11We characterise the aftershock sequence following the 2016 Mw=7.8 Pedernales earthquake. 12More than 10,000 events were detected and located, with magnitudes up to 6.9. Most of the 13 aftershock seismicity results from interplate thrust faulting, but we also observe a few normal 14 and strike-slip mechanisms. Seismicity extends for more than 300 km along strike, and is 15 constrained between the trench and the maximum depth of the coseismic rupture. The most 16 striking feature is the presence of three seismicity bands, perpendicular to the trench, which 17 are also observed during the interseismic period. Additionally, we observe a linear 18 dependency between the temporal evolution of afterslip and aftershocks. We also find a 19 temporal semi-logarithmic expansion of aftershock seismicity along strike and dip directions, 20 further indicating that their occurrence is modulated by afterslip. Lastly, we observe that the 21 spatial distribution of seismic and aseismic slip processes is correlated to the distribution of 22 bathymetric anomalies associated with the northern flank of the Carnegie Ridge, suggesting 23 that slip in the area could be influenced by the relief of the subducting seafloor. To explain 24 our observations, we propose a conceptual model in which the Ecuadorian margin is subject 25 to a bimodal slip mode, with distributed seismic and aseismic slip mechanically controlled by 26 the subduction of a rough oceanic relief. Our study sheds new light on the mechanics of 27 subduction, relevant for convergent margins with a complex and heterogeneous structure 28 such as the Ecuadorian margin. 29

SUMMARY The Ecuadorian convergent margin has experienced many large mega-thrust earthquakes in the past century, beginning with a 1906 event that propagated along as much as 500 km of the plate interface. Many subsections of the 1906 rupture area have subsequently produced Mw ≥ 7.7 events, culminating in the 16 April 2016, Mw 7.8 Pedernales earthquake. Interestingly, no large historic events Mw ≥ 7.7 appear to have propagated southward of ∼1°S, which coincides with the subduction of the Carnegie Ridge. We combine data from temporary seismic stations deployed following the Pedernales earthquake with data recorded by the permanent stations of the Ecuadorian national seismic network to discern the velocity structure of the Ecuadorian forearc and Cordillera using ambient noise tomography. Ambient noise tomography extracts Vsv information from the ambient noise wavefield and provides detailed constraints on velocity structures in the crust and upper mantle. In the upper 10 km of the Ecuadorian forearc, we see evidence of the deepest portions of the sedimentary basins in the region, the Progreso and Manabí basins. At depths below 30 km, we observe a sharp delineation between accreted fast forearc terranes and the thick crust of the Ecuadorian Andes. At depths ∼20 km, we see a strong fast velocity anomaly that coincides with the subducting Carnegie Ridge as well as the southern boundary of large mega-thrust earthquakes. Our observations raise the possibility that upper-plate structure, in addition to the subducting Carnegie Ridge, plays a role in the large event segmentation seen along the Ecuadorian margin.

The April 2016, Pedernales Earthquake ruptured a 100 km by 40 km segment of the subduction zone along the coast of Ecuador in an Mw 7.8 megathrust event east of the intersection of the Carnegie Ridge with the trench. This portion of the subduction zone has ruptured on decadal time scales in similar size and larger earthquakes, and exhibits a range of slip behaviors, variations in segmentation, and degree of plate coupling along strike. Immediately after the earthquake, an international rapid response effort coordinated by the Instituto Geofísico at the Escuela Politécnica Nacional in Quito deployed 55 seismometers and 10 OBS above the rupture zone and adjacent areas to record aftershocks. In this article we describe the details of the US portion of the rapid response and present an earthquake catalog from May 2016-May 2017 produced using data recorded by these stations. Aftershocks focus in distinct clusters within and around the rupture area and match spatial patterns observed in long term seismicity. For the first two and a half months, aftershocks exhibit a relatively sharp cutoff to the north of the mainshock rupture. In early July an earthquake swarm occurred ~100 km to the northeast of the mainshock in the epicentral region of a Mw 7.8 earthquake in 1958. In December, an increase in seismicity occurred ~70 km to the northeast of the mainshock in the epicentral region of the 1906 earthquake. Data from the Pedernales earthquake and aftershock sequence recorded by permanent seismic and geodetic networks in Ecuador and the dense aftershock deployment provide an opportunity to examine the persistence of asperities for large to great earthquakes over multiple seismic cycles, the role of asperities and slow slip in subduction zone Meltzer et al., SRL Data Mine: Pedernales RAPID Deployment 3 megathrust rupture, and the relationship between locked and creeping parts of the subduction interface.

On April 16 th 2016 a Mw 7.8 earthquake ruptured the central coastal segment of the Ecuadorian subduction zone. Shortly after the earthquake, the Instituto Geofisico de la Escuela Politecnica Nacional of Ecuador, together with several international institutions deployed a dense, temporary seismic network to accurately categorize the post-seismic aftershock sequence. Instrumentation included short-period and broadband sensors, along with Ocean Bottom Seismometers. This deployment complemented the permanent Ecuadorian seismic network and recorded the developing aftershock sequence for a period of one year following the main-shock. A subset of 345 events with M L > 3.5, were manually picked in the period of May to August 2016, providing highly accurate P-and S-onset times. From this catalogue, a high-quality dataset of 227 events, with an azimuthal gap < 200°, are simultaneously inverted for, obtaining the minimum 1D velocity model for the rupture region, along with hypocentral locations and station corrections. We observe an average Vp/Vs of 1.82 throughout the study region, with relatively higher Vp/Vs values of 1.95 and 2.18 observed for the shallowest layers down to 7.5 km. The high relative Vp/Vs ratio (1.93) of the deeper section, between 30 km and 40 km, is attributed to dehydration and serpentinization processes. For the relocated seismicity distribution, clusters of events align perpendicular to the trench, and crustal seismicity is also evidenced, along with earthquakes located close to the trench axis. We also compute Regional Moment Tensors to analyze the different sources of seismicity after the mainshock. Aside from thrust events related to the subduction process, normal and strike-slip mechanisms are detected. We suggest that the presence of subducting seamounts coming from the Carnegie Ridge act as erosional agents, helping to create a scenario which promotes locking and allows seismicity to extend up to the trench, along zones of weakness activated after large earthquakes.

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