Holocene liquefaction and soft-sediment deformation in Quito (Ecuador): A paleoseismic history recorded in lacustrine sediments

Abstract

We studied the Holocene fluvial-lacustrine sediments of the northern Quito Basin to determine a paleoseismic history for Quito, the capital of Ecuador. Fault-controlled sedimentation and coseismic deformation demonstrate the Holocene activity of the Quito reverse fault system. In the 450 years of historical seismicity, just a few events recorded in Quito could be related to local seismic sources. Numerous liquefaction horizons are possible evidence of paleoseismic activity in Quito. Potential paleoseismic horizons have a maximum thickness of 1.5 m and were produced in a lacustrine environment. Several horizons have been successfully correlated throughout the basin thanks to the presence of six volcanic marker beds. We found no evidence of widespread liquefaction processes originating in an aerial environment at water-table depth, clearly suggesting a present-day low liquefaction potential for the city. Assuming a seismic origin for contorted bedding features, we compared thickness distributions of these horizons and intensity distributions from the historical seismicity record, and proposed a methodology for the determination of seismic paleointensities. We constructed a paleoseismic history which suggests the occurrence of 28 earthquakes of intensity > V for a roughly 1500-year time span, prior to the historical record. From these 28 earthquakes, we determined the probable occurrence of a major event of intensity X (MSK) between the 10 th and the 16 th centuries. The correlation between its paleoseismic horizon and buried coseismic faulting in the Quito Basin suggests the occurrence of a local earthquake. The occurrence of a MM 6.5–7.0 event due to the rupture of the entire Quito fault appears to be the most probable origin for this high seismic intensity which exceeds the maximum recorded historical intensity by almost one degree.