We thank Graham Leslie (BGS Edinburgh) for arranging access to the Spireslack open cast pit, and the Scottish Mines Restoration Trust for permission to work there. Thanks to Clive Rice for showing us Souter Head. This work forms part of a NERC New Investigator award for DH (NE/I001743/1), which is gratefully acknowledged. We also thank Peter Chung at Glasgow for acquiring the SEM BSE images, and Mike Heap at EOST Strasbourg for deforming the Hopeman Sandstone sample. We acknowledge the use and incorporation of lineSegmentIntersect.m by U. Murat Erdem and readtext.m by Peder Axensten, both freely available on the Mathworks FileExchange. We acknowledge detailed and constructive reviews by Paul Gillespie and Mark Fischer, which, together with careful and detailed editorial comments by Bill Dunne, have greatly improved our manuscript.Peer reviewedPublisher PD

Catastrophic failure in brittle, porous materials initiates when smaller-scale fractures localise along an emergent fault zone in a transition from stable crack growth to dynamic rupture. Due to the rapid nature of this critical transition, the precise micro-mechanisms involved are poorly understood and difficult to image directly. Here, we observe these micro-mechanisms directly by controlling the microcracking rate to slow down the transition in a unique rock deformation experiment that combines acoustic monitoring (sound) with contemporaneous in-situ x-ray imaging (vision) of the microstructure. We find seismic amplitude is not always correlated with local imaged strain; large local strain often occurs with small acoustic emissions, and vice versa. Local strain is predominantly aseismic, explained in part by grain/crack rotation along an emergent shear zone, and the shear fracture energy calculated from local dilation and shear strain on the fault is half of that inferred from the bulk deformation.

The authors thank Andrew Hurst for support during the field work, and also Antonio Grippa, Giuseppe Palladino and Gustavo Zvirtes for help and constructive discussions during the field work. We are particularly grateful to Tom Manzocchi for his important comments that greatly improved the first version of this paper. We acknowledge constructive reviews by Julia Gale and Ken McCaffrey, which have greatly improved our manuscript. This work forms part of a NERC New Investigator award for David Healy (NE/I001743/1), which is gratefully acknowledged. Finally, Roberto Emanuele Rizzo is very appreciative of AFES (Aberdeen Formation Evaluation Society) for funding support during his PhD.Peer reviewedPublisher PD

Laboratory brittle deformation experiments have shown that a rapid transition exists in the behavior of porous materials under stress: At a certain point, early formed and spatially distributed tensile cracks interact and coalesce forming a shear plane. In this work, we present and apply a novel image processing tool that is able to quantify this transition between distributed (“stable”) damage accumulation and localized (“unstable”) deformation, in terms of the size, density, and orientation of cracks at the point of strain localization. Our technique, based on a two‐dimensional Morlet wavelet analysis, can recognize, extract, and visually separate the multiscale changes occurring in the crack network during the deformation process. We first performed a series of triaxial experiments (σ₁>σ₂=σ₃) on core plugs of Hopeman Sandstone (Scotland, UK) at different effective pressures (from 5 to 30 MPa). We then processed high‐resolution backscattered electron microscope images of thin sections of these core plugs and found differences in the strain localization process as effective pressure was increased. The critical length of tensile cracks required before the onset of strain localization was reduced from 0.24 to 0.15 mm as the effective pressure was increased to 30 MPa, resulting in a narrow fracture zone. Critically, by comparing patterns of fractures in these deformed sandstone samples, we can quantitatively explore the relationship between the observed geometry of the cracks and the inferred mechanical processes. This will eventually help us to better understand the physics underlying the initiation of catastrophic events, such as earthquakes, landslides, and volcanic eruptions.

Seismological and geodetic data from modern volcanic systems strongly suggest that magma is transported significant distance (tens of kilometres) in the subsurface away from central volcanic vents. Geological evidence for lateral emplacement preserved within exposed dykes includes aligned fabrics of vesicles and phenocrysts, striations on wall rocks and the anisotropy of magnetic susceptibility. In this paper, we present geometrical evidence for the lateral emplacement of segmented dykes restricted to a narrow depth range in the crust. Near-total exposure of three dykes on wave cut platforms around Birsay (Orkney, UK) are used to map out floor and roof contacts of neighbouring dyke segments in relay zones. The field evidence suggests emplacement from the WSW towards the ENE, and that the dykes are segmented over their entire vertical extent. Geometrical evidence for the lateral emplacement of segmented dykes is likely more robust than inferences drawn from flow-related fabrics, due to the prevalence of ubiquitous ‘drainback’ events (i.e. magmatic flow reversals) observed in modern systems.