DAS experiments to earthquake detection in the National Seismic Network of Spain

The National Seismic Network of Spain has developed four experiments to test the capability of fibre optic monitoring as another technique to contribute to earthquake detection. All the tests used pre-installed fibre for other purposes and were in a variety of realms: along a city with high seismic hazard in Spain, offshore the Mediterranean Sea, and along two railways, one following a sedimentary basin and, the other crossing a mountain range. Data from two of the experiments are freely available to download. The analysis of recorded earthquakes by DAS (Distributed Acoustic Sensing) show that the sensitivity of the cables used are around a 30% for local and a 10% for teleseismic earthquakes from the number of events detected by seismometers in the vicinity of the fibre. This low rate of detection might be mainly due to cable uncoupling with the soil. Also, the spatial heterogeneous response of the fibre shows regions with higher strain than other sectors of the optic cable. Despite this, these experiments are interesting to know the sensitivity of pre-installed fibre that might be available, and to investigate the possibility of using it when a seismic crisis arises. This specific use of the fibre could be possible through previous agreements between institutions. In this case, the last two experiments have been developed under the agreement of the National Seismic Network and the National Railway Infrastructure Administrator. DAS would increase the data coverage and provide additional earthquake records that complement the recordings from seismometers of the seismic network.