Radiated seismic energy based on dynamic rupture models of faulting

By modeling spontaneous ruptures, we study the mechanism dependence of radiated seismic energy from three hypothetical crustal events, 30 degrees dipping reverse fault, 60 degrees dipping normal fault, and a vertical strike-slip fault, and the 1994 blind-thrust Northridge earthquake. Embedded in a homogeneous half-space, all three hypothetical faults have the same area and are subjected to the same shear and normal stress conditions and frictional parameters. Dynamic simulations produce apparent stress of 0.53 MPa, 0.23 MPa, and 0.34 MPa for the reverse, normal, and strike-slip faults, respectively. The energy distribution on a distant surface shows that a large fraction of energy is concentrated in the forward direction of rupture propagation. We use spontaneous rupture models to compute the radiated energy from the 1994 Northridge earthquake. The initial stress drop distribution is based on a kinematic slip distribution. Using a linear slip-weakening friction law, we modify both the initial stress and yield stress until the dynamic rupture produces near-source synthetics that are consistent with the data. The total radiated seismic energy from our model is 6.0 x 10(14) J; seismic moment 1.47 x 10(19) Nm; apparent stress 1.5 MPa; fracture energy 3.2 x 10(14) J; and slip-weakening distance 0.25 m. The energy flux distribution is heterogeneous with strong directivity effects. These results suggest that correcting for directivity could be difficult, but necessary, for teleseismic and regional estimates of radiated energy. Dynamic source models constrained by ground motions can provide a stable and accurate energy estimate for large earthquakes.