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We develop a new inversion approach to construct a 3-D structural and shear-wave velocity model of the crust based on teleseismic P-to-S converted waves. The proposed approach does not require local earthquakes such as body wave tomography, nor a large aperture seismic network such as ambient noise tomography, but a three-component station network with spacing similar to the expected crustal thickness. The main features of the new method are: (1) a novel model parametrization with 3-D mesh nodes that are fixed in the horizontal directions but can flexibly vary vertically; (2) the implementation of both sharp velocity changes across discontinuities and smooth gradients; (3) an accurate ray propagator that respects Snell’s law in 3-D at any interface geometry. Model parameters are inverted using a stochastic method composed of simulated annealing followed by a pattern search algorithm. The first application is carried out over the Central Alps, where long-standing permanent and the temporary AlpArray Seismic Network stations provide an ideal coverage. For this study we invert 4 independent parameters, which are the Moho discontinuity depth, the Conrad discontinuity depth, the P-velocity change at the Conrad and the average Vp/Vs of the crust. The 3-D inversion results clearly image the roots of the Alpine orogen, including the Ivrea Geophysical Body. The lower crust's thickness appears fairly constant. Average crustal Vp/Vs ratios are relatively higher beneath the orogen, and a low-Vp/Vs area in the northern foreland seems to correlate with lower crustal earthquakes, which can be related to mechanical differences in rock properties, probably inherited. Our results are in agreement with those found by 3-D ambient noise tomography, though our method inherently performs better at localizing discontinuities. Future developments of this technique can incorporate joint inversions, as well as more efficient parameter space exploration.

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.

We report preliminary results from drill cores and logging from the ICDP Drilling the Ivrea-Verbano zonE (DIVE) project, Hole DT-1b in Ornavasso (Val d’Ossola, northern Italy). Characterized by pronounced geophysical anomalies, the exposed Ivrea-Verbano Zone in the Italian Alps represents an archetypal lower continental crust section. The first phase of DIVE is dedicated to the drilling and the petrological and geophysical characterization of the lowermost continental crust. Specifically, Hole DT-1b was set in the in the hinge zone of the Massone Antiform to explore the pre-Permian lithologies of the lower continental crust. Hole DT-1b was drilled using the diamond double tube continuous wireline coring method, from October 6 to December 10, 2022, and the retrieved rock cores were inspected and classified by the DIVE drilling project science team. Core recovery was effectively 100% throughout the entire drill hole. In total, 578m of the upper part of the lower continental crust in the Ivrea-Verbano Zone were drilled and cored.Here we summarize on site visual core descriptions and preliminary geophysical logging and microbiological investigations. The cores mostly consist of amphibolites and garnet-bearing metapelites with variable presence of migmatitic structures, and with local high and low temperature shear zones, pegmatitic dikes, and open fractures.Continuous monitoring of borehole fluids and gases (OLGA and miniRuedi devices, see EGU abstract by Dutoit and coworkers), and a suite of borehole logging measurements (see abstract by Li, Greenwood, Caspari and coworkers) were performed. They match very well the core logs performed on site (magnetic susceptibility and natural gamma radiation). The most prominent, directly observable deformation feature was a high-temperature foliation, with a dip angle between 30 and 60° in the upper part of the hole, becoming increasingly steeper in the deeper part.Along the entire drill hole fractures and open cracks were observed, some of them filled with precipitates of quartz, carbonates, sulphides and oxides. These fractures are potentially promising hosts for microbial communities and are currently under investigation. Additional samples for microbiological studies were taken every 20m from the drill cores and are currently cultivated for further investigations.Hole DT-1b provides detailed insights into the compositional, structural and geophysical variation of metasedimentary continental lower crust, including the distribution of sulphides. Relamination of metamorphic pelitic and mafic rocks may produce an important reservoir of sulphur bearing minerals in the lower continental crust. Further results emerging between abstract submission and the conference will be presented.

We present potential scenarios of the European and Adriatic plates’ collision that formed the Alps and the neighbouring mountain belts. Our results are based on teleseismic body-wave data from the AlpArray-EASI complementary experiment (2014-2015, Hetényi et al., Tectonophysics 2018) and the AlpArray Seismic Network (Hetényi et al., Surv. Geophys. 2018). Tomography of seismic velocities in the upper mantle along a ca. 200 km broad and 540 km long north-south transect images steady southward thickening of the lithosphere beneath the Bohemian Massif and northward dipping East-Alpine lithospheric keel. Thanks to the dense spacing of the AlpArray Seismic Network stations and high-quality data, the high-resolution tomography resolves for the first time two sub-parallel down-going high-velocity heterogeneities beneath the Eastern Alps, instead of a single, thick anomaly. The southern heterogeneity, which we relate to the subducted Adriatic plate, is more distinct than the northern one, which loses its connection with the shallow parts. Moreover, amplitudes and size of this heterogeneity decrease in cross-sections perpendicular to the strike of the Alps when shifting towards the Central Alps. The presented collision scenarios consider the smaller northern heterogeneity as (1) a remnant of a delaminated early phase subduction of the European plate with the reversed polarity relative to that in the Western Alps, (2) a piece of continental and oceanic lithosphere together, or, (3) a fragment of a quite extended lithosphere margin foundering in a preceding phase of the Adriatic subduction.