Developing stress-monitoring sites using cross-hole seismology to stress-forecast the times and magnitudes of future earthquakes

Abstract

A new understanding of rockmass deformation suggests that changing stress in the crust modifies the geometry of the distributions of fluid-saturated grain-boundary cracks and pores pervading almost all rocks in the crust. These stress-aligned micro cracks cause the widely observed splitting of seismic shear-waves, which are sensitive to the details of the microcrack geometry. This means that analysing shear-wave splitting along appropriate ray paths above small earthquakes can monitor the build up of stress before large earthquakes. This allowed the time and magnitude of an M=5 earthquake in Iceland to be successfully ‘stress-forecast’. Such forecasting, using small earthquakes as the source of shear-waves, is possible only on those rare occasions when the comparatively severe restrictions on the geometry of source, receiver, and large earthquake, allow shear-wave splitting to be analysed along appropriate ray paths. This paper suggests that in the absence of such pronounced local seismicity and optimum recording geometry, the time and magnitude of future earthquakes can be estimated by analysing shear-wave splitting in controlled-source cross-well seismology between three boreholes. We suggest that stress-forecasting at such stress-monitoring sites (SMSs) is the best deterministic option for reliable forewarning of large earthquakes near any earthquake-vulnerable township, or vibration-sensitive installation. A preliminary stress-monitoring site is currently being set up in Northern Iceland.