Abstract
Abstract. The Alpine mountains in central Europe are characterized by a heterogeneous crust accumulating different tectonic units and blocks in close proximity to sedimentary foreland basins. Centroid moment tensor inversion provides insight into the faulting mechanisms of earthquakes and related tectonic processes but is significantly aggravated in such an environment. Thanks to the dense AlpArray seismic network and our flexible bootstrap-based inversion tool Grond, we are able to test different setups with respect to the uncertainties of the obtained moment tensors and centroid locations. We evaluate the influence of frequency bands, azimuthal gaps, input data types, and distance ranges and study the occurrence and reliability of non-double-couple (DC) components. We infer that for most earthquakes (Mw≥3.3) a combination of time domain full waveforms and frequency domain amplitude spectra in a frequency band of 0.02–0.07 Hz is suitable. Relying on the results of our methodological tests, we perform deviatoric moment tensor (MT) inversions for events with Mw>3.0. Here, we present 75 solutions for earthquakes between January 2016 and December 2019 and analyze our results in the seismotectonic context of historical earthquakes, seismic activity of the last 3 decades, and GNSS deformation data. We study regions of comparably high seismic activity during the last decades, namely the Western Alps, the region around Lake Garda, and the eastern Southern Alps, as well as clusters further from the study region, i.e., in the northern Dinarides and the Apennines. Seismicity is particularly low in the Eastern Alps and in parts of the Central Alps. We apply a clustering algorithm to focal mechanisms, considering additional mechanisms from existing catalogs. Related to the N–S compressional regime, E–W-to-ENE–WSW-striking thrust faulting is mainly observed in the Friuli area in the eastern Southern Alps. Strike-slip faulting with a similarly oriented pressure axis is observed along the northern margin of the Central Alps and in the northern Dinarides. NW–SE-striking normal faulting is observed in the NW Alps, showing a similar strike direction to normal faulting earthquakes in the Apennines. Both our centroid depths and hypocentral depths in existing catalogs indicate that Alpine seismicity is predominantly very shallow; about 80 % of the studied events have depths shallower than 10 km.
Highlights
The Alpine mountains and surrounding areas are known for their complex tectonic setting with a highly heterogeneous lithospheric structure (e.g., Handy et al, 2010, 2015; Schmid et al, 2004; Hetényi et al, 2018)
Based on our methodological tests, we use a combination of time domain full waveforms and frequency domain amplitude spectra as input data for the centroid moment tensor (MT) inversion for earthquakes larger than Mw 3.0
Deviatoric inversions were generally favored over full moment tensors since we demonstrated that the isotropic and CLVD components can often not be distinguished reliably
Summary
The Alpine mountains and surrounding areas are known for their complex tectonic setting with a highly heterogeneous lithospheric structure (e.g., Handy et al, 2010, 2015; Schmid et al, 2004; Hetényi et al, 2018). Petersen et al.: CMT inversion in the Alps tectonically shaped by the interaction of the Adriatic microplate and the European plate in several stages of convergence between Europe and Africa (e.g., Schmid et al, 2004, 2008; Handy et al, 2010; Hetényi et al, 2018). Geological studies show that the Adriatic plate is the upper plate in the subduction of the Alpine Tethys in the Alps, while it is the lower plate of the thrust systems in the Apennines and the Dinarides (e.g., Schmid et al, 2008; Handy et al, 2015). Velocity anomalies in the crust and the upper mantle reflect the complexity of the crustal structure and the geodynamic setting (e.g., Diehl et al, 2009; Fry et al, 2010; Molinari et al, 2015; Kästle et al, 2018; Lu et al, 2020; Qorbani et al, 2020)
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