Insights into the Site Effects and Ground Motion at Portoviejo, Ecuador, During the 2016 Mw 7.8 Jama-Pedernales Earthquake

ABSTRACT The 2016 Mw 7.8 Jama-Pedernales event is the strongest earthquake in more than 45 yr along the Ecuador and Colombia subduction interface. It ruptured a ∼120 km long segment, severely damaging nearby cities and towns and producing thousands of casualties. Shortly after the earthquake, we deployed a seven-sensor mobile seismic array in the urban area of Portoviejo to record aftershocks. We collected data from 135 surveyed locations by systematically relocating the array. In this study, we analyze the site effects and ground motion at Portoviejo during the earthquake, recorded by APO1, a permanent accelerometric station located in the city. The analysis begins by normalizing the spectral amplitudes of aftershocks using location-specific, frequency-dependent factors to calculate each site’s 3D frequency response function (FRF), which characterizes the ground response at that site. This process enables the deconvolution of APO1 Jama-Pedernales earthquake records using APO1 FRFs to estimate the earthquake signal at the rock or stiff soil level. The ground shaking at the surface is revealed after convolving the rock-level earthquake signal with the FRFs of the 135 sites. Our main results are: (1) the interpolation of the 135 FRFs amplitudes at 1.1 Hz map the Portoviejo river valley, revealing more than tenfold amplification; (2) the deconvolution–convolution process generates 135 time series of acceleration, velocity, and displacement, approximating how the earthquake shook the ground; (3) higher vertical accelerations estimated at the City Center could trigger a buckling effect in tall structures leading to further damage; and (4) an intensity–duration index derived from surface velocities successfully maps the valley and surrounding slopes. The proposed methodology opens the opportunity to approximate ground motions in an urban environment for large, distant, damaging earthquakes by recording their aftershocks. Valuable information could be revealed to the engineering community, enabling a better understanding of damage and improving resilience.