Geodetic evidence for shallow creep along the Quito fault, Ecuador

Abstract

SUMMARY Quito, the capital city of Ecuador hosting ∼2 million inhabitants, lies on the hanging wall of a ∼60-km-long reverse fault offsetting the Inter-Andean Valley in the northern Andes. Such an active fault poses a significant risk, enhanced by the high density of population and overall poor building construction quality. Here, we constrain the present-day strain accumulation associated with the Quito fault with new Global Positioning System (GPS) data and Persistent Scatterer Interferometric Synthetic Aperture Radar (PS-InSAR) analysis. Far field GPS data indicate 3–5 mm yr–1 of horizontal shortening accommodated across the fault system. In the central segment of the fault, both GPS and PS-InSAR results highlight a sharp velocity gradient, which attests for creep taking place along the shallowest portion of the fault. Smoother velocity gradients observed along the other segments indicate that the amount of shallow creep decreases north and south of the central segment. 2-D elastic models using GPS horizontal velocity indicate very shallow (<1 km) locking depth for the central segment, increasing to a few kilometres south and north of it. Including InSAR results in the inversion requires locking to vary both along dip and along strike. 3-D spatially variable locking models show that shallow creep occurs along the central 20-km-long segment. North and south of the central segment, the interseismic coupling is less resolved, and data still allows significant slip deficit to accumulate. Using the interseismic moment deficit buildup resulting from our inversions and the seismicity rate, we estimate recurrence time for magnitude 6.5 + earthquake to be between 200 and 1200 yr. Finally, PS-InSAR time-series identify a 2 cm transient deformation that occurred on a secondary thrust, east of the main Quito fault between 1995 and 1997.

Detecting a master thrust system by magnetotelluric sounding along the western Andean Piedmont of Quito, Ecuador

“…Thrusting and folding systems are widely distributed throughout the Ecuadorian Andes (Égüez, Alvarado, & Yepes, 2003); however, only the flexural anticlinal ridges of the QFS have been studied with some detail on the surface (Villagómez, 2003). Several purely reverse‐slip and a mixture of strike‐slip and reverse‐slip focal mechanisms together with small‐magnitude seismicity and earthquake locations (Figure 2) have been reported along the Quito Basin (Alvarado et al., 2014; Mariniere et al, 2020), although a reliable underground geometric structuration is still lacking (e.g. Alvarado et al., 2014).…”

“…Alvarado et al., 2014). Recent studies (Mariniere et al, 2020) based on GPS and InSAR data use a model including a flat decollement at 10 km depth beneath the Quito Basin for the QFS, but without specifying the fault geometry at depth.…”

“…Faults, folds, focal mechanisms and earthquake locations of the QFS were taken from Alvarado et al. (2014) and Mariniere et al (2020). The geological cross‐section was modified from Villagómez (2003), which only includes the upper Quaternary sequences.…”

Active Tectonics and Earthquake Geology Along the Pallatanga Fault, Central Andes of Ecuador

“…When projecting the LOS onto the two fault planes, we end up with actual slip rates of 1.7 and 0.5 mm/yr for the Eastern and Western Boundary Faults, respectively. Those reverse faults slip rates are much lower than the one determined for the Quito Fault (3-5 mm/yr) in the same large tectonic block (Quito Latacunga block; Alvarado et al, 2014), a fault that is now recognized to absorb a portion of shallow creep in its central part (Marinière et al, 2020).…”

Post‐Earthquake Fold Growth Imaged in the Qaidam Basin, China, With Interferometric Synthetic Aperture Radar

“…In addition, although simplified models are valuable in many ways, modeling shortening and the associated uplift across a fold‐and‐thrust belt system with one or two elastic dislocations is an arguable simplification of the complicated history and geometry of a thrusting morphology, which may involve several interconnected faults and folds that interact with each other, multiple bends, and anelastic deformation as in the upper plate around the bends (e.g., Daout, Barbot, Peltzer, Doin, et al., 2016; Davis et al., 1983; Medwedeff & Suppe, 1997; Sathiakumar et al., 2020; Tapponnier, Meyer, et al., 1990; Whipple et al., 2016). Some other geodetic observations have recently documented or/and modeled episodic aseismic ground deformations (e.g., Mariniere et al., 2020) or long‐periods of afterslip that correlate with the present‐day geomorphology (e.g., Barnhart, Lohman, & Mellors, 2013; Copley, 2014; Daout, Sudhaus, Kausch, et al., 2019; Elliott, Bergman, et al., 2015; Fielding et al., 2004; Mackenzie et al., 2016; Wimpenny et al., 2017; Zhou et al., 2018). These measurements suggest that the permanent deformation in fold‐and‐thrust belts might be sometimes created by distributed off‐fault deformation (i.e., anelastic buckling of the medium), or aseismic slip on secondary faults branching from the main earthquake fault or around it, and which occur during stages of the earthquake cycle other than the seismic event.…”