One Year of Seismicity Recorded Through Ocean Bottom Seismometers Illuminates Active Tectonic Structures in the Ionian Sea (Central Mediterranean)

Seismological data recorded in the Ionian Sea by a network of seven Ocean BottomSeismometers (OBSs) during the 2017-2018 SEISMOFAULTS experiment provides aclose-up view of seismogenic structures that are potential sources of medium-highmagnitude earthquakes. The high-quality signal-to-noise ratio waveforms are observedfor earthquakes at different scales: teleseismic, regional, and local earthquakes as well assingle station earthquakes and small crack events. In this work, we focus on two differenttypes of recording: 1) local earthquakes and 2) Short Duration Events (SDE) associated tomicro-fracturing processes. During the SEISMOFAULTS experiment, 133 localearthquakes were recorded by both OBSs and land stations (local magnitude rangingbetween 0.9 and 3.8), while a group of local earthquakes (76), due to their low magnitude,were recorded only by the OBS network. We relocated 133 earthquakes by integratingonshore and offshore travel times and obtaining a significant improvement in accuracy,particularly for the offshore events. Moreover, the higher signal-to-noise ratio of the OBSnetwork revealed a significant seismicity not detected onshore, which shed new light onthe location and kinematics of seismogenic structures in the Calabrian Arc accretionaryprism and associated to the subduction of the Ionian lithosphere beneath the Apennines.Other signals recorded only by the OBS network include a high number of Short DurationEvents (SDE). The different waveforms of SDEs at two groups of OBSs and the closecorrelation between the occurrence of events recorded at single stations and SDEssuggest an endogenous fluid venting from mud volcanoes and active fault traces. Resultsfrom the analysis of seismological data collected during the SEISMOFAULTS experimentconfirm the necessity and potential of marine studies with OBSs, particularly in thosegeologically active areas of the Mediterranean Sea prone to high seismic risk.