We use the energy-to-moment ratio, as introduced by Newman and Okal  to examine the source characteristics of normal-faulting intraslab earthquakes, compared to nearby interplate thrust events, based on recent case studies in central Chile and southeastern Mexico. In Chile, we find that the 1997 intraslab event had an exceptionally large E/M0 ratio, 30 times greater than the nearby interplate shock. This suggests a very fast strain release at the source as the origin of the particularly destructive character of intraslab events. While the difference is less sharp in Mexico, we find a similar trend, which is in agreement with the observation that the locally most damaging earthquakes are indeed the intraslab events. We also document on the 1939 Chilean earthquake the feasibility
of extending this approach to historical earthquakes for which high-quality analog records have been archived.
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Energy-to-moment ratios for damaging intraslab earthquakes: preliminary results on a few case studies