Alexandra Alvarado

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 10th and the 16th 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.

Enhancing volcano‐monitoring capabilities in Ecuador

Ecuador has 55 active volcanoes in the northern half of the Ecuadorian Andes. There, consequences of active volcanism include ashfalls, pyroclastic flows (fast moving fluidized material of hot gas, ash, and rock), and lahars (mudflows), which result in serious damage locally and regionally and thus are of major concern to Ecuadorians. In particular, Tungurahua (elevation, 5023 meters) and Cotopaxi (elevation, 5876 meters) are high-risk volcanoes. Since 1999, eruption activity at Tungurahua has continued and has produced ashfalls and lahars that damage towns and villages on the flanks of the volcano. More than 20,000 people live on these flanks.

Seafloor margin map helps in understanding subduction earthquakes

Ecuador and southwest (SW) Colombia suffered widespread damage during the twentieth century as a result of some of the greatest subduction earthquakes and associated tsunamis ever recorded. In 1906, the Ecuador-SW Columbia margin, located at the transition between the continent and deep ocean, ruptured over a 500-kilometer length as a single great (Mw = 8.8) subduction earthquake (Figure 1a) [Kelleher, 1972]. The 1906 rupture zone was partially reactivated in 1942, 1958, and 1979 by earthquakes of Mw 7.7 to 8.2 (Figure 1b), with 100-200 kilometerlong rupture zones [Beck and Ruff, 1984]. Such considerable variation in earthquake rupture length and magnitude in this areas seismic cycles during the last century has raised questions about the nature and enduring significance of the boundaries that exist between rupture zones and about the long-term recurrence interval between earthquakes.

Large‐scale inflation of Tungurahua volcano (Ecuador) revealed by Persistent Scatterers SAR interferometry

The Tungurahua volcano, in Ecuador, has been experiencing a substantial activity period since 1999, with several eruptions, including those of 2006 and 2008. We use a persistent scatterers approach to analyze a time series of Envisat synthetic aperture radar (SAR) data over the period 2003–2009, to investigate surface deformation in the region of the volcano. We measure a continuous large-scale uplift with a maximum line of sight displacement rate of about 8 mm/yr, which is the first evidence of a sustained inflation in the Andes for an active volcano encompassing several eruptions. We model this signal as magma emplacement in a permanent storage zone at 11.5 km below sea level, with a net inflow rate of 7 million m3/yr. The paroxysmal eruptions in 2006 and 2008 did not seem to disrupt this long-term signal. However, we observe significant deformation during the 2006 eruption consistent with an additional intrusion of 4.5 million m3 of magma.

Depth‐dependent rupture mode along the Ecuador‐Colombia subduction zone

A large earthquake (Mw 7.7) occurred on 16 April 2016 within the source region of the 1906 earthquake in the Ecuador-Colombia subduction zone. The 1906 event has been interpreted as a megathrust earthquake (Mw 8.8) that ruptured the source regions of smaller earthquakes in 1942, 1958, and 1979 in this subduction. Our seismic analysis indicated that the spatial distribution of the 2016 earthquake and its aftershocks correlated with patches of high interplate coupling strength and was similar to those of the 1942 earthquake and its aftershocks, suggesting that the 2016 and 1942 earthquakes ruptured the same asperity. Our analysis of tsunami waveforms of the 1906 event indicated Mw around 8.4 and showed that large slip occurred near the trench off the source regions of the above three historical and the 2016 earthquakes, suggesting that a depth-dependent complex rupture mode exists along this subduction zone.

Probabilistic Seismic‐Hazard Assessment in Quito, Estimates and Uncertainties

The present study is focused on estimating the probabilistic seismic hazard for the capital city of Ecuador, Quito, the population of which currently exceeds 2 million inhabitants at present. Quito is located at 2800 meters above sea level within the Interandean Depression, bounded by the equatorial line to the north, in an earthquake‐prone environment (Chatelain et al., 1999; Fig. 1). The city and its suburbs have developed in a piggy‐back basin on the hanging wall of a reverse fault system (Fig. 2) that has been recognized as seismically active in historical, geomorphologic, geologic, and geodetic studies (Soulas et al., 1991; Ego and Sebrier, 1996; Hibsch et al., 1997; Egred, 2009; Champenois et al., 2013; Alvarado et al., 2014).

A New Seismic Hazard Model for EcuadorA New Seismic Hazard Model for Ecuador

We present a comprehensive probabilistic seismic hazard study for Ecuador, a country exposed to a high seismic hazard from megathrust subduction earthquakes and moderate-to-large shallow crustal earthquakes. Building on knowledge gained during the last decade about historical and contemporary seismicity, active tectonics, geodynamics, and geodesy, several alternative earthquake recurrence models have been developed. We propose an areal seismic zonation for the seismogenic crustal, inslab, and interface sources, modified from Yepes et al. (2016), to account for the information gained after the 2016Mw 7.8 Pedernales megathrust earthquake. Three different earthquake catalogs are used to account for uncertainties in magnitude–frequency distribution modeling. This first approach results in low hazard estimates for some areas near active crustal fault systems with low instrumental seismicity, but where geology and/or geodesy document rapid slip rates and high seismic potential. Consequently, we develop an alternative fault and background model that includes faults with earthquake recurrence models inferred from geologic and/or geodetic slip-rate estimates. The geodetic slip rates for a set of simplified faults are estimated from a Global Positioning System (GPS) horizontal velocity field from Nocquet et al. (2014). Various scenarios are derived by varying the percentage of motion that takes place aseismically. Combining these alternative earthquake recurrence models in a logic tree, and using a set of selected ground-motion models adapted to Ecuador’s different tectonic settings, mean hazard maps are obtained with their associated uncertainties. At the sites where uncertainties on hazard estimates are highest (difference between 84th and 16th percentiles > 0:4g), the overall uncertainty is controlled by the epistemic uncertainty on the source model.