New insights into the local seismicity and velocity structure of Albania from the application of an AI-based earthquake detection workflow on a large-N seismic dataset

Located within the active continental collision between the Adriatic microplate and Eurasia, Albania is an earthquake-prone country with one of the highest seismic hazard in Europe. A stark reminder of this was the occurrence of the Mw=6.4 Durrës earthquake in November 2019, which caused 51 fatalities and heavily damaged infrastructure in the port town of Durrës. Despite this, the country’s seismicity and velocity structure still remain poorly constrained. Our recent ANTICS large-N deployment aims to fill this knowledge gap by producing a high-quality seismic dataset from 382 temporary stations densely distributed along the southern half of the country during 2022-2024. Here we present the results of the processing of the continuous waveforms in order to extract a new catalogue of earthquakes and the inversion of a local velocity model for the region. Our semi-automatic workflow incorporates the detection and association of phases based on a fully automatic AI-based picker and associator (PhaseNet, HEX), the inversion of a 1D local velocity model for both P- and S-phases including station corrections terms, and the relocation of the entire catalogue using our newly derived velocity model and station corrections.

A total of 38 m phases were initially detected, of which 1.6 m were successfully associated to produce a catalogue of 18k events between October 2022 and May 2023. Magnitudes (M_L) vary between -1.0 to 4.5, with a magnitude of completeness of 1.5. The seismicity is concentrated in clusters and along major known structures, with hypocentres mostly occurring between 5 and 25 km depth. Two particular clusters are noteworthy for their productivity and higher magnitudes. The first one affected the town of Klos, in the northern centre of the country during mid January 2023. Up to 700 events per day were recorded during this sequence that lasted for two weeks starting on the 13 of January 2023. The mainshock magnitude was M_L=4.5, with up to 12 earthquakes with M_L>3.0 during the sequence. Seismogenic depths were constrained between 5 to 20 km depth, and the sequence can be spatially related to a NW-striking normal fault which is also supported by the mainshock focal mechanism. The second cluster occurred during March 2023 nearby the town of Erseke, in the south-east of the country. Up to 800 events per day were detected during this sequence that lasted for ten days between 23 of March and 2 of April 2023. The magnitude of the mainshock was M_L=4.3, with up to seven earthquakes with M_L>3.0 during the sequence. Seismogenic depths were constrained between 3 to 17 km depth, and the cluster is spacially associated to a NNE-striking oblique normal fault which is corroborated by the focal mechanism of the mainshock. Overall, in terms of number of earthquakes, our catalogue represents a 17-fold improvement over the local Albanian catalogue, which is manually picked and uses only the permanent stations. The detected seismicity highlights the active nature of shallow seismogenic sources in Albania, and could be used to update seismic hazard maps in the region.