A document by Sergio León-Ríos. Click on the document to view its contents.

Based on manually analyzed waveforms recorded by the permanent Ecuadorian network and our large aftershock deployment installed after the Pedernales earthquake, we derive three-dimensional Vp and Vp/Vs structures and earthquake locations for central coastal Ecuador using local earthquake tomography. Images highlight the features in the subducting and overriding plates down to 35 km depth. Vp anomalies (~4.5 – 7.5 km/s) show the roughness of the incoming oceanic crust (OC). Vp/Vs varies from ~1.75 to ~1.94, averaging a value of 1.82 consistent with terranes of oceanic nature. We identify a low Vp (~5.5 km/s) region extending along strike, in the marine forearc. To the North, we relate this low Vp and Vp/Vs (<1.80) region to a subducted seamount that might be part of the Carnegie Ridge (CR). To the South, the low Vp region is associated with high Vp/Vs (>1.85) which we interpret as deeply fractured, probably hydrated OC caused by the CR being subducted. These features play an important role in controlling the seismic behavior of the margin. While subducted seamounts might contribute to the nucleation of intermediate megathrust earthquakes in the northern segment, the CR seems to be the main feature controlling the seismicity in the region by promoting creeping and slow slip events (SSE) offshore that can be linked to the updip limit of large megathrust earthquakes in the northern segment and the absence of them in the southern region over the instrumental period.

The crust and upper mantle structure of the Greater and Lesser Antilles Arc provides insights into key subduction zone processes in a unique region of slow convergence of old slow-spreading oceanic lithosphere. We use ambient noise tomography gathered from island broadband seismic stations and the temporary ocean bottom seismometer network installed as part of the VoiLA experiment to map crustal and upper mantle shear-wave velocity of the eastern Greater Antilles and the Lesser Antilles Arc. We find sediment thickness, based on the depth to the 2.0 km/s contour in the Grenada and Tobago basins up to 15 km in the south, with thinner sediments near the arc and to the north. We observe thicker crust, based on the depth to the 4.0 km/s velocity contour, beneath the arc platforms with the greatest crustal thickness of around 30 km, likely related to crustal addition from arc volcanism through time. There are distinct low velocity zones (4.2-4.4 km/s) in the mantle wedge (30-50 km depth), beneath the Mona Passage, Guadeloupe-Martinique, and the Grenadines. The Mona passage mantle anomaly may be related to ongoing extension there, while the Guadeloupe-Martinique and Grenadine anomalies are likely related to fluid flux, upwelling, and/or partial melt related to nearby slab features. The location of the Guadeloupe-Martinique anomaly is slightly to the south of the obliquely subducted fracture zones. This feature could be explained by either three-dimensional mantle flow, a gap in the slab, variable slab hydration, and/or melt dynamics including ponding and interactions with the upper plate.

The north Ecuador subduction zone exhibits segmentation and clustering of seismicity through megathrust, interseismic, and aftershock seismicity. In 1906, a Mw 8.8 megathrust event ruptured a 500 km segment, portions of which were re-ruptured in ’42 (Mw 7.8), ’58 (Mw 7.7), ’79 (Mw 8.2) and 2016 (Mw 7.8 Pedernales event). Segmentation between the ruptures is caused in part by subducting topography and upper plate structure. Upper plate structure in north Ecuador includes major faults, sedimentary basins and accreted terranes. An international aftershock deployment and the Ecuador permanent network (RENSIG) recorded aftershocks of the 2016 Pedernales event. We performed finite difference tomography in a joint inversion for 3D velocity and earthquake location, using body wave arrivals of aftershocks. The Manabi, Manta-Jama and Borbon sedimentary basins are observed as high Vp/Vs features with the Manabi basin seen as a low Vp and Vs feature. High Vp and Vs are associated with accreted forearc terranes. Relocation of aftershocks in the 3D velocity results in previously described “bands” of seismicity collapsing to smaller clusters ranging from ~8-40 km across. South of the rupture area, a cluster near Manabi collapsed landward, and a cluster appeared west of the trench. Three clusters between the trench and directly south of the rupture contain lower plate and plate interface events. The cluster within the rupture area between the patches of greater slip became more focused, and a cluster became defined on the north side of the northern patch of slip. Two clusters outline subducting Atacames seamounts, with events in the lower plate and interface beneath and in front of the seamounts. North of the rupture, the clusters offshore and onshore near Galera contain mostly interface with some upper plate events. The onshore cluster focused around major faults in a transition from north/south to northeast/southwest structures along the coastal range. Events in the cluster near Atacames relocated mainly in the upper plate, and events in the cluster near Esmeraldas remained in the upper plate. Interseismic events cluster in the same locations as aftershock events. Existing features including upper plate structure and subducting features control and focus both postseismic and interseismic deformation across megathrust cycles.