Evidences of Surface Rupture Associated With a Low‐Magnitude (M_w5.0) Shallow Earthquake in the Ecuadorian Andes

Abstract

This study analyzes surface displacements generated by a low‐magnitude crustal earthquake in the Ecuadorian Andes by combining analysis of synthetic aperture radar (SAR) interferometry, geological field investigations, and seismological data. In March 2010, a significant surface faulting event occurred in the Pisayambo area (Eastern Cordillera), along the major dextral fault zone bounding the North Andean Sliver and the South America Plate. Interferograms were inverted to determine fault plane geometry and slip displacement distribution. The event affected a 9 km long previously unknown fault, referred as the Laguna Pisayambo Fault, with purely dextral movement reaching 45 cm and concentrated in the top 3 km of the crust. Geological investigations confirm both the fault mechanism and the amplitude of displacements. While these large displacements would be related to an event with a magnitude of 5.44 if using a standard crustal rigidity, we show that they can be convincingly associated with an Mw5.0 earthquake, that occurred on 26 March 2010. Reconciling the apparent differences in magnitude requires the existence of a low‐rigidity medium at shallow depths and/or postseismic activity of the fault. However, considering only the latter hypothesis would imply an unusually active postseismic process, in which 400–500% of the coseismic moment is released in the 6 days following the earthquake. Our observations highlight that the scaling laws relating surface observations to earthquake magnitude, classically used for seismic hazard assessment, should be carefully used. This study also illustrates how systematic InSAR analysis, even in places where no clues of ground deformation are present, can reveal tectonic processes.

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

“…Those are Rumipamba (R), Riobamba and Igualata/Huisla volcanoes, as well as the Patate river valley (IH), Laguna Pisayambo (P), and Salcedo/Latacunga (L) (Figures 1, 3). Our new mapping in IH area completes and improves those of Baize et al (2015) and Champenois et al (2017) on the Rumipamba and Pisayambo areas, respectively. In Supplementary Material, we provide five maps featuring 2 www.sigtierras.gob.ec the fault strands layered over the 4 m spatial resolution DEM ( Supplementary Figures S6-S11).…”

“…There is a lack of clear lineament on the DEM between the aforementioned neotectonic clues and the NE-SW active Laguna Pisayambo fault mapped and observed close to the Angahuana volcano ( Supplementary Figure S10), raising the possibility of a 15 km-wide active fault gap (Figure 3). Interestingly, this fault ruptured up to the surface in 2010 during a moderate earthquake (M∼5) (Champenois et al, 2017), and this historical surface rupture exactly sticks to a cumulative fault that dextrally displaces Pleistocene moraines of about ∼15 m and disrupts Holocene soils (Figures 9b,c; pictures 2014-18-1 and 2014-32 in Supplementary Material). A stratigraphic section in a handmade trench allowed the check of the 2010 and syn-andisol development (Holocene) activity of the fault, during the last 8 ky (see Supplementary Figure S4 in Champenois et al, 2017 and picture 2014-34, Supplementary Material).…”

“…These pioneer studies came out with significant slip rates (>1 mm/yr), raising questions on the seismic hazard they pose to the region. During the last decade, the franco-ecuadorian team ("Laboratoire Mixte International Séismes et Volcans des Andes du Nord") 1 launched new research that combine seismology, geodesy and geology (e.g., Font et al, 2013;Alvarado et al, 2014Alvarado et al, , 2016Champenois et al, 2014Champenois et al, , 2017Nocquet et al, 2014Nocquet et al, , 2016Baize et al, 2015) and consolidated the seismotectonic model (Yepes et al, 2016) at the national scale.…”

“…The 06/03/1987 Reventador earthquake sequence includes the largest crustal earthquake recorded in Ecuador (Mw = 7.1) that occurred within a restraining bend of the CCPP (Tibaldi et al, 1995). Besides, very few events of M > 4 were recorded along the CCPP and, according to Araujo (2016), the Pisayambo cluster is actually the major source of seismic energy release, with its strongest event being the Mw∼5.0 (26/3/2010) which caused a surface rupture on a CCPP segment (Champenois et al, 2017). Vaca et al (2019) provide new focal mechanisms of earthquakes within the 2009-2015 time span, based on seismic velocity structure and data from the Ecuadorian broadband seismic network.…”

Coseismic Rupture Geometry and Slip Rupture Process During the 2018 Mw 7.1 Anchorage, South‐Central Alaska Earthquake: Intraplate Normal Faulting by Slab Tear Constrained by Geodetic and Teleseismic Data

“…The recent InSAR measurement, advanced in high spatial resolution and large‐scale coverage, which overcome the drawback of time‐ and labor‐intensive in situ ground‐based observation, has become a routine response in earthquake investigations (He et al, ). Present InSAR can provide high‐quality coseismic displacement caused not only by large and moderate earthquakes but also by small earthquakes, for example, the 2011 Mw 5.3 Trinidad earthquake (Barnhart et al, ), the 2010 Mw 5.0 Ecuadorian Andes earthquake (Champenois et al, ), and the 2016 Mw 6.0 Australia earthquake (Polcari et al, ), suggesting it should be available for this Mw 7.1 earthquake, even though the earthquake is in intermediate depth. We collect SAR data from the Sentinel‐1 radar satellite constellation, which is operated by the Europe Space Agency (ESA) and launched on 3 April 2014.…”