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 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.

The north Ecuador subduction zone has a history of experiencing a range of slip modes including megathrust and other fast slip, slow, and aseismic slip. In 1906, a Mw 8.8 megathrust ruptured 500 km along the north Ecuador/Colombia margin. Parts of this region re-ruptured in events (south to north): ‘42 (Mw 7.8), ‘58 (Mw 7.7), and ‘79 (Mw 8.2). The April 16, 2016 Pedernales megathrust rupture overlapped the ‘42 rupture. Postseismic deformation following the 2016 event exhibited a range of slip behaviors and associated seismicity. A dense temporary land and offshore deployment augmented permanent stations of the national network (RENSIG) to record postseismic deformation for one year. Aftershocks concentrate spatially in bands or clusters mirroring patterns in background seismicity marking persistent asperities which cause variations in plate coupling. Bands of aftershocks outline the 2016 rupture and two patches of larger slip within the rupture; additional bands are observed to the south and to the north. North of the rupture, bands and clusters are observed near Punta Galera, Atacames, and Esmeraldas. Seismicity near Punta Galera outlines the north edge of a patch of aseismic slip that occurred in the month following the mainshock. One month after the mainshock, Mw 6.7 and 6.9 aftershocks occurred. Calibrated relocations show these are interface events north of the 2016 rupture, downdip of the aseismic slip. On 7/11/ 2016, Mw 5.9 and 6.3 interface events occurred, causing an increase in local seismicity. In June intermittent seismicity began in Esmeraldas, near the 1958 rupture. An earthquake swarm and a transient in GPS data in July 2016 suggests possible slow slip in the region. Relocations of earthquakes in the swarm outline a splay fault in the upper plate. An increase in seismicity near Atacames in December suggests fast slip. Calibrated relocations of the 5 largest events (M 4.7-5.2) and automatic locations of the remaining 246 events show they are upper plate events. In the months following the Pedernales event, fast, aseismic, and slow slip occur north of the rupture. Near Atacames and Esmeraldas upper plate seismicity is predominant.