Complex Rupture of the 2015 Mw 8.3 Illapel Earthquake and Prehistoric Events in the Central Chile Tsunami Gap

Abstract

On 16 September 2015, the Mw 8.3 Illapel megathrust earthquake broke the subduction contact of the Nazca plate beneath the South American plate, causing unexpectedly high tsunami waves that strongly impacted the coast along the Coquimbo region in central–northern Chile. Here, we report results from a postearthquake and tsunami survey, evidencing a complex coastal geological response driven by the near coast and near trench seismic patches that ruptured during this event. Systematically, high tsunami run-ups, up to 10–11 m a.s.l., were measured on the western coast of the Punta Lengua de Vaca Peninsula in front of the near trench rupture patch, whereas tsunami run-ups up to 5–6 m were measured close to the near coast rupture patch. Detailed measurements conducted in the Coquimbo area revealed tsunami run-ups systematically higher than 6–6.5 m and up to 7.8 m. Field observations of bleached coralline algae supported by ulterior laboratory experiences were compared with geodetic measurements that evidenced complex and variable alongshore uplift-subsidence responses of the coast, suggesting a relative consistency with respect to its modern geomorphological configuration. From the analysis of pit dug in the Tongoy area, which was strongly impacted by this last tsunami, we evaluated distinctive sandy layers with benthic foraminifera tests interbedded between alluvial fine sediments, which we interpreted as paleotsunami events dated before 1108 ± 77 C.E., shortly before 1346 ± 50 C.E., and approximately 1473 ± 37 C.E. In addition to the historical massive tsunamis that occurred in 1730, 1877, and 1922 C.E. in central and northern Chile, we interpreted prehistoric tsunamis revealed here as events triggered by large megathrust earthquake ruptures, evidencing the high exposition of these coasts to near fields as well as to distant tsunamis produced along the Chilean subduction margin but also in the pan-Pacific region.

Updated concepts of seismic gaps and asperities to assess great earthquake hazard along South America

“…The 2015 Illapel earthquake is of particular note because rapid seismic magnitude estimation of the event prompted a tsunami warning and evacuation notifications within 8 to 11 min of the origin time, resulting in large-scale evacuation along the Chile coast ( 45 ). Seismic, geodetic, and tsunami waveform analyses of the 2015 Illapel earthquake indicate concentrations of ~3-m coseismic slip below the coast and a large patch with up to ~10-m slip at shallow depths ( 46 – 48 ). Studies with the best offshore resolution are consistent with the large-slip patch having extended up-dip to near the trench.…”

IonosphericTotal Electron Content Changes during the 15 February 2018 and 30 April 2022 Solar Eclipses over South America and Antarctica

“…The frequent occurrence of large earthquakes (EQs) on the west coast of South America is due to the fact that this region is on top of the subducting Nazca plate [64]. Therefore, we also investigated whether superficial EQs (depths less than 70 km) with moment magnitudes (M w ) greater than 5 occurred from 10 to 21 February 2018 (by PSE2018), and from 25 April to 6 May 2022 (by PSE2022).…”