Frequency-dependent relocation of the 1992 Nicaragua slow earthquake: An empirical Green's function approach

Abstract

We present a two-step empirical Green's function analysis of the slow 1992 Nicaragua earthquake (Mw = 7.7). In the first step, a narrow-band cross-correlation method recovers the relative source amplitude and phase-delay spectra between the main shock and a suitable empirical Green's function (EGF). The analysis is performed at 2 mHz intervals from 4 to 44 mHz, using first- and second-orbit fundamental-mode Rayleigh waves. From the low-frequency relative amplitudes, we obtain the differences in mechanism and depth between the main shock and the EGF, and we present a method to correct for such differences. In the second step, we use the relative spectra to relocate the main shock relative to the EGF position at a set of discrete frequencies. In addition, we invert the relative spectra for the relative source time functions, using both a positivity and a smoothing constraint. Unlike standard EGF deconvolution, the spectral resolution achieved in our cross-correlation method allows us to determine the conditions of validity of the EGF approach. From our analysis, it appears that the centre of low-frequency radiation (4-10 mHz) during the slow 1992 Nicaragua earthquake is situated about 40 km trenchwards of the epicentre. Between 20 and 44 mHz, centroids occupy intermediate positions between the low-frequency points and the epicentre. Relative source time functions have an average duration of about 120 s. They reveal a slow (∼ 1 km s-1), smooth component of rupture with unilateral directivity towards the SE along the trench axis. A large amount of slip occurred at shallow depths at the interface between the subducted Cocos plate and the Caribbean plate, in agreement with tsunami models of the event. The low-frequency radiation appears to originate from a zone devoid of teleseismic aftershocks. This suggests that unusual frictional properties at the plate interface, perhaps related to subducted sediments, are responsible for the slow and smooth nature of the 1992 Nicaragua earthquake.