A stress-forecast earthquake (with hindsight), where migration of source earthquakes causes anomalies in shear-wave polarisations

The stress-aligned shear-wave splitting observed throughout most of the crust is a three-dimensional function of the effective elastic anisotropy of distributions of fluid-saturated grain-boundary cracks and preferentially oriented pores. Fluid-saturated microcracks are highly compliant and crack geometry responds readily to changes in stress. Monitoring with shear-wave splitting allows the accumulation of stress before earthquakes to be recognised and the time and magnitude of the impending earthquake to be stress-forecast. Such increases in stress have been observed some 15 times with hindsight and one successful real-time stress-forecast. Here we report a further with-hindsight stress-forecast in Northern Iceland, where increases of time-delays implying stress accumulation are observed before a 4.9 earthquake at two seismic stations at epicentral distances of 50 km and 92 km. There are, however, two anomalies. Changes in shear-wave polarisations at the nearest station appear to be due to migration of shear-wave source earthquakes, in a uniform anisotropic symmetry, into ray path directions with different anisotropic shear-wave polarisations. Such directional changes in shear-wave polarisations are occasionally observed above small earthquakes and may be misinterpreted as structural or temporal changes in anisotropy. A further anomaly is that the station at 92 km displays variations in time-delays, but a station just 4 km farther away on the other side of a major fault appears to be in the stress shadow of the fault and does not display temporal variations despite having adequate source data.