Plate-wide deformation before the Sumatra–Andaman Earthquake

Abstract

Rock is weak to shear-stress and the energy released by the 26th December, 2004, M≈9.2 Sumatra–Andaman Earthquake, the largest earthquake for four decades, must have accumulated over enormous volumes of crust and mantle, certainly plate-wide, possibly world-wide. Here we report evidence for plate-wide stress accumulation. Changes in seismic shear-wave splitting monitor stress-induced changes in the geometry of the microcrack distributions in almost all rocks in the Earth’s crust. Such changes observed in Iceland show stress-accumulation beginning at least four years before the Sumatra–Andaman Earthquake. These changes were recognised as monitoring stress-accumulation before an impending large earthquake and 10 ‘stress-forecasts’ were emailed to Iceland Meteorological Office for some 29months forecasting an impending large earthquake. The remarkable sensitivity of critical-systems of microcrack geometry to miniscule changes of stress had not been recognised at that time and the stress-accumulation was expected to lead to a M⩾7 earthquake somewhere in Iceland. Only now is it recognised that the changes in shear-wave splitting were monitoring stress-accumulation which would eventually lead to the Sumatra–Andaman Earthquake at a distance of some 10,500km on the opposite side of the Eurasian Plate. This extreme sensitivity confirms the critical nature of fluid-saturated stress-aligned microcracks in the Earth’s crust.