Implications on recent tectonics in the Alps from centroid moment tensor inversion of weak to moderate seismicity
Authors: Petersen Gesa Maria , Cesca Simone , Heimann Sebastian , Niemz Peter , Dahm Torsten , Kühn Daniela , Kummerow Jörn , Plenefisch Thomas , AlpArray working group the 
Published: March 2021

&lt;p&gt;&lt;span&gt;Despite recent tectonic activity, the Alpine mountain range in central Europe is &lt;/span&gt;&lt;span&gt;mostly &lt;/span&gt;&lt;span&gt;characterized by weak to moderate seismicity. &lt;/span&gt;&lt;span&gt;L&lt;/span&gt;&lt;span&gt;ow &lt;/span&gt;&lt;span&gt;earthquake &lt;/span&gt;&lt;span&gt;magnitudes &lt;/span&gt;&lt;span&gt;and&lt;/span&gt;&lt;span&gt; the heterogeneous crust comprising of different tectonic units &lt;/span&gt;&lt;span&gt;challenge&lt;/span&gt;&lt;span&gt; centroid moment tensor inversions in &lt;/span&gt;&lt;span&gt;this region.&lt;/span&gt;&lt;span&gt; Thanks to the dense AlpArray seismic network, comprising more than 600 stations across the Alps, as well as the &lt;/span&gt;&lt;span&gt;adoption of a &lt;/span&gt;&lt;span&gt;flexible, bootstrap-based inversion tool, &lt;/span&gt;&lt;span&gt;we were able to reduce the magnitude threshold for moment tensor inversion to Mw 3.0. The &lt;/span&gt;&lt;span&gt;inversion set-up&lt;/span&gt;&lt;span&gt; was implemented after &lt;/span&gt;&lt;span&gt;systematic &lt;/span&gt;&lt;span&gt;tes&lt;/span&gt;&lt;span&gt;ts&lt;/span&gt; &lt;span&gt;of different &lt;/span&gt;&lt;span&gt; frequency bands, distance ranges, input data types &lt;/span&gt;&lt;span&gt;and&lt;/span&gt;&lt;span&gt; azimuthal gaps. &lt;/span&gt;&lt;span&gt;We &lt;/span&gt;&lt;span&gt;quantif&lt;/span&gt;&lt;span&gt;ied&lt;/span&gt; &lt;span&gt;the &lt;/span&gt;&lt;span&gt;uncertainties&lt;/span&gt; &lt;span&gt;of &lt;/span&gt;&lt;span&gt;centroid location&lt;/span&gt;&lt;span&gt;s&lt;/span&gt;&lt;span&gt; and moment tensor&lt;/span&gt;&lt;span&gt;s&lt;/span&gt;&lt;span&gt;, &lt;/span&gt;&lt;span&gt;and assess&lt;/span&gt;&lt;span&gt;ed&lt;/span&gt;&lt;span&gt; the&lt;/span&gt;&lt;span&gt; reliability of &lt;/span&gt;&lt;span&gt;potential &lt;/span&gt;&lt;span&gt;non &lt;/span&gt;&lt;span&gt;double couple&lt;/span&gt;&lt;span&gt; components. &lt;/span&gt;&lt;span&gt;Here, w&lt;/span&gt;&lt;span&gt;e p&lt;/span&gt;&lt;span&gt;resent &lt;/span&gt;&lt;span&gt;~&lt;/span&gt;&lt;span&gt;80&lt;/span&gt;&lt;span&gt; deviatoric &lt;/span&gt;&lt;span&gt;moment tensor solutions &lt;/span&gt;&lt;span&gt;and compare &lt;/span&gt;&lt;span&gt;our results&lt;/span&gt;&lt;span&gt; to &lt;/span&gt;&lt;span&gt;strain rates, &lt;/span&gt;&lt;span&gt;historic &lt;/span&gt;&lt;span&gt;and &lt;/span&gt;&lt;span&gt;recent&lt;/span&gt;&lt;span&gt; seismic activity &lt;/span&gt;&lt;span&gt;as well as to other&lt;/span&gt;&lt;span&gt; published focal mechanisms. We identify t&lt;/span&gt;&lt;span&gt;hree&lt;/span&gt;&lt;span&gt; main seismically active subregions, namely the Western Alps, the Lake Garda &lt;/span&gt;&lt;span&gt;region&lt;/span&gt; &lt;span&gt;and &lt;/span&gt;&lt;span&gt;the SE Alps, a&lt;/span&gt;&lt;span&gt;nd two&lt;/span&gt;&lt;span&gt; clusters further &lt;/span&gt;&lt;span&gt;away &lt;/span&gt;&lt;span&gt;from the study region, in the Dinarides and the Apennines. Seismicity is particularly low in the &lt;/span&gt;&lt;span&gt;N&lt;/span&gt;&lt;span&gt;E Alps and in parts of the central Alps. &lt;/span&gt;&lt;span&gt;Additionally, w&lt;/span&gt;&lt;span&gt;e apply a &lt;/span&gt;&lt;span&gt;focal mechanism &lt;/span&gt;&lt;span&gt;clustering algorithm to &lt;/span&gt;&lt;span&gt;the joint catalog, including &lt;/span&gt;&lt;span&gt;our moment tensor &lt;/span&gt;&lt;span&gt;solutions and those &lt;/span&gt;&lt;span&gt;from existing catalogs. While typical E-W to ENE-WSW striking thrust faulting is observed in the Friuli area in the SE Alps, strike-slip faulting with a similarly oriented pressure axis is observed &lt;/span&gt;&lt;span&gt;along &lt;/span&gt;&lt;span&gt;the central Alps and in the Dinarides. NW-SE striking normal faulting is observed in the NW Alps with a similar strike direction as the dominant normal faulting events in the Apennines. &lt;/span&gt;&lt;span&gt;In the W Alps as well as in the SE Alps, rotations of mechanisms are observed. &lt;/span&gt;&lt;span&gt;Both, our centroid depths as well as hypocentral depths in existing catalogs indicate that Alpine seismicity is predominantly very shallow, &lt;/span&gt;&lt;span&gt;with&lt;/span&gt;&lt;span&gt; 80 % of the studied events &lt;/span&gt;&lt;span&gt;being &lt;/span&gt;&lt;span&gt;shallower than 10 km.&lt;/span&gt;&lt;/p&gt;