Seven Questions about Fracking in Scotland

This IPPI Policy Brief is intended as a comment on what we view as the urgent need to improve on the quality of the ‘fracking debate’ that has been conducted in the public domain over the past year. Our argument is that, not only has the debate been somewhat polarised, but the questions raised and debated have been very narrowly focussed and lacking a wider contextual view. In particular, we argue that the issue needs to be set in terms of the broader question of ‘if we don’t get gas from shale, where do we get it from, and what are the alternatives?’, but also with the over-arching question of ‘what could fracking do for Scotland?’ We have identified seven questions that we believe must be answered in order for the process of consultation to come to a well-informed conclusion. We consider each of these in turn

Improving earthquake ground-motion predictions for the North Sea

Estimates of the expected ground motion are essential for the design, assessment and decommissioning of offshore critical infrastructure. The North Sea is an area of moderate seismic hazard that contains many high-value offshore structures (e.g. oil, gas and wind-turbine facilities). The most recent seismic hazard assessment for the North Sea is about 20 years old, before many innovations in ground-motion modelling were developed. In this study, firstly we investigate which ground-motion model from more than a dozen recent models is the most appropriate for this area based on a residual analysis of ground-motion data from onshore seismic stations surrounding the North Sea. The limited data that are available for this area and the poor magnitude and distance coverage are inherent weaknesses of this residual analysis. A recent model developed for Europe and the Middle East is the model that shows the lowest bias and minimal statistical trends with respect to magnitude and distance. Following this, we develop adjustments to this best-performing model to relax the ergodic assumption, i.e. to make the model more site- and path-specific thereby allowing a smaller aleatory variability (sigma) to be used within a probabilistic seismic hazard assessment. The use of this adjusted model within seismic hazard assessments for the North Sea should lead to better estimates of the expected ground motion for critical offshore infrastructure sites, although this would require the effects of the geotechnical properties of the seafloor to be accounted for

Effective crustal permeability controls fault evolution: An integrated structural, mineralogical and isotopic study in granitic gneiss, Monte Rosa, Northern Italy

Two dextral faults within granitic gneiss in the Monte Rosa nappe, northern Italy reveal key differences in their evolution controlled by evolving permeability and water/rock reactions. The comparison reveals that identical host rock lithologies develop radically different mineralogies within the fault zones, resulting in fundamentally different deformation histories. Oxygen and hydrogen isotope analyses coupled to microstructural characterisation show that infiltration of meteoric water occurred into both fault zones. The smaller Virgin Fault shows evidence of periodic closed system behaviour, which promoted the growth of hydrothermal K-feldspar, whilst the more open system behaviour of the adjacent Ciao Ciao Fault generated a weaker muscovite-rich fault core, which promoted a step change in fault evolution. Effective crustal permeability is a vital control on fault evolution and, coupled to the temperature (i.e. depth) at which key mineral transformations occur, is probably a more significant factor than host rock strength in controlling fault development. The study suggests that whether a fault in granitic basement grows into a large structure may be largely controlled by the initial hydrological properties of the host rocks. Small faults exposed at the surface may therefore be evolutionary “dead-ends” that typically do not represent the early stages in the development of larger faults

Increasing the quality of seismic interpretation

Acknowledgments E. Macrae was funded by an NERC Open CASE Ph.D. award (NE/F013728/1) with Midland Valley Exploration Ltd. as the industry partner. We thank 763 geoscientists for their participation, and in particular, the REs who gave their time freely to the project. M. Scott (University of Glasgow, UK) is thanked for assisting with the statistical analysis. Four reviewers are thanked for their constructive comments that improved the manuscript.Peer reviewedPublisher PD

Seismic slip on the west flank of the Upper Rhine Graben (France-Germany) : evidence from tectonic morphology and cataclastic deformation bands

