Subducted oceanic relief locks the shallow megathrust in central Ecuador

• Published in: Journal of geophysical research. Solid earth 2017-05, Vol.122 (5), p.3286-3305
• Main Authors: Collot, Jean‐Yves, Sanclemente, Eddy, Nocquet, Jean‐Mathieu, Leprêtre, Angélique, Ribodetti, Alessandra, Jarrin, Paul, Chlieh, Mohamed, Graindorge, David, Charvis, Philippe
• Format: Article
• Language: English
• Subjects: Angle of reflection; Area; Asperity; Banks (topography); Bathymetry; Coastal environments; Correlation; Deformation; Depth; Earth Sciences; Earthquakes; Friction; Geology; Geophysical data; Geophysics; Height; Hosts; Imaging; Imaging techniques; interseismic coupling; Language; Lines; Locking; Locks; megathrust earthquake; Reflection; Rupture; Rupturing; Sciences of the Universe; seamount subduction; Seamounts; Sediment; Seismic activity; Seismic data; seismic imaging; Seismic surveys; Seismological data; Shearing; Slip; Slope; slow slip event; Subduction; Subduction (geology); Topography; Topography (geology); Uplift; Width
• ISSN: 2169-9313; 2169-9356
• DOI: 10.1002/2016JB013849

Whether subducted oceanic reliefs such as seamounts promote seismic rupture or aseismic slip remains controversial. Here we use swath bathymetry, prestack depth‐migrated multichannel seismic reflection lines, and wide‐angle seismic data collected across the central Ecuador subduction segment to reveal a broad ~55 km × 50 km, ~1.5–2.0 km high, low height‐to‐width ratio, multipeaked, sediment‐bare, shallow subducted oceanic relief. Owing to La Plata Island and the coastline being located, respectively, ~35 km and ~50–60 km from the trench, GPS measurements allow us to demonstrate that the subducted oceanic relief spatially correlates to a shallow, ~80 km × 55 km locked interplate asperity within a dominantly creeping subduction segment. The oceanic relief geometrical anomaly together with its highly jagged topography, the absence of a subduction channel, and a stiff erosive oceanic margin are found to be long‐term geological characteristics associated with the shallow locking of the megathrust. Although the size and level of locking observed at the subducted relief scale could produce an Mw >7+ event, no large earthquakes are known to have happened for several centuries. On the contrary, frequent slow slip events have been recorded since 2010 within the locked patch, and regular seismic swarms have occurred in this area during the last 40 years. These transient processes, together with the rough subducted oceanic topography, suggest that interplate friction might actually be heterogeneous within the locked patch. Additionally, we find that the subducted relief undergoes internal shearing and produces a permanent flexural bulge of the margin, which uplifted La Plata Island. Plain Language Summary Subducted seamounts play an important but still uncertain role in earthquake rupture processes, as seamounts are considered to subduct either aseismically or seismically. We use marine geophysical data across the central Ecuador convergent margin to reveal a broad ~55 km × 50 km, ~1.5–2.0 km high, shallow subducted oceanic relief of the Carnegie Ridge. New GPS measurements collected on La Plata Island and along the Ecuador coast allow us to demonstrate that the subducted oceanic relief spatially correlates to a shallow, ~80 km × 55 km locked interplate asperity within a dominantly creeping subduction segment. The oceanic relief geometrical anomaly together with its rough topography and the stiff oceanic Ecuador margin are found to be long‐term geological characterist