Imaging topographic growth by long-lived postseismic afterslip at Sefidabeh, east Iran

This paper describes observations and models of the postseismic deformation following the 1994 Sefidabeh earthquake sequence in east Iran, which shed light on the nature of the earthquake cycle and the mechanisms of topographic growth in the region. Interferometric synthetic aperture radar observations show creeping fault motion (“postseismic afterslip”) on an array of faults. Some of these faults probably represent the extensions of those that ruptured in the blind thrust-faulting earthquakes in 1994, and cut through the entire seismogenic layer, while others are shallow and break up the hanging walls of the coseismic faults. The postseismic slip accommodates at least part of the vertical displacement gradient resulting from the buried coseismic slip, which was concentrated at depths of greater than ∼5 km. The postseismic afterslip is visible for over 16 years following the earthquakes. Agreement between the areas of postseismic uplift and indications of long-term motion preserved in the geomorphology suggest that shallow fault slip during seismic cycles similar to the one we have observed governs the development of the landscape in the region. Slip on an array of shallow faults provides a mechanism for the development of short-wavelength topography and geological structures above active thrust faults and has important implications for the interpretation of shallow geological features produced in regions experiencing similar seismic cycles to that at Sefidabeh.This study forms part of the NERC- and ESRC-funded project “Earthquakes without Frontiers.”This is the final published version of an article which originally appeared in Tectonics. It is also available from the journal's website at http://onlinelibrary.wiley.com/doi/10.1002/2013TC003462/abstract. © 2014 American Geophysical Unio

Estimates of fault strength from the Variscan foreland of the northern UK

We provide new insights into the long-standing debate regarding fault strength, by studying structures active in the late Carboniferous in the foreland of the Variscan Mountain range in the northern UK. We describe a method to estimate the seismogenic thickness for ancient deformation zones, at the time they were active, based upon the geometry of fault-bounded extensional basins. We then perform calculations to estimate the forces exerted between mountain ranges and their adjacent lowlands in the presence of thermal and compositional effects on the density. We combine these methods to calculate an upper bound on the stresses that could be supported by faults in the Variscan foreland before they began to slip. We find the faults had a low effective coefficient of friction (i.e. 0.02–0.24), and that the reactivated pre-existing faults were at least 30% weaker than unfaulted rock. These results show structural inheritance to be important, and suggest that the faults had a low intrinsic coefficient of friction, high pore-fluid pressures, or both.This work forms part of the NERC- and ESRC-funded project 'Earthquakes without Frontiers', and was partially supported by the NERC grant 'Looking Inside the Continents from Space'

Lateral Variations in Lower Crustal Strength Control the Temporal Evolution of Mountain Ranges: Examples From South-East Tibet

Controversy surrounds the rheology of the continental lithosphere, and how this rheology controls the evolution and behavior of mountain ranges. In this study, we investigate the effect of lateral contrasts in the strength of the lower crust, such as those between cratonic continental interiors and weaker rocks in the adjacent deforming regions, on the evolution of topography. We combine numerical modeling with recently published results from stable-isotope palaeoaltimetry in south-east Tibet. Stable-isotope palaeoaltimetry in this region provides constraints on vertical motions, which are required to distinguish between competing models for lithosphere rheology and deformation. We use numerical modeling to investigate the effect of lateral strength contrasts on the shape and temporal evolution of mountain ranges. In combination with palaeoaltimetry results, our modeling suggests that lateral strength contrasts provide a first-order control on the evolution of topography in south-east Tibet. We find that the evolution of topography in the presence of such strength contrasts leads to laterally varying topographic gradients, and to key features of the GPS- and earthquake-derived strain-rate field, without the need for a low-viscosity, lower-crustal channel. We also find that palaeoaltimetric samples may have been transported laterally for hundreds of kilometers, an effect which should be accounted for in their interpretation. Our results are likely to be applicable to the evolution of mountain ranges in general and provide an explanation for the spatial correlation between cratonic lowland regions and steep mountain range-fronts

Findings of the International Subarachnoid Aneurysm Trial and the National Study of Subarachnoid Haemorrhage in context.

Concern has been expressed about the applicability of the findings of the International Subarachnoid Aneurysm Trial (ISAT) with respect to the relative effects on outcome of coiling and clipping. It has been suggested that the findings of the National Study of Subarachnoid Haemorrhage may have greater relevance for neurosurgical practice. The objective of this paper was to interpret the findings of these two studies in the context of differences in their study populations, design, execution and analysis. Because of differences in design and analysis, the findings of the two studies are not directly comparable. The ISAT analysed all randomized patients by intention-to-treat, including some who did not undergo a repair, and obtained the primary outcome for 99% of participants. The National Study only analysed participants who underwent clipping or coiling, according to the method of repair, and obtained the primary outcome for 91% of participants. Time to repair was also considered differently in the two studies. The comparison between coiling and clipping was susceptible to confounding in the National Study, but not in the ISAT. The two study populations differed to some extent, but inspection of these differences does not support the view that coiling was applied inappropriately in the National Study. Therefore, there are many reasons why the two studies estimated different sizes of effect. The possibility that there were real, systematic differences in practice between the ISAT and the National Study cannot be ruled out, but such explanations must be seen in the context of other explanations relating to chance, differences in design or analysis, or confounding