Source model of the 1906 Ecuador-Colombia earthquake (Mw8.4) based on tsunami waveforms and seismic intensity data; Implications for megathrust earthquake potential in Northern South-America

The 1906/01/31 Ecuador-Colombia earthquake is one of the largest megathrust earthquakes that have occurred at the interface of the Nazca and South-American plates. Recently the source process of the earthquake has been re-examined using historical tsunami waveforms, yielding a slip distribution mainly near to the trench (Yoshimoto et al. 2017, Y17), and a smaller moment magnitude than previous estimations (Mw8.8). Many studies have shown that tsunami data can effectively constrain the long wavelength characteristics of slip during megathrust-earthquakes. However to fully understand the strong motion generation processes of earthquakes the study of the shorter wavelength slip is also necessary. In this study we use the tsunami-slip model the 1906 earthquake (Y17), as well as comprehensive macroseismic intensity estimations of the earthquake (Sarabia et al., 2007), to elaborate a multi-wavelength source model appropriate for the generation of broadband frequency ground motions as well as tsunami simulations. First, we investigated the spectral characteristics of the Y17 slip model (PSD), and applied a low pass filter to obtain a smooth slip distribution (LWN slip). We then used a Von-Karman PSD function to generate shorter wavelength slip models (HWN slip), and obtain broadband wavelength slip models of the earthquake. The HWN slip models are constrained to have a given magnitude, correlation length and spectral decay, and a random phase. We simulated strong ground motions from the LWN and HWN slip models at the available intensity points. We first calculate strong motions at a seismic bedrock condition (Pulido et al. 2015), and then add site amplifications to simulated PGV values based on empirical relationships, and an AVS30 map of Colombia (Eraso, 2015). Finally, we obtain simulated intensities based on our computed PGV values and empirical relationships. Our multi-wavelength slip model of the earthquake is able to reproduce observed intensities but requires a slight increase in moment magnitude ( 8.5), which represents the contribution of short-wavelength slips for strong motion generation. Based on these results and on the evaluation of seismic coupling at the plate interface in the Ecuador-Colombia Pacific margin, we estimate the seismic potential for future megathrust earthquakes in the region.