Sensing Optical Fibers for Earthquake Source Characterization Using Raw DAS Records

Distributed Acoustic Sensing (DAS) is becoming a powerful tool for earthquake monitoring, providing continuous strain-rate records of seismic events along fiber optic cables. However, the use of standard seismological techniques for earthquake source characterization requires the conversion of data in ground motion quantities. In this study we provide a new formulation for far-field strain radiation emitted by a seismic rupture, which allows to directly analyze DAS data in their native physical quantity. This formulation naturally accounts for the complex directional sensitivity of the fiber to body waves and to the shallow layering beneath the cable. In this domain, we show that the spectral amplitude of the strain integral is related to the Fourier transform of the source time function, and its modeling allows to determine the source parameters. We demonstrate the validity of the technique on two case-studies, where source parameters are consistent with estimates from standard seismic instruments in magnitude range 2.0-4.3. When analyzing events from a 1-month DAS survey in Chile, moment-corner frequency distribution shows scale invariant stress drop estimates, with an average of Delta sigma = (0.8 +/- 0.6) MPa. Analysis of DAS data acquired in the Southern Apennines shows a dominance of the local attenuation that masks the effective corner frequency of the events. After estimating the local attenuation coefficient, we were able to retrieve the corner frequencies for the largest magnitude events in the catalog. Overall, this approach shows the capability of DAS technology to depict the characteristic scales of seismic sources and the released moment. A new formulation for far-field strain radiation from seismic ruptures is derived, leading to a direct interpretation of Distributed Acoustic Sensing (DAS) data to retrieve source properties (seismic moment and source size), via a spectral modeling. This approach is validated on real data recorded in two different tectonic environments, the Chilean margin and the southern Apennines, in Italy. Despite the unique directional sensitivity and peculiar signal characteristics, we demonstrated the high potential of DAS systems in characterizing the seismic ruptures over different space scales, with accuracy increased by redundancy of information from the very-high spatial resolution in the recording of seismic waves. A theoretical description of strain far field radiation from a seismic rupture is introduced Source parameters were evaluated from strain data for earthquakes in magnitude range 2.0-4.3 Distributed Acoustic Sensing allows for investigation of source parameters and site effects with fine spatial resolution