Wavefield Modeling and Analysis of Lightning Quakes Measured by a Distributed Acoustic Sensing Array

Abstract

Lightning-induced seismic waves, termed “lightning quakes,” are frequent natural sources in many storm-prone regions. Lightning quakes have been clearly observed in numerous environments by both seismic and acoustic instruments, for example, by distributed acoustic sensing (DAS) array. Despite these numerous observations, the physical nature of lightning quake wavefields detected by ground-based arrays remains poorly understood. The possibility of electroseismic (ES) conversion due to lightning’s powerful electromagnetic fields was, until now, unstudied. This investigation uses 3D numerical simulations of acoustic-to-seismic and ES wavefields alongside a novel data-driven azimuthal strain-rate variation analysis technique to robustly reveal the complex nature of lightning quakes. We show that lightning quakes begin as airborne acoustic waves before coupling with the solid earth as air-coupled Rayleigh waves and Love waves that are generated by local sources near the receiver, such as topography or urban infrastructure. These conclusions suggest thunder observations from a DAS array can be used to infer the structure of the near surface around the receiver, but care needs to be taken in understanding the role of local sources. An estimate of the Rayleigh- and Love-wave phase velocities is produced using a novel data analysis method unique to DAS. Furthermore, we demonstrate that electroseismic coupling does not play a significant role in the lightning quake wavefields. Although these simulations do not fully capture the realistic frequency of the electroseismic coupled wavefield, theory suggests that the wavefield is high frequency and thus quickly attenuated in the saturated near-surface soils. Electroseismic coupled wavefields from lightning may be detectable very close to the bolt.