Monte Carlo slip inversion in the frequency domain: Application to the 1992 Nicaragua slow earthquake

The spatio-temporal complexity of moment release of the 1992 Nicaragua slow earthquake (M(w) = 7.7) is estimated by simulated annealing. Contrary to linearized inversions, Monte Carlo techniques do not make assumptions about a priori features of the model, in particular, large variations in rupture velocities can be imaged. Relative spectra between the main shock and an empirical Green's function were obtained from 4-44 mHz using first and second orbit fundamental mode Rayleigh waves. The spectra were inverted for the moment distribution on a 2D horizontal fault, under the physical constraints of slip positivity and causality. The non-linear inversion revealed two main regions of slip. One centered 20 km to the NW of the epicenter, the other 160 km to the SE, but delayed by 70 s. Rupture velocities in the slipping patches were found to be low, around 1 km/s. Large amount of slip occurred trenchward of the epicenter, accounting for the large tsunami excitation. Analysis of the slip distribution suggests that the slow and smooth nature of the 1992 Nicaragua event may be related to the frictional properties at the plate interface.