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

Abstract

The 2015 Illapel earthquake sequence in Central Chile, occurred along the subductionzone interface in a known seismic gap, with moment magnitudes of Mw 8.3, Mw 7.1, andMw 7.6. The main event triggered tsunami waves that damaged structures along the coast,while the surface ground motion induced localized liquefaction, settlement of bridgeabutments, rockfall, debris flow, and collapse in several adobe structures. Because of thestrict seismic codes in Chile, damage to modern engineered infrastructure was limited,although there was widespread tsunami-induced damage to one- and two-storiesresidential homes adjacent to the shoreline. Soon after the earthquake, shear wavemeasurements were performed at selected potentially liquefiable sites to test recent Vsbasedliquefaction susceptibility approaches. This paper describes the effects that thisearthquake sequence and tsunami had on a number of retaining structures, bridgeabutments, and cuts along Chile’s main highway (Route 5). Since tsunami wavesredistribute coastal and near shore sand along the coast, liquefaction evidence in coastalzones with tsunami waves is sometimes obscured within minutes because the tsunamiwaves entrain and deposit sand that covers or erodes evidence of liquefaction (e.g. lateralspread, or sand blows). This suggests that liquefaction occurrence and hazard may beunder estimated in coastal zones. Importantly, the areas that experienced the greatestcoseismic 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 majorevents.