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International audience; Repeating earthquakes repeatedly rupture the same seismic asperity and are strongly linked to aseismic slip. Here, we study the repeating aftershocks of the April 16, 2016 MW 7.8 Pedernales earthquake in Ecuador, which generated a large amount of afterslip. Using temporary and permanent stations, we correlate waveforms from a one‐year catalog of aftershocks. We sort events with a minimum correlation coefficient of 0.95 into preliminary families, which are then expanded using template‐matching to include events from April 2015 to June 2017. In total, 376 repeaters are classified into 62 families of 4–15 events. They are relocated, first using manual picks, and then using a double difference method. We find repeating earthquakes during the whole period, occurring primarily within large aftershock clusters on the edges of the areas of largest afterslip release. Their recurrence times, shortened by the mainshock, subsequently increase following an Omori‐type law, providing a timeframe for the afterslip's deceleration. Although they are linked temporally to the afterslip, repeater‐derived estimates of slip differ significantly from GPS‐based models. Combined with the fact that repeaters appear more spatially correlated with the afterslip gradient than with the afterslip maxima, we suggest that stress accumulation at the edge of the afterslip may guide repeater behavior.

Repeating earthquakes are earthquakes that repeatedly break a single, time-invariant fault patch. They are generally associated with aseismic slip, which is thought to load asperities, leading to repeated rupture. Repeating earthquakes are therefore useful tools to study aseismic slip and fault mechanics, with possible applications to earthquake triggering, loading rates and earthquake forecasting.In this study, we analyze one year of aftershocks following the 16th April 2016 Mw 7.8 Pedernales earthquake in Ecuador to find repeating families, using data recorded by permanent and temporary seismological stations. In our area, seismicity during both the inter-seismic and post-seismic periods has been previously linked to aseismic slip. We calculate waveform cross-correlation coefficients (CC) on all available catalogue events, which we use to sort events into preliminary families, using a minimum CC of 0.95. These events were then stacked and used to perform template-matching on the continuous data. In total, 376 earthquakes were classified into 62 families of 4 to 15 earthquakes, including 8 from the one-year period before the mainshock. We later relocated these earthquakes using a double-difference method, which confirmed that most of them did have overlapping sources.Repeating earthquakes seem to concentrate largely around the areas of largest afterslip release, where afterslip gradient is the highest. We also find an increase in the recurrence time of repeating events with time after the mainshock, over the first year of the postseismic period, which highlights a possible timeframe for the afterslip’s deceleration. Our results suggest that while most repeating aftershocks are linked to afterslip release, the afterslip gradient may play a bigger role in determining their location than previously thought.

International audience; The north Ligurian margin is a complex geological area in many ways. It has witnessedseveral phases of highly contrasting deformation styles, at both crustal scale and that of shallower covertectonics, simultaneously or in quick succession, and with significant spatial variability. This complexinterplay is mirrored in the resulting intricate structures that make it hard to identify active faultsresponsible for both, the significant seismicity observed, and the tectonic inversion undergone bythe margin, identified at longer time scales on morphostructural grounds. We present here the firstpreliminary results of the leg 1 of SEFASILS cruise, conducted in 2018 offshore Monaco, in an effort toanswer these questions by means of modern deep seismic acquisitions, using multichannel reflectionand wide-angle sea-bottom records. Some first interpretations are provided and point towards anactive basement deformation that focuses at the limits between main crustal domains.

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

International audience

International audience; The Mw7.8 Pedernales earthquake is associated with the subduction of the Nazca Plate beneath the South American Plate. The mainshock caused many casualties and widespread damages across the Manabi province. The 150 km-long coseismic rupture area is found beneath the coastline, near 25 km depth. The rupture propagated southward and involved the successive rupture of two discrete asperities, with a maximum slip (~ 6 m) on the southern patch. The rupture area is consistent with the highly locked regions observed on interseismic coupling models, overlaps the 7.2 Mw rupture zone, and terminates near where the 1906 Mw 8.8 megathrust earthquake rupture zone ends. Two neighboring highly coupled patches remain locked: (A) southern to and updip of the coseismic rupture zone and (B) northern and downdip. In this study, we relocate the aftershocks and compare the seismicity distribution to the interseismic coupling and the rupture area. We use continuous seismic traces recorded on the permanent network partly installed in the frame of the collaboration between l’Institut de Recherche pour le Développement (IRD-France) and the Instituto Geofísico, Escuela Politécnica Nacional, Quito, Ecuador. Detections are conducted using Seiscomp in play-back mode. Arrival-times are manually picked. To improve earthquake location, we use the MAXI technique and a heterogeneous a priori P-wave velocity model that approximates the large velocity variations of the Ecuadorian subduction system. Aftershocks align along 3 to 4 main clusters that strike perpendicularly to the trench, and mostly updip of the co-seismic rupture. Aftershock seismicity develops indifferently over portions of plate interface that are known to be strongly locked or almost uncoupled. The seismicity pattern is similar to the one observed during a decade of observation during the interseismic period with swarms such as the Galera alignment, Jama and Cabo Pasado, and between Manta and Puerto Lopez.

