A M = 5 earthquake in Iceland has been successfully ‘stress forecast’ by using variations in time delays of seismic shear wave splitting to assess the time and magnitude at which stress‐modified microcracking reaches fracture criticality within the stressed volume where strain is released. Local investigations suggested the approximate location of the forecast earthquake. We report the criteria on which this stress forecast was based.

This chapter reports temporal variations in the time-delays between split shear-waves before both earthquakes and volcanic eruptions in Iceland. The hypothesis is that during a buildup of stress, crack distributions in a large volume surrounding the immediate source zone are modified until the level of cracking reaches fracture criticality, when shear strength is lost, rocks fracture, and earthquakes, or some types of eruptions occur. In one two-year period, when volcanic and magmatic activity appeared to be low, changes in shear-wave splitting in SW Iceland were observed routinely before earthquakes with magnitudes between M3.5 and M5.1. Assuming a linear relationship between earthquake magnitude and the rate of increasing crack aspect-ratio in this comparatively narrow amplitude range, the time and magnitude of a M5 earthquake was successfully stress-forecast. These results confirm a new understanding of pre-fracturing deformation of in situ rock that has implications over a wide range of situations where the crust undergoes changes at low levels of deformation below those at which rocks fracture. Potential applications of this new understanding include monitoring hydrocarbon production, as well as stress-forecasting earthquakes and some volcanic eruptions.This chapter is the second part of a report of a fouryear study of shear-wave splitting in Iceland as part of the European Commission funded PRELAB

Summary Polarisations of seismic shear‐wave splitting observed above small earthquakes in Iceland are typically approximately NE to SW, parallel to the direction of maximum horizontal stress. In contrast, the polarisations of shear‐waves at three new stations sited over the Húsavík–Flatey Fault, a major seismically‐active transform fault in northern Iceland, are approximately NW to SE, orthogonal to the stress‐aligned polarisations elsewhere. Modelling suggests that these 90°‐flips in polarisations are caused by propagation through cracks containing fluids at high pore‐fluid pressures within one or two MPa of the critical stress. These observations suggest that high pore‐fluid pressures, which play a key role in earthquake source mechanisms, can be monitored by analysing shear‐wave splitting above seismically‐active fault planes.

A four-year study of seismic shear-wave splitting in Iceland was designed to seek temporal variations before earthquakes. Shear-wave splitting is observed routinely in Iceland whenever shear-waves arrive within the shear-wave window of seismic stations, and whenever adequate data are available, temporal and spatial variations in shear-wave splitting are observed before both earthquakes and volcanic eruptions. Shear-wave splitting is caused principally by the stress-aligned fluid-saturated microcracks and pore throats in almost all in situ rocks. Fluid-saturated microcracks are the most compliant elements of the rock mass, and changes in splitting can be directly interpreted and modelled as the effects of changing stress on the microcrack geometry in the rock mass often at considerable distances from the immediate earthquake source zone. Such changes were found and are reported in Part 2 of this study. This chapter presents the background, preliminary observations, and analysis of shear-wave splitting in Iceland.