Structural Control on Megathrust Rupture and Slip Behavior: Insights From the 2016 Mw 7.8 Pedernales Ecuador Earthquake

The heterogeneous seafloor topography of the Nazca Plate as it enters the Ecuador subduction zone provides an opportunity to document the influence of seafloor roughness on slip behavior and megathrust rupture. The 2016 M$_{w}$ 7.8 Pedernales Ecuador earthquake was followed by a rich and active postseismic sequence. An internationally coordinated rapid response effort installed a temporary seismic network to densify coastal stations of the permanent Ecuadorian national seismic network. A combination of 82 onshore short and intermediate period and broadband seismic stations and six ocean bottom seismometers recorded the postseismic Pedernales sequence for over a year after the mainshock. A robust earthquake catalog combined with calibrated relocations for a subset of magnitude ≥4 earthquakes shows pronounced spatial and temporal clustering. A range of slip behavior accommodates postseismic deformation including earthquakes, slow slip events, and earthquake swarms. Models of plate coupling and the consistency of earthquake clustering and slip behavior through multiple seismic cycles reveal a segmented subduction zone primarily controlled by subducted seafloor topography, accreted terranes, and inherited structure. The 2016 Pedernales mainshock triggered moderate to strong earthquakes (5 ≤ M ≤ 7) and earthquake swarms north of the mainshock rupture close to the epicenter of the 1906M$_{w}$ 8.8 earthquake and in the segment of the subduction zone that ruptured in 1958 in a M$_{w}$ 7.7 earthquake

1D-velocity structure and seismotectonics of the Ecuadorian margin inferred from the 2016 Mw7.8 Pedernales aftershock sequence

International audienceOn April 16th 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 ML > 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

Earthquake And Tectonic Processes In The Mid-Atlantic Us Passive Margin And The Ecuador Subduction Zone

Our ability to minimize the risks associated with earthquake hazards is directly linked to our fundamental understanding of the causes, processes and dynamics associated with this type of natural event. This dissertation presents my research work towards a better understanding of earthquake and tectonic processes in two distinct settings: the Mid-Atlantic US passive margin and the Ecuador subduction zone.The first part of my dissertation focused in studying intraplate seismicity and crustal structure in the Mid-Atlantic US. Our understanding of the characteristics of and mechanisms responsible for seismicity in passive margins is limited. To assess a plausible mechanism for intraplate seismicity and deformation in the Mid-Atlantic US, I calculated receiver functions to image the crustal structure, and characterized the spatio-temporal distribution of seismicity. Receiver functions imaged a steep gradient in crustal thickness, in the transition from the Coastal Plain-Piedmont into the Valley and Ridge. The spatial distribution of earthquakes coincides with the observed crustal thickness gradient. This suggests that lateral variations in crustal structure, inherited from past orogenic and rifting processes, play a role in the concentration of stress, and the spatial distribution of seismicity.The second part of my dissertation focused on an active tectonic margin. On April 16, 2016, a Mw7.8 megathrust earthquake ruptured a ~100x40km segment of the Ecuador subduction zone. An automatic catalog of 9,036 events was created to analyze the post-seismic activity associated with this large earthquake. Calibrated multiple-event relocations were calculated for M\u3e4 earthquakes. This seismic sequence is dominated by pronounced spatio-temporal clustering in regions that have remained active during the interseismic period and/or experienced slow slip events, repeaters and swarms. The mainshock appears to have induced stress transfer towards the northern 1958 M7.7 rupture area. Event locations show a sharp downdip limit in seismic activity where the slab reaches a depth of ~30km. This suggests a well-defined transition in frictional properties from the seismogenic zone into the conditionally stable downdip region. A better definition of the updip and downdip limits of the seismogenic zone helps to assess which inland population centers may be more affected by large co-seismic slip in the future

The Mw7.8 2016 Pedernales, Ecuador earthquake aftershock sequence: a detailed spatio-temporal analysis of the rupture processes, stress patterns and slip behavior

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Postseismic deformation: the Esmeraldas, Ecuador Seismic sequence Following the 2016 Pedernales Megathrust Earthquake

