How Alpine seismicity relates to lithospheric strength

Abstract

Despite the amount of research focussed on the Alpine orogen, different hypotheses still exist regarding varying spatial seismicity distribution patterns throughout the region. Previous measurement-constrained regional 3D models of lithospheric density distribution and thermal field facilitate the generation of a data-based rheological model of the region. In this study, we compute the long-term lithospheric strength and compare its spatial variation to observed seismicity patterns. We demonstrate how strength maxima within the crust (~ 1 GPa) and upper mantle (> 2 GPa) occur at temperatures characteristic of the onset of crystal plasticity in those rocks (crust: 200–400 °C; mantle: ~ 600 °C), with almost all seismicity occurring in these regions. Correlation in the northern and southern forelands between crustal and lithospheric strengths and seismicity show different patterns of event distribution, reflecting their different tectonic settings. Seismicity in the plate boundary setting of the southern foreland corresponds to the integrated lithospheric strength, occurring mainly in the weaker domains surrounding the strong Adriatic plate. In the intraplate setting of the northern foreland, seismicity correlates to modelled crustal strength, and it mainly occurs in the weaker and warmer crust beneath the Upper Rhine Graben. We, therefore, suggest that seismicity in the upper crust is linked to weak crustal domains, which are more prone to localise deformation promoting failure and, depending on the local properties of the fault, earthquakes at relatively lower levels of accumulated stress than their neighbouring stronger counterparts. Upper mantle seismicity at depths greater than modelled brittle conditions, can be either explained by embrittlement of the mantle due to grain-size sensitive deformation within domains of active or recent slab cooling, or by dissipative weakening mechanisms, such as thermal runaway from shear heating and/or dehydration reactions within an overly ductile mantle. Results generated in this study are available for open access use to further discussions on the region.

Stable equilibrium dynamics explains continental lithosphere deformation

“…On geological time scales, the deformation of the continental lithosphere is described by a continuum description of the plate deforming as a viscous fluid 5,6 . Orogens that deform in response to an applied convergence are considered to be mechanically weaker than their foreland lithosphere 7 , a feature confirmed by data-driven lithospheric scale models of the Alps and Andes 8,9 . The physics driving this weakening has been ultimately related to mantle processes during plate-plate collision (e.g.…”

“…1d). Hence, a thickened orogenic crust is also associated with a higher than average HPE content 8,9 . Such a thermochemical configuration has a direct influence on the mechanical state of the lithosphere, therefore, on its mode of deformation during orogenesis.…”

Stress Patterns and Crustal Anisotropy in the Eastern Alps: Insights From Seismological and Geological Observations

“…In 1976, a major earthquake sequence occurred in the Friuli region, with a mainshock of moment magnitude M W 6.4 followed by several months-long aftershocks sequence (Pondrelli et al, 2001;Slejko et al, 1999;Figure 2). Conversely, the regions to the north and west of the DI display sparse and low-magnitude earthquakes (Reiter et al, 2005;Viganò et al, 2008Viganò et al, , 2015 often superimposed on diffuse background microseismicity (Spooner et al, 2022). However, this region is also known to be occasionally affected by strong historical earthquakes (details in: Reiter et al, 2018).…”

Present‐Day Upper‐Mantle Architecture of the Alps: Insights From Data‐Driven Dynamic Modeling

“…A thicker crust beneath the Alps with intermediate average densities has been shown to correlate with observed surface uplift (Spooner et al., 2019a). Beneath the Alps, upper‐crustal seismicity (Cattaneo et al., 1999; Eva et al., 2015; Singer et al., 2014) occurs within a weaker orogenic lithosphere, while weaker crustal domains are found preferentially around the stronger Adriatic microplate in the southern foreland (Marotta & Splendore, 2014; Spooner et al., 2022; Willingshofer & Cloetingh, 2003). In the northern forelands, the entirety of crustal seismicity (Deichmann, 1992; Singer et al., 2014) is also bound to weaker crustal areas, which underwent extensive thinning beneath the Upper Rhine Graben.…”