Abstract
Abstract. Understanding the development of post-glacial faults and their associated seismic activity is crucial for risk assessment in Scandinavia. However, imaging these features and their geological environment is complicated due to special challenges of their hardrock setting, such as weak impedance contrasts, often high noise levels and crooked acquisition lines. A crooked-line geometry can cause time shifts that seriously de-focus and deform reflections containing a cross-dip component. Advanced processing methods like swath 3-D processing and 3-D pre-stack migration can, in principle, handle the crooked-line geometry but may fail when the noise level is too high. For these cases, the effects of reflector cross-dip can be compensated for by introducing a linear correction term into the standard processing flow. However, existing implementations of the cross-dip correction rely on a slant stack approach which can, for some geometries, lead to a duplication of reflections. Here, we present a module for the cross-dip correction that avoids the reflection duplication problem by shifting the reflections prior to stacking. Based on tests with synthetic data, we developed an iterative processing scheme where a sequence consisting of cross-dip correction, velocity analysis and dip-moveout (DMO) correction is repeated until the stacked image converges. Using our new module to reprocess a reflection seismic profile over the post-glacial Burträsk fault in northern Sweden increased the image quality significantly. Strike and dip information extracted from the cross-dip analysis helped to interpret a set of southeast-dipping reflections as shear zones belonging to the regional-scale Burträsk Shear Zone (BSZ), implying that the BSZ itself is not a vertical but a southeast-dipping feature. Our results demonstrate that the cross-dip correction is a highly useful alternative to more sophisticated processing methods for noisy datasets. This highlights the often underestimated potential of rather simple but noise-tolerant methods in processing hardrock seismic data.
Highlights
IntroductionThere are a number of intraplate earthquakes connected to the post-glacial rebound, but only very few of them exceed a magnitude of 4 (Bödvarsson et al, 2006)
Today, northern Scandinavia is generally considered to be a low seismic hazard area
Throughout northern Scandinavia, up to 15 m high fault scarps extending for tens of kilometers (Fig. 1) suggest the occurrence of violent earthquakes at the end or directly after the last glacial retreat (e.g., Lagerbäck and Sundh, 2008; Olesen et al, 2013; Kuivamäki et al, 1998)
Summary
There are a number of intraplate earthquakes connected to the post-glacial rebound, but only very few of them exceed a magnitude of 4 (Bödvarsson et al, 2006). Throughout northern Scandinavia, up to 15 m high fault scarps extending for tens of kilometers (Fig. 1) suggest the occurrence of violent earthquakes at the end or directly after the last glacial retreat (e.g., Lagerbäck and Sundh, 2008; Olesen et al, 2013; Kuivamäki et al, 1998). Based on sediment deformation and liquefaction features, the magnitude of the earthquakes associated with some of these post- or end-glacial faults has been estimated to be on the order of magnitude of 7–8 (Arvidsson, 1996; Mörner, 2005). The most important question is, if the large intraplate earthquakes are repeatable or unique events
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