A review of retrospective stress-forecasts of earthquakes and eruptions

Changes in shear-wave splitting (SWS) monitor stress-induced changes to the geometry of the stress-aligned fluid-saturated microcracks pervading almost all sedimentary, igneous, and metamorphic rocks in the Earth’s crust and upper mantle. Changes in SWS implying stress–accumulation and stress–relaxation (suggesting crack-coalescence) before large earthquakes have been observed retrospectively in the rock mass surrounding large or larger earthquakes. In one case, the time, magnitude, and fault-plane of a M 5 earthquake in SW Iceland, was successfully stress-forecast 3 days before it occurred. Similar characteristic behaviour of shear-wave splitting has been observed retrospectively before ∼17 other earthquakes and before three volcanic eruptions. These retrospective stress-forecasts have been published in different formats in different journals. For clarification, this paper redraws all observations of stress–accumulation and stress–relaxation in a consistent normalised format that allows the overall similarities in behaviour to be recognised before earthquakes and volcanic eruptions. Such behaviour, inconsistent with conventional sub-critical geophysics, confirms the compliance of the New Geophysics of a critically microcracked Earth, where the microcracks are so closely-spaced that they verge on failure and hence are critical-systems that impose a range of fundamentally-new properties on conventional sub-critical geophysics. One of the implications of New Geophysics is that there are similarities in the behaviour of stress before earthquakes and volcanic eruptions. The normalised formats show such similarities and include the opportunity to stress-forecast both earthquakes and eruptions.