Intraplate large and moderate earthquakes have occurred along the Upper Rhine Graben (URG) in the past but no coseismic surface faulting has been reported so far. We investigate the 25-km-long linear Riedseltz-Landau normal fault scarp affecting late Pleistocene and Holocene deposits of the western edge of the northern URG. The fault zone with cataclastic deformation textures is exposed in the Riedseltz quarry where it affects Pliocene and late Pleistocene (Wurm) units. Cataclasis is demonstrated by spalling and transgranular fractures in quartz grains concentrated in deformation bands with reduced grain size. The observed microstructures suggest multiple phases of deformation with cataclasis followed by emplacement of Fe-oxide matrix into deformation bands, and later emplacement of a clay-rick matrix into fractures. Previous studies along the fault show late Pleistocene (Wurm) loess deposits and early Holocene sand-silty deposits with 1.5 m and 0.7 m surface slip, respectively. New and previous results provide a minimum 0.15 mm/yr slip rate. A dislocation model suggests a minimum Mw 6.6 earthquake as a plausible scenario in the western edge of northern URG. Surface faulting in young sediments associated with cataclasis provides new evidence for assessing the occurrence of large earthquakes and seismic hazard assessment in the northern URG

Scale-dependent influence of pre-existing basement shear zones on rift faulting : a case study from NE Brazil

Rifting of continental crust initiates faults that are commonly influenced by pre-existing structures. We document newly identified faults cutting Precambrian units in the interior of the NE Brazilian margin to assess the effects of structural inheritance on both rift geometry and fault architecture. Stratigraphic and structural data indicate that the faults were active in the main phase of rifting of Gondwana. The influence of pre-existing structures on the Mesozoic rift faulting is scale dependent. Regionally, the faults trend parallel to subvertical, crustal-scale Brasiliano (c. 750–540 Ma) shear zones. Mylonitic foliations and broadly distributed low strain in the lower crust indicated by shear-wave splitting controlled the overall orientation and kinematics of the rift faults. However, outcrop observations of the faults show that at scales up to hundreds of metres, mylonitic foliations have little influence on fault architectures. Faults cross-cut shear zones and do not commonly utilize foliation planes as shear fractures. Instead, slip zones and fractures have a range of orientations that form acute angles to the local foliation orientation. This observation explains the range of focal mechanisms associated with seismicity that coincides with ancient shear zones in intra-continental areas

Fracking bad language – hydraulic fracturing and earthquake risks

We thank all conference and event organis- ers for supporting our work, as well as survey participants. We also thank Stella Pytharouli, James Verdon, and Stephen Hicks, for their insights into earthquake magnitudes and seismological terminology, and Juan Alcalde for comments about language nuance and trans- lation. We would also like to thank Brigitte Nerlich for the early discussion about the relevance of this work.Peer reviewedPublisher PD

The growth of faults and fracture networks in a mechanically evolving, mechanically stratified rock mass : a case study from Spireslack Surface Coal Mine, Scotland

Fault architecture and fracture network evolution (and resulting bulk hydraulic properties) are highly dependent on the mechanical properties of the rocks at the time the structures developed. This paper investigates the role of mechanical layering and pre-existing structures on the evolution of strike–slip faults and fracture networks. Detailed mapping of exceptionally well exposed fluvial–deltaic lithologies at Spireslack Surface Coal Mine, Scotland, reveals two phases of faulting with an initial sinistral and later dextral sense of shear with ongoing pre-faulting, syn-faulting, and post-faulting joint sets. We find fault zone internal structure depends on whether the fault is self-juxtaposing or cuts multiple lithologies, the presence of shale layers that promote bed-rotation and fault-core lens formation, and the orientation of joints and coal cleats at the time of faulting. During ongoing deformation, cementation of fractures is concentrated where the fracture network is most connected. This leads to the counter-intuitive result that the highest-fracture-density part of the network often has the lowest open fracture connectivity. To evaluate the final bulk hydraulic properties of a deformed rock mass, it is crucial to appreciate the relative timing of deformation events, concurrent or subsequent cementation, and the interlinked effects on overall network connectivity