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Structure of the Ecuadorian forearc from the joint inversion of receiver functions and ambient noise surface waves

International audienceThe Ecuadorian forearc is a complex region of accreted terranes with a history of largemegathrust earthquakes. Most recently, aMw7.8 megathrust earthquake ruptured the plateboundary offshore of Pedernales, Ecuador on 16 April 2016. Following this event, an inter-national collaboration arranged by the Instituto Geofisico at the Escuela Polit ́ecnica Nacionalmobilized a rapid deployment of 65 seismic instruments along the Ecuadorian forearc. Wecombine this new seismic data set with 14 permanent stations from the Ecuadorian nationalnetwork to better understand how variations in crustal structure relate to regional seismichazards along the margin. Here, we present receiver function adaptive common conversionpoint stacks and a shear velocity model derived from the joint inversion of receiver functionsand surface wave dispersion data obtained through ambient noise cross-correlations for theupper 50 km of the forearc. Beneath the forearc crust, we observe an eastward dipping slowvelocity anomaly we interpret as subducting oceanic crust, which shallows near the projectedcentre of the subducting Carnegie Ridge. We also observe a strong shallow positive conversionin the Ecuadorian forearc near the Borbon Basin indicating a major discontinuity at a depth of∼7 km. This conversion is not ubiquitous and may be the top of the accreted terranes. We alsoobserve significant north–south changes in shear wave velocity. The velocity changes indicatevariations in the accreted terranes and may indicate an increased amount of hydration beneaththe Manab ́ı Basin. This change in structure also correlates geographically with the southernrupture limit of multiple high magnitude megathrust earthquakes. The earthquake record alongthe Ecuadorian trench shows that no event with aMw>7.4 has ruptured south of∼0.5◦Sinsouthern Ecuador or northern Peru. Our observations, along with previous studies, suggestthat variations in the forearc crustal structure and subducting oceanic crust may influance theoccurrence and spatial distribution of high magnitude seismicity in the region

Upper-plate structure in Ecuador coincident with the subduction of the Carnegie Ridge and the southern extent of large mega-thrust earthquakes

International audienceThe 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 Manabı´ 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

1D-velocity structure and seismotectonics of the Ecuadorian margin inferred from the 2016 Mw7.8 Pedernales aftershock sequence

International audienceOn April 16th 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 ML > 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

Triggered crustal earthquake swarm across subduction segment boundary after the 2016 Pedernales, Ecuador megathrust earthquake

International audienceMegathrust ruptures and the ensuing postseismic deformation cause stress changes that may induce seismicity on upper plate crustal faults far from the coseismic rupture area. In this study, we analyze seismic swarms that occurred in the north Ecuador area of Esmeraldas, beginning two months after the 2016 Mw7.8 Pedernales, Ecuador megathrust earthquake. The Esmeraldas region is 70 km from the Pedernales rupture area in a separate segment of the subduction zone. We characterize the Esmeraldas sequence, relocating the events using manual arrival time picks and a local a-priori 3D velocity model. The earthquake locations from the Esmeraldas sequence outline an upper plate fault or shear zone. The sequence contains one major swarm and several smaller swarms. Moment tensor solutions of several events include normal and strike-slip motion and non-double-couple components. During the main swarm, earthquake hypocenters increase in distance from the first event over time, at a rate of a few hundred meters per day, consistent with fluid diffusion. Events with similar waveforms occur within the sequence, and a transient is seen in time series of nearby GPS stations concurrent with the seismicity. The events with similar waveforms and the transient in GPS time series suggest that slow aseismic slip took place along a crustal normal fault during the sequence. Coulomb stress calculations show a positive Coulomb stress change in the Esmeraldas region, consistent with seismicity being triggered by the Pedernales mainshock and large aftershocks. The characteristics of the seismicity indicate that postseismic deformation involving fluid flow and slow slip activated upper plate faults in the Esmeraldas area. These findings suggest the need for further investigation into the seismic hazard potential of shallow upper plate faults and the potential for megathrust earthquakes to trigger slow-slip and shallow seismicity across separate segments of subduction zones