Seismo Lab Brown Bag Seminar

Measuring stress drop offers insight into the physical mechanism of earthquakes. While extensively studied for shallow earthquakes, the stress drop of deep earthquakes remains less explored. Based on the empirical Green's function approach and using the spectral ratio method, we estimated the stress drop of deep-focus (>350 km) earthquakes. We found the shear stress of faults in the mantle transition zone (MTZ) is an order of magnitude higher than those in the crust. The observed large variation suggests the possibility of multiple physical mechanisms coexisting in the MTZ. In stress drop estimation, the single-pulse Brune source model assumption has been commonly used. We further investigated how stress drop estimates of multiple-subevent earthquakes are biased if using the Brune source model. Our analysis showed when the Brune model is used, the stress drop of a complex earthquake is determined primarily by the largest subevent. This implies that, in addition to the simplified assumption of rupture area and rupture velocity, the stress variation of asperities, rather than the average stress change of the entire fault, contributes to the large variance of stress-drop estimates.