Mount Pandan, Indonesia

Abstract

Indonesia experiences intense volcanic and tectonic activity due to its strategic location between 6° N–11° S latitude and 95° E–141° E longitude, where several major tectonic plates converge. Physiographically, Mount Pandan is located in the modern Sunda Arc region within the Anticlinorium or Kendeng Zone. Previous research recorded a minor earthquake in 2016, indicating ongoing tectonic activity in the Kendeng Zone, connected to sub-magmatic activity observed in several hot springs in Banyukuning, Jari, and Selogajah. Using gravity data, we identify the relationship between tectonic and magmatic activity through 3D inversion modelling of subsurface structures. This analysis correlates with fault fracture density (FFD) for surfaces with faults or fractures. Identifying the focal mechanism is essential for constructing the fault model of the earthquake source. The movement of Earth’s crust along the Kendeng Fault influences underlying magmatic processes. Evidence of this interaction includes low-density zones and sub-magmatic features, such as the presence of hot springs. Earthquakes around the mountain with magnitudes below 4.0 SR suggest a relationship between the movement of strike-slip faults and oblique reverse faults with magma ascent. 3D inversion modelling reveals four layers:

• At a depth of 0–0.46 km, with an estimated density range of 1.69–2.69 g/cm³, we interpret the layer as caprock, composed of pyroxene and host rock types.
• The layer at 0.46–1.14 km, with a density range of 1.31–2.23 g/cm³, is interpreted as a reservoir containing sand and clay rock types.
• Andesite and volcanic breccia rocks make up the layer at 1.14–1.59 km, with a density range of 2.4–2.8 g/cm³. It is thought to be caprock and intrusion (active fault).
• We interpret the layer at 1.59–2.43 km, with a density range of 1.43–3.45 g/cm³, as a heat source with basalt rock types and magma content.

These findings provide new insights into the subsurface structure and fault dynamics of the Kendeng Zone, contributing to a better understanding of tectono-magmatic interactions in seismically active regions.