Analysis of Strong-Motion Accelerograph Records of the 16 April 2016 Mw 7.8 Muisne, Ecuador Earthquake

Abstract eng:
On April 16th, 2016, the Mw 7.8 Muisne earthquake ruptured a large area of the tectonic plate along Ecuador’s pacific coast, causing several damages in many locations along the coast. The major city, near the epicentral area, was Pedernales with a maximum EMS-98 intensity reported of IX. Previous studies had shown that the northern coastal region, along the cities of Muisne and Pedernales (latitude 1ºN to 0º, approximately), was coupled and remained unbroken since the January 1906 megathrust earthquake, suggesting the potential occurrence of a M > 7.5 event the next decades. In addition, Carnegie Ridge enters the trench between 2.5ºN and 1ºS and its inferred subducted continuation is interpreted to be a “flat slab” configuration. With this background, Muisne earthquake hypocenter was located 20 km deep and its projection to surface was located over the continent. Hence, this event is one of the few subduction zone earthquakes well registered in terms of the closeness of the accelerographic stations to the seismic source. A comprehensive analysis of the accelerograms for the 21 RENAC stations of the 2016 Muisne earthquake, originated by the subduction of the Nazca plate under the South American plate, is presented in this paper. The youth of the Nazca plate means that its surface has high density of asperities, which combined with its high convergence velocity, produces the highest seismicity zone of the world. Researchers had found Chile and Peru subduction accelerograms are characterized by high accelerations. For the Ecuadorian event, higher values of peak ground acceleration (PGA) and acceleration response spectra were observed. Moreover, the study of the destructiveness capacity of the accelerograms during this earthquake was performed by comparing their horizontal destructiveness potential factor (Pdh) with those calculated for the latest Chilean and Mexican seismic events. Two-peak response spectra were previously observed in some stations of Chilean interplate thrust earthquakes in 1985 and 2010. When the accelerations are recorded at the epicentral zone stations, the response spectra are more complex, having two peaks in some cases. Similar response spectra with two predominant peaks were observed for Pedernales station, which was located 36 km SW of the epicenter. Furthermore, free soil vibrations were detected at Chone station analyzing the spectrogram of its acceleration record. Finally, the period of the second peak in the response spectra of these two representative stations of the 2016 Muisne earthquake agrees approximately with the soil deposit natural period determined by the Nakamura’s H/V spectral ratio from their accelerogram codas. The Muisne earthquake shows greater PGA and Pdh values than other important subduction earthquakes, like 1985 Mw 7.8 Valparaiso - Chile, 1985 Mw 8.1 Michoacán - Mexico and 2010 Mw 8.8 El Maule – Chile. In addition, soil effects could be inferred due to the high destructiveness potential, the low zero crossing intensity and free soil vibration phenomenon in some accelerograms. All these remarks evidence the high seismic hazard in Ecuador and represents a warning voice for the future development of high-rise building projects in many cities settled over soft soils along the country.

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