Earthquake location based on Distributed Acoustic Sensing (DAS) as a seismic array

Abstract

We present a simple and fast method to estimate preliminary earthquake location coordinates using Distributed Acoustic Sensing (DAS). Strain rate data is recorded on a 25-km long fibre-optic cable located in the northern suburbs of Athens (Greece). We apply frequency - wavenumber analysis on two segments of the cable forming an L-shaped seismic array, which offers good azimuthal sensitivity, whereas, the spatial resolution (spacing) and signal coherency control the ability to reliably resolve the apparent slowness of an incoming wavefield, and hence, to estimate the hypocentral distance to the array. We attempt to locate a local earthquake (2021/9/23, $M_L$=3.4, Thiva) NW to the DAS array (approximately 50 km NW of Athens), and a regional earthquake (2021/10/12, $M_W$=6.3, off the East coast of Crete) using $S$-wave onsets filtered within two frequency bands (0.5 Hz - 2.5 Hz and 1.0 Hz - 5.0 Hz). The obtained backazimuths agree with the observed backazimuths based on the locations reported by the Institute of Geodynamics, National Observatory of Athens (NOA) for both earthquakes, with our location for the local earthquake being roughly 10 km South of the NOA location, whereas, the location that we obtained for the regional earthquake suggested larger errors in distance, projected in our slowness estimation, possibly due to the array spatial resolution and the complex structure of the Hellenic subduction zone.