Stress transfer during the 2016 Pedernales earthquake

Recently, in Ecuador, the amount of fault slip during the Pedernales crisis (2016, Mw7.8) and its dynamic evolution (spatial and temporal) has been determined and modeled after GPS and teleseismic data (Ye et al., 2016 ; Nocquet et al., subm). The southward directivity of the slip along the interface and its abrupt stop nearby Canoa suggests an elastic rebound or release of stress along an oblique inherited fault in the overlying plate. Since permanent plate deformation is not easily observed at subduction zones, some strain accumulation may occur over a small region during the earthquake cycle. Modeling of this type of interaction and feedbacks on the behaviour of the coastal region during a subduction earthquake will be the goal of this Master project. To do so, we will use the slip distributions inverted from geodetic data during the Pedernales crisis, and perform Coulomb modeling of the stress transfer on inherited faults in the coastal region. This will help constrain the interactions between co-seismic slip at the subduction interface and tectonic processes in the upper plate.

Accumulation of strain by far field tectonic loading and its release by major earthquake along the subduction megathrust is known to induce mainly fault slip at the inferface, as frequently observed by the geodetic networks in South America. However, if the oceanic plate subduction is driven by distant tectonic forces, local strain accumulation during the interseismic phase must somehow affect the continental upper plate and inherited tectonic structures.

The 2016 pedernales MW 7.8 earthquake ruptured the subduction zone between the Nazca oceanic and South American continental plates in Ecuador. Joint modeling of seismic and tsunami observations indicates an ∼120 km long rupture area beneath the coastline north of the 1998 MW 7.2 rupture (Ye et al., 2016). The slip distribution reveals two discrete asperities, one near the hypocenter and one south of the rupture zone around the equator line. Their locations and the patchy pattern are consistent with the prior interseismic geodetic strain, which showed highly locked patches beneath the coastline. Aftershocks cluster along two streaks, one aligned nearly parallel to the plate convergence direction up-dip of the main slip patches, and the other on a trench-perpendicular lineation south of the 1958 rupture zone. The 2016 earthquake ruptured the southernmost portion of the larger earthquake of 1906 (MW 8.6, MS 8.6). The seismic behavior reflects persistent heterogeneous frictional properties of the Colombia–Ecuador megathrust and the high interseismic coupling suggest potential release of strain along the upper plate heterogeneities during the co seismic phase.