Comparison of Observed Ground‐Motion Attenuation for the 16 April 2016 M w 7.8 Ecuador Megathrust Earthquake and Its Two Largest Aftershocks with Existing Ground‐Motion Prediction Equations

Description

A megathrust subduction earthquake (Mw 7.8) struck the coast of Ecuador on 16 April 2016 at 23:58 UTC. This earthquake is one of the best‐recorded megathrust events to date. Besides the mainshock, two large aftershocks have been recorded on 18 May 2016 at 7:57 (Mw 6.7) and 16:46 (Mw 6.9). These data make a significant contribution for understanding the attenuation of ground motions in Ecuador. Peak ground accelerations and spectral accelerations are compared with four ground‐motion prediction equations (GMPEs) developed for interface earthquakes, the global Abrahamson et al. (2016) model, the Japanese equations by Zhao, Zhang, et al. (2006) and Ghofrani and Atkinson (2014), and one Chilean equation (Montalva et al., 2017). The four tested GMPEs are providing rather close predictions for the mainshock at distances up to 200 km. However, our results show that high‐frequency attenuation is greater for back‐arc sites, thus Zhao, Zhang, et al. (2006) and Montalva et al. (2017), who are not taking into account this difference, are not considered further. Residual analyses show that Ghofrani and Atkinson (2014) and Abrahamson et al. (2016) are well predicting the attenuation of ground motions for the mainshock. Comparisons of aftershock observations with the predictions from Abrahamson et al. (2016) indicate that the GMPE provide reasonable fit to the attenuation rates observed. The event terms of the Mw 6.7 and 6.9 events are positive but within the expected scatter from worldwide similar earthquakes. The intraevent standard deviations are higher than the intraevent variability of the model, which is partly related to the poorly constrained VS30 proxies. The Pedernales earthquake produced a large sequence of aftershocks, with at least nine events with magnitude higher or equal to 6.0. Important cities are located at short distances (20–30 km), and magnitudes down to 6.0 must be included in seismic‐hazard studies. The next step will be to constitute a strong‐motion interface database and test the GMPEs with more quantitative methods.

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