Geotechnical Aspects of the 2015 M_w 8.3 Illapel Megathrust Earthquake Sequence in Chile

Abstract

The 2015 Illapel earthquake sequence in Central Chile, occurred along the subduction zone interface in a known seismic gap, with moment magnitudes of M w 8.3, M w 7.1, and M w 7.6. The main event triggered tsunami waves that damaged structures along the coast, while the surface ground motion induced localized liquefaction, settlement of bridge abutments, rockfall, debris flow, and collapse in several adobe structures. Because of the strict seismic codes in Chile, damage to modern engineered infrastructure was limited, although there was widespread tsunami-induced damage to one-story and two-stories residential homes adjacent to the shoreline. Soon after the earthquake, shear wave measurements were performed at selected potentially liquefiable sites to test recent V S-based liquefaction susceptibility approaches. This paper describes the effects that this earthquake sequence and tsunami had on a number of retaining structures, bridge abutments, and cuts along Chile's main highway (Route 5). Since tsunami waves redistribute coastal and near shore sand along the coast, liquefaction evidence in coastal zones with tsunami waves is sometimes obscured within minutes because the tsunami waves entrain and deposit sand that covers or erodes evidence of liquefaction (e.g., lateral spread or sand blows). This suggests that liquefaction occurrence and hazard may be under estimated in coastal zones. Importantly, the areas that experienced the greatest coseismic slip, appeared to have the largest volumes of rockfall that impacted roads, which suggests that coseismic slip maps, generated immediately after the shaking stops, can provide a first order indication about where to expect damage during future major events.

Comment on “Crustal faults in the Chilean Andes: geological constraints and seismic potential” by Santibáñez et al. (2019), Andean Geology 46 (1): 32-65

“…This means megathrust earthquake have less accelerations when they reach the Earth's surface. For example, during the Mw 8.3 2015 Illapel Earthquake in central Chile, maximum PGA's of 0.83 g were recorded at one station while most stations had recorded PGA's of less than 0.2 g (Candia et al, 2017). Santibáñez et al (2019) do carefully outline a number of case studies, that have estimates of PGA values associated with them from crustal events, that also outlines these stark differences between megathrust events and crustal events regarding intensity.…”

Developing conceptual models for the recognition of coseismic landslides hazard for shallow crustal and megathrust earthquakes in different mountain environments – an example from the Chilean Andes

“…The events are unevenly distributed in the study area, the majority of landslides located in the Principal Andean Cordillera and a very constrained region near the coast on the Arauco Peninsula, forming landslide clusters (Serey et al, 2019). Additionally, Candia et al (2017) demonstrated that there were more coseismic landslides that impacted critical infrastructure in areas with the largest fault slip at the plate boundary during the 2015 M8.4 Illapel earthquake (31.6°S).…”

“…Return periods for M ∼ 8 (and above) events are of the order of 80-130 years for any given region in Chile, and about a dozen years when the country is considered as a whole (Barrientos, 2018). The latest examples of these type of earthquakes were the 2010 M 8.8 Maule, the 2014 M 8.2 Iquique, and the 2015 M 8.4 Illapel earthquakes (Barrientos et al, 2004;Candia et al, 2017;Barrientos, 2018). Megathrust earthquakes seem to have much longer return periods, of the order of a few centuries for any given region (Cifuentes, 1989;Barrientos and Ward, 1990).…”

“…Landslides tend to form clusters that may be related to geological conditions or ground motion parameters (e.g. Serey et al, 2019;Sepúlveda et al, 2010) or to the influence of strong ground motions coincident with fault slip distributions (Candia et al, 2017)). Furthermore, most occur in the Cordilleran environment where high relief and steeper slopes prevail.…”

Shaking table test and theoretical analysis of the pile–soil–structure interaction at a liquefiable site

“…The performance of structures above piles depends on the behavior of pile foundations under earthquake loading . Moreover, post‐earthquake site investigations have confirmed that liquefaction is one of the most predominant causes of damage to engineering structures . Therefore, specific studies are required to better understand the effect of liquefaction on the seismic performance of pile–soil–structure interaction (PSSI) …”