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

Stéphane Baize,Pedro Espin,Alexandra Alvarado,Xavier Quidelleur,Pablo Samaniego,Hervé Jomard,Johann Champenois,Jean-Luc Le Pennec,Mathilde Bablon,Laurence Audin

doi:10.3389/feart.2020.00193

Abstract

Based on new geological data and the analysis of a 4 m spatial resolution Digital Elevation Model (DEM), we provide a detailed and comprehensive description of section of the Chingual Cosanga Pallatanga Puna Fault System, a major active fault system in Ecuador. This work allows estimating new slip rates and large earthquakes parameters (displacement, recurrence) along a ∼100 km-long section of the continental-scale dextral shear zone that accommodates the extrusion of the North Andean Sliver with respect to the South America continental Plate. We focus on the NE-SW Pallatanga strike-slip fault zone and related contractional and transcurrent features that extend to the north in the Inter-Andean valley and the Cordillera Real, respectively. The detailed analysis of the available DEM allowed mapping a series of lineaments at the regional scale and along the entire fault system. Field studies on key areas show valley deflections, aligned and elongated hills of Tertiary or Quaternary sediments, as well as faulted Holocene deposits and even preserved coseismic free-face ruptures in some places. Such morphological anomalies strongly suggest that those landscape scars represent long-living (Holocene to historical times) earthquake faults. Altogether, these new data confirm that very large crustal earthquakes (M∼7.5) have been generated along the fault system, probably during multiple segment ruptures. This conclusion agrees with reports of large earthquakes during historical times (post-1532 CE) in 1698, 1797, and 1949. They all occurred in the vicinity of the Pallatanga fault, causing catastrophic effects on environmental and cultural features. Based on new sample dating of both soils and volcanic series, we infer that the NE-SW dextral Pallatanga fault slips at rates ranging from ∼2 to 6 mm/yr for southern and central strands of the studied area, respectively. Further north, surface faulting is distributed and the deformation appears to be partitioned between sub-meridian fault-related folds (∼2 mm/yr) and NE- SW strike-slip fault(s), like the ∼1 mm/yr Pisayambo Fault that ruptured the surface in 2010. All this information offers the opportunity to size the earthquake sources for further seismic hazard analyses.

Highlights

GENERAL BACKGROUNDEarthquake hazard analyses in continental areas, especially those with sustained seismic activity, are based on fault models supported by detailed studies of active faults (e.g., Field et al, 2014)
We focused on a 100 kmlong and ∼20 km-wide area covering a section of the fault zone that delineates the eastern margin of the NNE-moving North Andean Sliver (Figure 1) and which has been defined as the Chingual Cosanga Pallatanga Puna Fault System (CCPP; Nocquet et al, 2014; Alvarado et al, 2016; Yepes et al, 2016) between Colombia to the Pacific Ocean
In the frame of the geodynamics and active tectonics of the Northern Andes of Ecuador, we present our major findings based on morphological analysis of a 4 m-high resolution Digital Elevation Model (DEM) and field investigations

Summary

Introduction

GENERAL BACKGROUNDEarthquake hazard analyses in continental areas, especially those with sustained seismic activity, are based on fault models supported by detailed studies of active faults (e.g., Field et al, 2014). Beauval et al (2018) produced the hazard calculation for the entire country, defining a fault model with first-order fault geometries and earthquake parameters They only considered two distinct simplified source faults (Pallatanga and Latacunga faults), named according to the surrounding cities (Figure 1) settled in between the Western Cordillera and the Eastern Cordillera. In this work, their slip rates were derived from Global Navigation Satellite System (GNSS) velocity field (Nocquet et al, 2014). This approach is relevant for national scale hazard estimation, but it could be significantly misleading for site-specific approaches nearby the fault sources

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