Precise Earthquake Distribution and Seismic Velocity Models in the Western Saronic Gulf, Greece, based on the MeMaX Experiment

The western Saronic Gulf is part of the active South Aegean Volcanic Arc and hosts the dormant Methana volcanic system and the adjacent submarine Pausanias Volcanic Field. Although Methana last erupted around 230 BCE, ongoing hydrothermal activity and the proximity to densely populated regions, including the greater Athens metropolitan area, motivate detailed seismic investigations. A key prerequisite for the precise location of microseismicity and potentially magmatic seismicity in this region is the availability of accurate regional P- and S-wave velocity models.

Within the framework of the Methana Magmatic Observational Experiment (MeMaX), we densified the regional seismic network to improve event detection, ray coverage and hypocentral resolution. Since 2019, six permanent seismic stations operated by the National Observatory of Athens and the University of Patras have been recording seismicity on Methana and the nearby Peloponnese mainland. In March 2024, this network was expanded by 15 temporary short-period seismic stations deployed across Methana, the islands of Aegina, Agistri, Kyra, and Poros, and the Peloponnese mainland, resulting in a dense network geometry. MeMaX is well suited for local earthquake detection, location and the inversion for seismic velocity models to outline active faults and possible magmatic activity.

Noise analyses indicate low background noise levels at most temporary stations, allowing the detection of low magnitude earthquakes. Using the recorded waveform data, we compile a high-quality dataset of local earthquakes for an enhanced event catalog. We apply machine learning for phase picking (PhaseNet) and robust event association (PyOcto). Hypocenter parameters are determined with NonLinLoc and quality is controlled by sorting out events with too large location uncertainties. The seismic arrival times provide the basis for the inversion of new minimum 1-D P- and S-wave velocity models and corresponding station delay times using VELEST. Numerous starting models are tested to sample the model space and assess uncertainties together with the best-fit models.

The resulting velocity models are used to relocate the seismicity with improved accuracy and to refine the spatial distribution of earthquakes beneath Methana and the western Saronic Gulf. MeMaX thus establishes a robust seismological framework for future high resolution relative relocations, fault imaging, and the investigation of potential deep low frequency seismicity in this part of the South Aegean Volcanic Arc.

This study was supported by grant no. FKZ: 03F0952C of the German Federal Ministry of Research, Technology and Space (BMFTR) as part of the DAM mission “mareXtreme”, project MULTI-MAREX.