Rethinking Earthquakes

In Gutenberg’s era, earthquakes were understood primarily as phenomena involving the release and propagation of seismic wave energy. Since the 1960s, seismic wave radiation has been explained by fault slip, and various characteristics of earthquakes have been successfully described by slip processes governed by friction laws on fault surfaces. As a result, the view that “earthquakes are fault slip” became widely accepted, and earthquake size is now usually represented not by radiated seismic energy but by seismic moment, a measure of fault slip. However, while earthquakes certainly involve fault slip, it is incorrect to assume that all fault slip represents an earthquake.The discovery and increasing understanding of slow earthquakes in recent decades have made it necessary to reconsider a fundamental question: What exactly is an earthquake? What distinguishes slow earthquakes from regular earthquakes? Under what conditions does this distinction arise? Do regular earthquakes begin in a universal manner? Addressing such questions leads to a view of earthquakes as a coupled process of rock fracture and wave radiation that cascades through hierarchical heterogeneities spanning a wide range of spatial and temporal scales. I aim to discuss approaches for understanding—and ultimately forecasting—such complex, multiscale phenomena.