Characterizing Rupture Directivity of Small Earthquakes with Distributed Acoustic Sensing

Rupture directivity is a fundamental property of earthquake source dynamics, where seismic waves display higher amplitudes and richer high frequency content in the direction of rupture propagation, and lower amplitudes and lower frequency content in the opposite direction. Characterizing this behavior offers important insights into the physical processes of rupture kinematics and contributes to seismic hazard assessment. Although small earthquakes are known to exhibit directivity, resolving their patterns has been limited by the relatively low spatial density and restricted azimuthal coverage of conventional seismic arrays. The emergence of Distributed Acoustic Sensing (DAS) significantly overcomes these limitations by providing continuous measurements over tens of kilometers, yielding both higher spatial density and improved azimuthal sampling of the wavefield. This work presents the first systematic investigation of rupture directivity using DAS alongside the dense Israeli Seismic Network, focusing on two small repeating Mw 3.3 and Mw 2.8 earthquakes recorded along a 66-kilometer DAS fiber and 26 accelerometers. We calculated relative source spectra using the spectral ratios technique and extracted the corner frequencies of the larger event. DAS measurements yield significantly smaller uncertainties compared to accelerometers, suggesting that dense fiber networks can capture directivity effects even for weak or complicated rupture patterns. We found significant azimuthal variation in S-wave corner frequencies, with systematically higher corner frequencies toward the ENE. The observed patterns indicate distinct rupture directivity, demonstrating that DAS alongside a dense seismic network can resolve such signatures and improve characterization of source complexity.