Observational constraints on the fracture energy of subduction zone earthquakes

We relate seismologically observable parameters such as radiated energy, seismic moment, rupture area, and rupture speed to the dynamics of faulting. To achieve this objective, we computed the radiated energy for 23 subduction zone earthquakes recorded between 1992 and 2001; most of these earthquakes have a magnitude M-w > 7.5, but we also included some smaller (M-w similar to 6.7) well-studied subduction zone earthquakes and six crustal earthquakes. We compiled static stress drop estimates for these 29 earthquakes from literature and used a slip-weakening model to determine the radiation efficiency of these earthquakes. We also determined the rupture speed of these earthquakes from literature. From fracture mechanics, fracture energy, and hence radiation efficiency, can be related to the rupture speed. The radiation efficiencies estimated from the partitioning of energy as given by the slip-weakening model are consistent with the rupture speed estimated for these earthquakes. Most earthquakes have radiation efficiencies between 0.25 and 1 and are hence efficient in generating seismic waves, but tsunami earthquakes and two deep earthquakes, the 1994 Bolivia and the 1999 Russia-China border earthquakes, have very small radiation efficiencies (<0.25) and hence dissipate a large amount of energy during faulting. We suggest that differences in the radiation efficiencies of different types of earthquakes could be due to fundamental differences in their rupture mechanics. In deep events, the energy is probably dissipated in thermal processes in the fault zone, while it is possible that the morphology of the trench causes large energy dissipation during the rupture process of tsunami earthquakes.