Spatial relation of surface faults and crustal seismicity: a first comparison in the region of Switzerland

Abstract

The deformation pattern in active orogens is in general diffuse and distributed, and is expressed by spatially scattered seismicity and fault network. We select two relating datasets in the region encompassing Switzerland and analyse how they compare with each other. The datasets are not complete but are the best datasets currently available which fully cover the investigated area at a uniform scale. The distribution of distances from each earthquake to the nearest fault suggests that about two-thirds of the seismicity occurs near faults, yet about 10% occurs far from known faults. These numbers are stable for various selections of earthquakes and even when considering location uncertainties. Earthquake magnitudes in the catalogue are smaller than what could be expected from faults lengths. This suggests that the deep fracture pattern is more segmented than the superficial one, or mostly partial rupture during earthquakes, and (partly) the impropriety of the scaling law. Statistics on the distances from each fault to the nearest earthquake reveal that all supposedly-active faults in Switzerland have experienced a typically felt (magnitude 2.5 or larger) event, and only one out of six has not done so in the past four decades. Future applications of the presented approach to more complete or comprehensive fault databases may result in revised numbers regarding the connection between deep and superficial fracture patterns, representative of the stress regime of the region. The public and educational message: (1) in the region of Switzerland, earthquakes can happen in areas without known or mapped faults; (2) not all faults produce earthquakes within a human lifetime, but they seem to do so over long times.

Density distribution across the Alpine lithosphere constrained by 3-D gravity modelling and relation to seismicity and deformation

“…Our model shows that the Adriatic crust is denser than the European crust and seismic velocities are also higher in the Adriatic crust than in the European crust (e.g. IESG, 1978;IESG and ETH Zuerich, 1981;Strößenreuther, 1982;Scarascia and Cassinis, 1997;Bleibinhaus and Gebrande, 2006;Brückl et al, 2007). Higher densities and velocities indicate an on average more mafic lithology for these domains, potentially suggesting that they may be stronger than the European crust.…”

“…The data integrated to constrain subsurface lithospheric features are shown in Fig. 1b and include regional-scale, gravitationally and seismically constrained models of the TRANSALP study area (Ebbing, 2002), the Molasse Basin (Przybycin et al, 2014) and the Upper Rhine Graben (Freymark et al, 2017); regional-scale, seismically constrained models of the Po Basin, such as MAMBo (Turrini at al., 2014;Molinari et al, 2015); and seismic reflection or conversion depths and their associated P wave velocity from projects such as ALP'75, EGT'86, TRANSALP, ALP 2002, andEASI (IESG, 1978;IESG and ETH Zuerich, 1981;Strößenreuther, 1982;Mechie et al, 1985;Zucca, 1984;Prodehl, 1985, 1987;Deichmann et al, 1986;Yan and Mechie, 1989;Zeis et al, 1990;Aichroth et al, 1992;Guterch et al, 1994;Ye et al, 1995;Scarascia and Cassinis, 1997;Enderle et al, 1998;Bleibinhaus and Gebrande, 2006;Brückl et al, 2007;Hetényi et al, 2018a). The Lithosphere-Asthenosphere Boundary (LAB) was utilised unaltered from Geissler (2010), which was obtained by S receiver functions of teleseismic events.…”

“…As we are working with a coarse-resolution crustal model to identify major features, we found it appropriate to compare it to a sparse seismic catalogue comprising only the largest (M>6) events allowing for a first-order identification of this correlation. Due to the complex structural nature of an active orogen it is difficult to relate seismicity purely to density contrasts in the crust at higher spatial resolution, as the interplay of faults and collisional processes plays a major role in localising seismicity (Serpelloni et al, 2016), while a non-negligible part of earthquakes occurs away from known faults (Hetényi et al 2018b). However, due to the inherently different properties of crustal blocks of different provenance, it presents the likelihood that major faults and other structures likely to accommodate seismicity would form at the boundaries between these blocks.…”

“…Brückl et al, 2007), tending to result in simple models. However, lateral differences in crustal structure have been demonstrated, even at short wavelength, for example through the deployment of parallel seismological profiles, spaced 15 km apart across the Eastern Alps (Hetényi et al, 2018a), indicating the need for more complex models. Studies that have integrated multiple geoscientific datasets to create 3-D models of the region have either focussed on smaller subsections of the Alps (Ebbing, 2002;Ebbing et al, 2006) or included the Alps as part of a much larger study area (e.g.…”

“…The generation of such an Alpine-wide specific model is made possible by the existence of seismological results from numerous published deep seismic surveys (e.g. IESG and ETH Zuerich, 1981;Gajewski and Prodehl, 1985;Yan and Mechie, 1989;Ye et al, 1995;Brückl et al, 2007) that have been completed throughout the region and avail-able high-quality global gravity field models (e.g. Förste et al, 2014).…”