Comparison of observed ground-motion attenuation for the 2016/04/16 Mw7.8 Ecuador megathrust earthquake and its two largest aftershocks with existing ground-motion prediction equations

TL;DR

This study compares observed ground-motion data from the 2016 Ecuador megathrust earthquake and its aftershocks with existing ground-motion prediction equations, highlighting their accuracy and regional differences in attenuation.

Contribution

It provides a detailed comparison of observed ground motions with multiple GMPEs for a well-recorded megathrust event and its aftershocks, assessing model performance and regional attenuation characteristics.

Findings

GMPEs predict ground motions accurately up to 200 km for the mainshock.

High-frequency attenuation is greater at backarc sites, affecting model predictions.

G&A14 and Aetal2016 models effectively predict attenuation for the mainshock.

Abstract

A megathrust subduction earthquake (Mw7.8) struck the coast of Ecuador on April 16th, 2016 at 23h58 UTC. This earthquake is one of the best-recorded megathrust events up to date. Besides the mainshock, two large aftershocks have been recorded on May 18th, 2016, at 7h57 (Mw 6.7) and 16h46 (Mw6.9). 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 developed for interface earthquakes, the global Abrahamson et al. (2016) model, the Japanese equations Zhao et al. (2006) and Ghofrani and Atkinson (2014), and one Chilean equation Montalva et al. (2016). The four tested GMPEs are providing rather close predictions for the mainshock at distances up to 200km. However, our results show that high-frequency attenuation is greater for backarc sites, thus Zetal2016 and Metal2016, which are not taking into account this difference, are not considered further. Residual analyses show that G&A14 and Aetal2016 are well predicting the attenuation of ground motions for the mainshock. Comparisons of aftershock observations with Aetal2016 predictions indicate that the GMPE provide reasonable fit to the attenuation rates observed. The event terms of the Mw6.7 and Mw6.9 events are positive but within the expected scatter from worldwide similar earthquakes. The intra-event standard deviations are higher than the intraevent variability of the model, which is partly related to the poorly constrained VS30 proxys.