Lightning-Induced Implosive Microseismic Events at Depth Detected by Downhole Distributed Acoustic Sensing in Taiwan

Abstract Cloud-to-ground (CG) lightning can generate seismic waves that travel along the ground surface, but direct evidence for its subsurface energy dissipation has been lacking. Here, we report seismic impulsive waveforms detected by a distributed acoustic sensing (DAS) system deployed in a borehole in Taiwan, immediately following a CG lightning-strike time, which may be induced by the CG lightning. The strain-rate waveforms exhibit only one distinct seismic phase, with moveouts propagating toward the surface between DAS channels at depths of 50–86 m, and another set propagating downward between channels at depths of 86–140 m. The maximum peak amplitude corresponds to a strain rate as low as ∼3×10−6·s−1. The wave amplitudes show strong attenuation during propagation. Analysis of the source mechanism for the observed seismic impulses using the finite-difference strain-rate simulation technique indicates that these waves were generated by an implosive event of magnitude Mw−1.5 to −1.0 (Mxx=Myy=Mzz<0, all other components = 0), located ∼10 to 20 m from the borehole at a depth of 84 m. Our simulations of the implosive source also explain the unique strain-rate polarities observed, with downward first motion at channels closer to the source and upward first motion at channels farther away. These results provide the first direct evidence that lightning’s subsurface impact can penetrate to depths far greater than previously recognized, highlighting the need to further investigate the underlying physical mechanisms.