Integrating a seismic station 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 investigate an earthquake sequence offshore northwest of Kefalonia, Greece that initiated in March 2024 and returned to background levels by November–December of the same year. The sequence was recorded by a permanent seismic network throughout its duration and by DAS deployed along a ~15 km fiber-optic cable connecting northern Kefalonia and Ithaki between July and September 2024. We focus on a two-week period of elevated seismicity (1–15 August 2024) identified during routine earthquake catalog monitoring by the National Observatory of Athens. The integration of seismic and DAS observations increases the number of detected earthquakes by approximately a factor of 40 and reveals detailed source and statistical properties of the sequence. The enhanced catalog resolves clear mainshock–aftershock sequences and captures source spectra up to ~50 Hz for events with M < 3, frequencies not well-resolvable using seismic stations alone. DAS waveforms exhibit signal-to-noise ratios exceeding 3 at frequencies up to ~70 Hz for representative events, enabling spectral stress drop estimates consistent with typical earthquake values of 1–10 MPa.

We apply semblance-based detection to DAS data and manually review 5,734 events occurring within ~50 km of the cable to construct an initial catalog. By combining DAS and seismic-station data, we locate 356 events with signal-to-noise ratios greater than 12 dB and determine their local magnitudes from seismic stations. Waveform cross-correlation is then used to associate additional detections with template events, enabling relative magnitude estimation and further expansion of the catalog. This approach yields 2,871 earthquakes with assigned locations and magnitudes and a completeness magnitude between -0.4 and -0.3. Approximately 97% of events (2,780 of 2,871) cluster within a ~5 km radius located ~10 km offshore of northwestern Kefalonia, with peak seismicity rates exceeding 100 events per hour.

Our results demonstrate how integrating DAS with conventional seismic networks can substantially increase event detection rates and improve constraints on earthquake locations and source properties in regions with sparse station coverage. The enhanced resolution resolves clear mainshock–aftershock clustering that would likely be misclassified as swarm-like activity in standard catalogs, highlighting how limited observations can bias interpretations of earthquake sequence behavior.