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Earthquake detection and phase picking are crucial steps in the analysis of earthquakes. With the increasing number of seismic instruments available, large amounts of seismic data are generated, requiring the use of automatic algorithms to process earthquake series and to include events that would not be discovered with manual approaches. The Petrinja earthquake series started with local magnitude ML5.0 earthquake on December 28, 2020, followed by ML6.4 earthquake one day later. In the two years of this earthquake series, human analysts picked a total of 16,000 earthquakes smaller than M2.0, 1528 with magnitudes M2.0-2.9, 156 with magnitudes M3.0-3.9, 17 with magnitudes M4.0-4.9, 2 with magnitudes M5.0-5.9 and one earthquake with magnitude greater than M6.0. While the seismic network at the onset of this sequence counted only a few instruments in the epicentral area, the rapid aftershock deployment of 5 stations in the near vicinity of the fault zone, and the further gradual yet still remarkable growth of the seismic network to more than 50 instruments, produced an extraordinary amount of data, which are perfectly suited for employing machine learning (ML) methods for seismic phase picking and earthquake detection.In this study we present application of various ML methods to the Petrinja earthquake series. We also compare how the results change when we train a model using a subset of data from this earthquake series. Our results show that these machine learning methods are promising approaches for accurately detecting and picking phases in such earthquake series, and also delineate tectonic features responsible for generating them.

On 2021 March 27th an Mw 5.2 earthquake occurred in the Adriatic Sea, between the Italia and Croatian coast. The earthquake sequence lasted for several months and consisted of more than 150 seismic events with a magnitude above 2. Analyzing offshore seismic sequences is challenging both for the lack of optimal seismic monitoring networks and detailed enough velocity models. These conditions strongly limit the data analysis procedures, leading to inaccurate results that may have severe effects on the identification of the seismogenic structure associated with the seismic sequence, bringing to wrong seismo-tectonic interpretations, with direct consequences in the seismic hazard assessment of an area. In this study, we analyze the March 2021 Mw 5.2 earthquake sequence that occurred in the Adriatic Sea with recently developed location techniques. Our workflow allows achieving a higher location accuracy, even when dealing with suboptimal monitoring conditions. We analyze this dataset using waveform-based location techniques and a recently developed location technique based on Distance Geometry Solvers (DGS). This last approach uses inter-event distances between earthquake pairs estimated at one or two seismic stations to get high-resolution locations of seismicity clusters. The application of such techniques led to different improvements in locating the seismic sequence, which is more clustered and clearly shows an N-S trending compatible with the geological setting of the area.

At present time, the formerly much larger Adriatic Microplate is still actively being subducted beneath the Apennines and the Dinarides-Hellenides zone with continental collision and related processes occurring under the Alps and the Dinarides. These tectonic processes along with the large-scale component of the northward moving African Plate resulted in a complex 3D stress field. In the light of the complex tectonic processes accompanying the movement of the Adriatic Plate, we aim to investigate the three-dimensional stress field in that area by stress inversion using focal mechanism data from the available CMT and RCMT earthquake catalogues. The focal mechanisms are inverted to better understand the stress regime in that region and how the stress pattern is depending on the current tectonic setting. A staggered grid algorithm was used for binning the focal mechanisms before the inversion.The calculated 3D stress field indicates that the direction of the large-scale convergence of Africa and Eurasia is similar to the dominating direction of the maximum horizontal stress axis in the western central Mediterranean, with the exception of the Apennines, where the subduction of the Adriatic Plate beneath the northern Apennines is the primary source of stress. On the eastern margin of the Adriatic Plate the lack of deeper seismicity and a back arc basin, as well as the orogen normal orientation of the maximum horizontal stress axis in the Dinarides is pointing towards a continental subduction zone with an aseismic delaminating slab of lower lithosphere without a significant slab pull component.Changes of the stress pattern within the Adriatic Plate may result from intraplate deformation, which points towards a fragmentation of Adria along the Mid Adriatic Ridge into two subplates, Adria Sensu Strictu in the north and Apulia in the south. While Adria Sensu Strictu is moving independently from Africa, Apulia is depending on the larger plates movement.The inversion of the focal mechanisms from the Hellenic Subduction Zone yields results about the rotation of the stress field with depth, as the maximum horizontal stress rotates from trench normal at shallow depths to trench parallel deeper down.