Local earthquake tomography of the Alpine region from 24 years of data

We present a new 3-D crustal P-wave velocity (V-P) model for the greater Alpine region (GAR). We use and merge three different high-quality datasets for local earthquake tomography covering 24 yrs, starting from 1st January 1996, up to 31st December 2019. We processed and repicked the waveforms from the events reported by the European-Mediterranean Seismological Centre with M > 3.0 inside the greater Alpine region for the period between May 2007 and December 2015 using a recently developed automated arrival time-picking procedure (ADAPT framework). This allows bridging the data gap between previously published (pre-2007) datasets and the recently published AlpArray research seismicity catalogue and thus provides a high-quality, highly consistent set of P-wave arrival times covering 24 yrs. With this dataset we derived a new minimum 1-D V-P model and associated station delays covering the entire GAR. Subsequently, we performed a series of local-earthquake-tomography (LET) inversions obtaining a 3-D V-P model with a horizontal node spacing of 20x20 km and between 7 and 15 km variable vertical spacing in the well-resolved area of investigation, thus improving the spatial and uniformly high-resolution coverage compared to previous LET studies in the area. For well-known major crustal structures, such as, for example, the geophysical Ivrea body, deep foreland basins and main orogenic crustal roots, our tomographic results correlate well with features documented by various previous seismic studies in the region. This correlation increases our confidence in the model's accuracy throughout the well-resolved area. Additionally, our model reveals previously poorly known, or unknown crustal features and it documents details in the Moho topography throughout the region. Eventually, we present a LET-Moho map (V-P isoline of 7.25 km s(-1)) for the GAR with spatially nearly uniform resolution and document its comparison with previously published Moho maps. The new regional 3-D V-P crustal model also correlates well with a previously published V-S crustal model obtained by ambient noise tomography. These comparisons document the new LET results of combined 3-D V-P crustal velocities and Moho topography being intrinsically consistent and reliable within the region of high resolution. Hence, in addition to further improving our understanding of crustal structure geometries in the GAR, our results also provide pivotal information for a future reference seismic 3-D crustal model of the region.