Enhancing earthquake location in deep ocean environments with DAS

Abstract

Earthquake location schemes for regional oceanic earthquakes within archipelago regionsare limited by the azimuthal resolution that land stations provide for each specific event. Althoughocean bottom seismometers (OBS) can help, they are not suited for real time monitoring and thuscan only provide a posteriori improvements. An alternative approach to improve coverage, notstrictly limited to a temporal increase of onshore stations, is to use Distributed Acoustic Sensing(DAS) on oceanic fiber-optic cables. With DAS, real time monitoring in areas up to 100 kmoffshore is possible, which can potentially improve azimuthal resolution by reaching previouslyinaccessible locations within the archipelago. A great example of such a region are the CanaryIslands, where land-based coverage is already phenomenal yet volcanic activity monitoring fromunderwater sources still has poor coverage in some cases.Recently, the deployment of additional land stations at the islands of Gran Canaria andFuerteventura has revealed existing seismicity between the two islands. When locating the source ofthis seismicity, there is a considerable east-west bias on the azimuthal distribution of those stations.An existing fiber-optic cable that connects the islands of Tenerife and Gran Canaria has beeninterrogated with DAS before, on both sides. The most recent experiment, which was set up on theGran Canaria side, recorded such seismicity from late 2022 to early 2023.This work attempts to use the DAS-recorded onsets of some events for location purposes.First, we test DAS by itself, and subsequently, DAS in combination with land stations. Automatic picking methods are tested for this purpose, since manual picking of onsets is not feasible for DAS. The possibility of catalog revisions using just a few hand-picked onsets atpreviously selected DAS channels is also considered, since there is a lot of redundancy in DAS dataand visual inspection is not unreasonable if limited to a number of channels.