Using a hybrid seismic and Distributed Acoustic Sensing (DAS) network to study microseismicity in high spatiotemporal resolution offshore of Kefalonia Island, Greece

Combining traditional seismic networks with Distributed Acoustic Sensing (DAS) to record ground-motion on telecommunications cables provides new opportunities to study small earthquakes with unprecedented spatial and temporal resolution. Here we present a detailed study of an earthquake sequence offshore northwest of Kefalonia island, Greece that began in March 2024 and returned to background levels by November–December. The sequence was recorded by both a permanent seismic network for its duration and by DAS on a fiber-optic telecommunications cable between 1 - 15 August 2024.  The two-week DAS dataset provides continuous strain measurements along ~15 km of optical fiber between northern Kefalonia and Ithaki during a period that captured elevated seismic activity. Combining seismic station and DAS data reveals distinct physical features of the sequence that are not observable with seismic stations alone, including details of mainshock-aftershock clustering and well-resolved source spectra at frequencies of up to ~50 Hz for M < 3 events. The signal-to-noise-ratio > 3 at frequencies of up to 50 Hz observed on DAS waveforms for a representative group of events suggests consistency with typical earthquake stress-drop values that range from 1-10 MPa. It further suggests that DAS data may be used to augment detailed studies of microearthquake source parameters.

We apply semblance-based detection to DAS waveforms and manually inspect 5,734 earthquakes that occurred within ~50 km of the fiber to build an initial earthquake catalog. We then combine DAS and seismic-station data to locate 284 events with high signal-to-noise ratios and compute their local magnitudes with seismic station data to create a detailed subset of the initial catalog. We apply waveform cross-correlation to offshore DAS data for events in the detailed catalog to associate unlocated detections with template events and estimate relative magnitudes from amplitude ratios and further enhance the detailed catalog. This approach adds an additional 2,496 earthquakes (2,780 events in total) with assigned locations and magnitudes and leads to an enhanced catalog with completeness magnitude M_c = -0.5. Most earthquakes (2,718 of 2780) cluster within a ~5 km radius approximately 10 km offshore of northwestern Kefalonia and exhibit local rates exceeding 100 events per hour.

Our enhanced catalog provides a detailed spatiotemporal record of seismicity in a region with limited station coverage and demonstrates the effectiveness of integrating DAS with seismic networks for earthquake monitoring of active seismic sequences. Furthermore, it resolves details of mainshock–aftershock clustering that would have otherwise likely have been erroneously classified as swarm-like with standard monitoring, highlighting how observational resolution influences the interpretation of the physics driving earthquake sequences.