Abstract
Seismological data recorded in the Ionian Sea by a network of seven Ocean Bottom Seismometers (OBSs) during the 2017–2018 SEISMOFAULTS experiment provides a close-up view of seismogenic structures that are potential sources of medium-high magnitude earthquakes. The high-quality signal-to-noise ratio waveforms are observed for earthquakes at different scales: teleseismic, regional, and local earthquakes as well as single station earthquakes and small crack events. In this work, we focus on two different types of recording: 1) local earthquakes and 2) Short Duration Events (SDE) associated to micro-fracturing processes. During the SEISMOFAULTS experiment, 133 local earthquakes were recorded by both OBSs and land stations (local magnitude ranging between 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 integrating onshore 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 OBS network revealed a significant seismicity not detected onshore, which shed new light on the location and kinematics of seismogenic structures in the Calabrian Arc accretionary prism 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 Duration Events (SDE). The different waveforms of SDEs at two groups of OBSs and the close correlation between the occurrence of events recorded at single stations and SDEs suggest an endogenous fluid venting from mud volcanoes and active fault traces. Results from the analysis of seismological data collected during the SEISMOFAULTS experiment confirm the necessity and potential of marine studies with OBSs, particularly in those geologically active areas of the Mediterranean Sea prone to high seismic risk.
Highlights
The lithosphere beneath the Ionian Sea is an oceanic embayment (Catalano et al, 2001), which is subducting contemporaneously beneath the Apennines and the Hellenides (Carminati et al, 2012; Carminati et al, 2020)
Prior to the SEISMOFAULTS experiment, available seismological data, collected both by onshore seismic station and by the NEMO-SN1 seafloor observatory (Sgroi et al, 2021), have shown that the submarine Calabrian Arc is characterized by a pattern of small magnitude, scattered earthquakes, mainly due to slow rates of NW-directed Ionian subduction
The new data acquired during the SEISMOFAULTS project show a significant number of seismic events not detected by the land stations (Figure 8) that could provide a more complete picture of the active seismicity in the region
Summary
The lithosphere beneath the Ionian Sea is an oceanic embayment (Catalano et al, 2001), which is subducting contemporaneously beneath the Apennines and the Hellenides (Carminati et al, 2012; Carminati et al, 2020). We cannot accurately locate the SDE source since we only have data from one seismic station: as demonstrated in previous studies performed in other areas, the SDE source distance from the sensor is estimated in the order of hundreds of meters (Sultan et al, 2011), or even tens of meters (Tary et al, 2012), whereas their amplitude is roughly higher than the one of single station earthquakes (see amplitude values in Figure 10 for single station events and SDE) and their distribution is indicative of the state of activity of structures placed in the Ionian Sea. As differences in the waveforms and spectrograms of SDE are observed at each OBS (Figures 10, 11), we can argue that different processes generate these signals.
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