Earthquake Stress Drop and Arias Intensity

We determine earthquake stress drops directly from the Arias intensity database of NGA-West2. Arias intensity (Arias, 1970, ) is an engineering measure proportional to the integral of the absolute value of acceleration squared, over the significant duration of the signal. As such, it is closely related to root mean square acceleration and can readily be connected to earthquake stress drop (Hanks & McGuire, 1981, ). Arias intensity records out to 100-km yield stable stress drops for moderate-to-large magnitude earthquakes, M6.5+; for smaller events similar to M4.5-6.5, only closer-in records yield stable results. For the 116 events considered, stress drops are about 35% larger for class 1 mainshocks than for traditional on-fault class 2 aftershocks, and smaller for those aftershocks close to the main fault plane. Aftershock stress drops show large variability, however, implying that on average they rerupture weakened patches but can also rupture intact rock or high-stress asperities. We observe an increase of stress drop with earthquake depth similar to that of other studies but do not find any significant faulting mechanism dependence. The variability of the Arias intensity-based stress drop is lower than that of eGf-based stress drops from Baltay et al. (2010, ; 2011, ), and nearly on par with variability seen in ground-motion prediction equations. The Arias intensity stress drop is a novel and promising method to estimate stress drop without the need for path and site corrections and yields further insight into the connection between source physics and ground motion.