Finite Difference Modeling of Seismic Wave Resonance and Vertical Reverberation in the Toba Volcanic Caldera

Abstract

The Toba volcanic caldera represents a highly heterogeneous volcanic system where seismic wave propagation is strongly influenced by complex subsurface structures. This study presents an integrated analysis of resonance and wavefield evolution using the Finite Difference Time Domain (FDTD) method combined with observed seismic data. A physics‑based subsurface model derived from the CRUST 1.0 dataset incorporates spatial variations in P‑wave velocity (Vp), S‑wave velocity (Vs), and density (𝜌). Frequency‑domain analysis reveals a dominant resonance band at 1.7‑1.9 Hz, in close agreement with theoretical predictions from a layered medium, with deviations below 10%, highlighting the primary control of near‑surface sediment thickness and shear‑wave velocity. Time‑domain simulations further exhibit prolonged coda waves, particularly in the central caldera, indicating efficient energy trapping and sustained multiple reflections. Cumulative energy analysis confirms that seismic energy is not rapidly attenuated but redistributed over time within the subsurface structure. Importantly, this study demonstrates that the observed wavefield complexity is governed by the coupled effects of structural resonance and vertical reverberation rather than true lateral scattering, aligning with the limitations of 1D horizontal layer extensions. By integrating frequency and time‑domain perspectives within a unified FDTD framework, this work provides new insights into seismic wave behavior in volcanic basins and offers a robust basis for improving seismic hazard assessment in the Toba caldera region.