Don't Call it Stress Drop

O ne of the most battle-weary parameters in seismology today is stress drop. Seismologists argue over everything about this parameter, including its name, meaning, measurement, and scaling with magnitude. The original concept of stress drop was introduced as a static measure of final fault slip, as a fraction of fault dimension (Δσ ∼ u/r ), and was estimated from measurements or inferences of these geologically-based parameters (as, for example, by Kanamori and Anderson, BSSA, 1975, pp. 1,073–1,095). Stress drop became an important earthquake source parameter following Brune's classic paper ( JGR , 1970, pp. 4,997–5,009), showing that the radiated far-field spectrum of shear waves could be interpreted in terms of a simple point-source model with just two source parameters—seismic moment ( M ) and stress drop (Δσ). This paved the way for measurments of stress drop from seismic signals. In Brune's model, the acceleration source spectrum has a simple `omega-squared' shape...