A 3D Velocity Model for the European Alps: New Insights Into the Crustal Structure

Abstract We present the first crustal 3D P ‐ and S ‐wave velocity model covering the entire Greater Alpine region (GAR) based on Local Earthquake tomography. Applying the deep neural network PhaseNet to broad‐band waveforms from 989 stations in the GAR between 2016 and 2022, we determined 173,841 P ‐ and 68,967 S ‐phase onsets from 2,553 events with 1.5 recorded at epicentral distances up to 1,000 km. With the SIMUL2023 algorithm we simultaneously relocate the seismicity and invert for 3D velocity structure using an approximate bayesian approach to minimize the influence of the starting model. The excellent overall agreement with previously published geophysical transects gives us the confidence that our new 3D model is representative for the whole Alpine region. We find a consistent decrease in P ‐ and S ‐wave velocity at mid crustal depths beneath the Western, Central and Eastern arc which we name the Alpine Mid Crustal Low Velocity (AMCLV) anomaly. The AMCLV is terminated sharply by the Dolomites indenter east of the Giudicarie line and is visible again east of 12E, but considerably less pronounced in the Eastern Alps. Due to its partial connection to the upper crust in the European foreland we interpret this material to be formerly part of the European upper crust which has been stacked during the collision process. Based on 1D velocity profiles we suggest that the southern part of the Dolomites indenter consists mainly of undeformed northward dipping Adriatic mantle, whereas the northern part shows a thickened lower crust maybe caused by Permian intrusions.