Evidences of surface rupture associated with a low magnitude (Mw5.0) shallow earthquake in the Ecuadorian Andes

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 9km long previously unknown fault, referred as the Laguna Pisayambo Fault, with purely dextral movement reaching 45cm and concentrated in the top 3km 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 M(w)5.0 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 6days 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.