Can Seismic Event Size (Mw) be Measured with Distributed Acoustic Sensing (DAS)?

Abstract:

During a geothermal monitoring experiment at Brady Hot Springs, Nevada both Quantitative Distributed Acoustic Sensing (DAS) and nodal geophones were deployed on the surface for two weeks in March 2016. DAS array consisted of ~8 km of fiber-optic cable deployed in zig-zag pattern in shallow trench and the conventional seismometer array was composed of shallowly buried three-component geophones (Fairfield Nodal systems). Both DAS and geophones recorded continuously for fifteen days during which two moderate-sized earthquakes with a local magnitude of Mw=4.3 (3/21/16) and Mw=4.1 (3/22/16) were recorded. The epicenters of both events were at similar epicentral distances (129 km and 156 km) and backazimuth (172° and 159°) with respect to Bradys Hot Springs. These earthquakes provided a unique opportunity to investigate coda amplitude response similarity of envelopes between DAS and geophone and potential to estimate Mw using coda waves. Several DAS line segments with co-located geophone stations were used to compare the amplitude variation using narrow-band S-wave coda envelopes. Initial calculations of DAS coda envelope decay at each point showed remarkable similarity with geophone coda envelopes at all frequencies. Final source spectra and Mw matched source spectra from regional stations without any major scatter. As coda waves arrive from a range of azimuths, the azimuthal sensitivity of DAS is somewhat ameliorated. This purposely recorded quantitative DAS data was compared to opportunistic field data from 2009 recorded in an installed Intensity only DAS system deployed in Turkey for pipeline monitoring. We tested the pipeline data from Turkey combined with seismic stations of national seismic network at Kandilli Observatory and Earthquake Research Institute (KOERI).