From stress drop mapping to earthquake nucleation conditions in the northern Chilean subduction zone

Abstract

The characteristic stress drop of an earthquake is indicative of its slip to fault dimension. Its value is affected by fault strength, fault topography, the presence of fluids, and other properties. By estimating stress drops throughout an entire subduction zone, namely for the seismically highly active northernmost part of Chile, and combining it with mapped b-values and their corresponding magnitude distribution, this work aims to better constrain the conditions under which earthquakes of different provenances may nucleate.Database is a recent seismicity catalog, containing over 180,000 events and covering 15 years of seismicity, for which more than 50,000 stress drop estimates were computed. Their class wise spatial average segments the subduction zone into different parts. This difference, however, is small compared to the natural scatter of stress drop values. By considering stress drop variations, b-value map, magnitude distribution, and thermal modeling, I describe a variety of mechanisms of earthquake nucleation which might explain the observed stress drop variation. This is done for 1) the plate interface in general; 2) local shallow interface features, i.e., asperities and creeping sections; 3) the highly active intermediate depth seismicity region. In all three cases, the combination of stress drop distribution and b-value mapping helps to better understand the differences in earthquake nucleation and to formulate hypotheses on the controlling factors of earthquake nucleation.