UC Berkeley

It is of paramount importance to independently assess the spatiotemporal uniqueness of a proposed regional precursor initiated a few months before the 2011 Mw 9.0 Tohoku-Oki earthquake. The precursor has been inferred from GRACE-derived gravitational gradient changes and has been interpreted as a large-scale aseismic movement of the subducting plate along the Japan Trench. We design a hypothesis test, which enables the rigorous assessment of the statistical significance of short-term gradient anomalies at any time in any place and the quantitative comparison of the resolved anomalies with the proposed precursory signal. We find that the proposed precursory changes are not statistically unique either in time or in space. Therefore, the precursor cannot be attributed to the proposed dynamic acceleration of the subduction process. Instead, such transient features more likely represent temporally correlated GRACE observation errors or signals associated with other processes.