11:15 AM - 11:30 AM
[SSS03-08] Off-shore Fault Geometry Revealed from Earthquake Locations Using Inland Seismic Stations: The Case of the 2022 Adriatic Sea Earthquake Sequence
Keywords:Adriatic Sea, Interevent distance, Earthquake location
Introduction. On the 9th of November 2022 two M5+ earthquakes occurred 30 km NE off the coast of Fano, Italy. According to the Italian National Institute of Geophysics and Volcanology catalogue, by midnight (UTC+01:00) on the 15th, 360 aftershocks were observed, most of them being as shallow as 10 km, similar to the depth of the two M5+ earthquakes. The occurrence of these M5+ earthquakes in the Adriatic Sea region is rather uncommon because, in the past 24 years until 2021, only 14 M4+ earthquakes occurred in the entire Adriatic region (Orecchio et al., 2023). Since the coverage of inland stations is often insufficient to accurately locate offshore earthquake events, we have taken this opportunity to develop a relocation strategy that could provide reliable faulting information.
Data and method. To reconstruct the earthquake cluster geometry, we apply a new earthquake location method developed by Grigoli et al. (2021) to this earthquake sequence, using only the data recorded inland. The method relies on the precise location of several master events at only 2 seismic stations: the cluster is reconstructed based on the relative distances to the master events. To constrain the absolute location of 24 M3+ earthquakes, which are used as master events in the geometry reconstruction, we used a travel time stacking tool (Grigoli et al., 2013) in combination with a teleseismic depth phase tool that has been successfully used before (e.g., Cesca et al., 2021).
Results. By using a velocity model derived from the tomography data (Di Stefano and Ciaccio, 2020), we relocated 24 master events that are as shallow as determined by the Italian National Institute of Geophysics and Volcanology, but slightly closer to the shoreline after relocation. Based on the error estimation of the travel time stacking tool, the good master events are determined with a coherence as high as 80% and errors less than 1 km. By using 5 well-located master events as references and the P and S arrivals at just 2 inland stations, the cluster geometry is well reconstructed showing a SW dipping fault, at an angle of around 30 - 40 degrees. Such dipping direction and angle align with the dipping of the fault plane that was revealed from the seismic reflection data (Finetti et al., 2001) and the focal mechanism determined by Italian National Institute of Geophysics and Volcanology.
Data and method. To reconstruct the earthquake cluster geometry, we apply a new earthquake location method developed by Grigoli et al. (2021) to this earthquake sequence, using only the data recorded inland. The method relies on the precise location of several master events at only 2 seismic stations: the cluster is reconstructed based on the relative distances to the master events. To constrain the absolute location of 24 M3+ earthquakes, which are used as master events in the geometry reconstruction, we used a travel time stacking tool (Grigoli et al., 2013) in combination with a teleseismic depth phase tool that has been successfully used before (e.g., Cesca et al., 2021).
Results. By using a velocity model derived from the tomography data (Di Stefano and Ciaccio, 2020), we relocated 24 master events that are as shallow as determined by the Italian National Institute of Geophysics and Volcanology, but slightly closer to the shoreline after relocation. Based on the error estimation of the travel time stacking tool, the good master events are determined with a coherence as high as 80% and errors less than 1 km. By using 5 well-located master events as references and the P and S arrivals at just 2 inland stations, the cluster geometry is well reconstructed showing a SW dipping fault, at an angle of around 30 - 40 degrees. Such dipping direction and angle align with the dipping of the fault plane that was revealed from the seismic reflection data (Finetti et al., 2001) and the focal mechanism determined by Italian National Institute of Geophysics and Volcanology.