A comparison of strain rates and seismicity for Fennoscandia: depth dependency of deformation from glacial isostatic adjustment

Abstract

We investigate the influence of the glacial isostatic adjustment (GIA) on the deformation at the surface and at seismogenic depths in Fennoscandia. The surface strain rate field, derived from geodetic data, is controlled by GIA which causes NW-SE extension of up to 4 × 10 −9 yr −1 in most of mainland Fennoscandia, surrounded by regions of radial shortening towards the centre of uplift. The seismic deformation field, derived from a new compilation of focal mechanisms, shows consistent NW-SE compression on the Norwegian continental margin and a tendency towards tension in mainland Fennoscandia. The seismic moment rate is at least two orders of magnitude smaller than the geodetic moment rate. We propose that the low level of seismicity and the tendency towards tensional focal mechanisms in mainland Fennoscandia may be explained by the destructive interference of the regional stress from ridge push with the flexural stress due to GIA.

Evidence for the release of long‐term tectonic strain stored in continental interiors through intraplate earthquakes

“…Following the removal of the major ice load, and the end of the 'unclamping' triggering mechanism, the ongoing glacially induced strain-rate field acts counter to the orientation of both the cumulative glacially-driven strain and the tectonically driven field in central Fennoscandia, resulting in the ongoing reduction of the overall compressive stress and strain, and likely contributing to the relatively low rates of seismicity in present day Fennoscandia relative to the geodetically-observed rates of deformation [Keiding et al, 2015], and increasing the contrast to the pulse of seismicity at 11 -9 ka. Additionally, the pulse at 11 -9 ka further stands out against the background seismicity rate, due to the predicted inhibition of sub-ice sheet seismicity on faults similar to the observed end-glacial fault during the loading and initial unloading phase [Johnston, 1987], as predicted from the negative cumulative Coulomb failure stresses predicted prior to mid glaciation ( Figure 4; see also Lund et al 2009).…”

Image log analysis of in situ stress orientation, breakout growth, and natural geologic structures to 2.5 km depth in central Scandinavian Caledonides: Results from the COSC‐1 borehole

“…The pattern of the strain rate field modeled by Craig et al . [] at 11–10,000 years does not look so different from the present‐day strain rate field calculated from geodetic observations [ Keiding et al , ], except that it is 1 order of magnitude lower today. However, the recent mapping of postglacial fault scarps in Sweden by Mikko et al [] shows a remarkably homogeneous NE‐SW orientation of these reverse faults that are consistent with the dominance of a linear rather than radial stress pattern in Scandinavia also at that time.…”

“…Thus, COSC‐1 provides an opportunity to study in situ stresses where few stress orientation and focal mechanism data are available [ Heidbach et al , ]. The stress orientation observations from COSC‐1 are contextualized with respect to tectonic drivers of crustal stresses from the ridge push hypothesis [e.g., Gregersen , ; Zoback , ], mantle‐driven stresses [e.g., Bird and Baumgardner , ; Forsyth and Uyeda , ], glacial unloading after the last glacial maximum [e.g., Keiding et al , ], and gravitational loading due to the influence of topography [e.g., Pascal and Cloetingh , ].…”

Gravitational slope deformation, not neotectonics: Revisiting the Nordmannvikdalen feature of northern Norway

“…Although the impact upon fracturing in the uppermost levels depends on a variety of conditions, it is generally accepted that rapid deglaciation can drive seismicity (e.g., Arvidsson, 1996;Wu & Hasagawa, 1996). This hypothesis is commonly applied to Fennoscandia, either in full (e.g., Gudmundsson, 1999) or in part (e.g., Muir Wood, 2000; Bungum et al, 2010;Keiding et al, 2015;Redfield & Osmundsen, 2015). The downward tapering fissure could plausibly have opened during or immediately after deglaciation.…”

“…1; see Osmundsen et al, 2009Osmundsen et al, , 2010. Because earthquakes in onshore Norway tend to be normal to normal-oblique Keiding et al, 2015), repeat intervals for medium-large (M w ≥ 6.0) events are uncertain in Scandinavia (e.g., Bungum et al, 2005), and such tremors are known to cause landsliding (see Keefer, 1984), its importance to Norwegian geology becomes that much the greater. Yet since 2000, no new work on the NVDF has been formally reported (e.g., Olesen et al, 2013).…”