International audience; We study source parameters of a cluster of 21 very shallow (

Subduction margins, as in the case of south-central Chile, are active seismotectonic environments and locus of the world largest earthquakes. In this thesis, two segments of the south-central Chilean subduction margin are studied: (A) the southernmost portion, at the termination of the Nazca-South America convergence (~46ºS), and (B) the segment located between 34º-38ºS, where the Mw 8.8 Maule Earthquake took place in 2010. Analysis of data from a local seismic network deployed in 2004-2005 in area A, indicates low levels of background seismicity with magnitudes ranging 0-3.4 Ml. The seismicity corresponds to shallow crustal events, mostly occurring within the upper 10 km. A third of the seismicity is associated to volcanic activity present in the area, while scarce seismicity is associated with a large strike-slip fault, the Liquiñe-Ofqui Fault System (LOFS), that intersects the region along the arc in a N-S-trend. In 2007, this region was affected by a seismic sequence with a peak of activity associated with a Mw 6.2 earthquake in April that year. A local seismic network was deployed after this main event in order to study its sequence of aftershocks, which provided a unique opportunity to characterise seismotectonically this area that usually lacks intermediate magnitude seismicity, including the calculation of a new local velocity model, accurate aftershock locations and computation of focal mechanisms. The results show P-wave velocities of ~5 km/s for the upper 5 km in accordance with the geology of the area, and low S-wave velocities for the upper 3 km of crust due to rock fracturing and the presence of fluids. An average Vp/Vs ratio of 1.76 was calculated for the region. The alignment of most of the aftershocks within the LOFS plus obtained focal mechanisms, indicate that this sequence had tectonic origin related to the re-activation of the LOFS. Further, a maximum seismogenic depth of about 15 km was determined for the entire region. Regarding area B, affected by a large megathrust earthquake in 2010, the study of moment tensor solutions for the sequence of aftershocks provided new insight into the distribution of postseismic activity relative to co-seismic slip and the release of seismic afterslip. Thrust aftershocks dominate the postseismic activity, but also normal faulting was detected in the outer-rise area and in the overriding plate near the coastline. The largest seismically released afterslip is located between the two main patches of co-seismic slip. Large aftershocks (M>4) occur along the megathrust interface, in zones of intermediate co-seismic slip associated to stress introduced on dislocation tips with high co-seismic slip contrast. On the other hand, smaller events (M

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 (1.85) 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 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.

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 (1.85) 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 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., Plain Language Summary: Using seismic data recorded by the permanent Ecuadorian network and the large emergency installation after the 2016 Pedernales earthquake, we obtained the seismic velocity structure together with precise earthquake locations for the coastal Ecuadorian margin. Our images highlight the heterogeneities of the subduction zone affected by seamounts and ridges comprising the oceanic crust. These features play an important role in controlling the seismic behavior of the margin. While seamounts can contribute to the occurrence of intermediate (M ∼ 7–7.5) megathrust earthquakes in the north, the Carnegie Ridge seems to be the main feature controlling the seismicity in the region by promoting creeping and slow slip events 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., Key Points: 3D Vp and Vp/Vs models were calculated using local earthquake tomography in the region affected by the 2016 Pedernales, Ecuador earthquake Tomographic images highlight the heterogeneities of the margin affected by seamounts and ridges comprising the oceanic crust Carnegie Ridge seems the main feature controlling the seismic activity and the offshore extent of large megathrust earthquakes in the region, IGEPN, IRD, INSU‐CNRS, ANR, NERC, IRIS PASSCAL and NSF RAPID Program Award, ANID under Programa Formación de Capital Humano Avanzado, Becas Chile, UCA/JEDI project

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 (1.85) 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 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., Plain Language Summary: Using seismic data recorded by the permanent Ecuadorian network and the large emergency installation after the 2016 Pedernales earthquake, we obtained the seismic velocity structure together with precise earthquake locations for the coastal Ecuadorian margin. Our images highlight the heterogeneities of the subduction zone affected by seamounts and ridges comprising the oceanic crust. These features play an important role in controlling the seismic behavior of the margin. While seamounts can contribute to the occurrence of intermediate (M ∼ 7–7.5) megathrust earthquakes in the north, the Carnegie Ridge seems to be the main feature controlling the seismicity in the region by promoting creeping and slow slip events 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., Key Points: 3D Vp and Vp/Vs models were calculated using local earthquake tomography in the region affected by the 2016 Pedernales, Ecuador earthquake Tomographic images highlight the heterogeneities of the margin affected by seamounts and ridges comprising the oceanic crust Carnegie Ridge seems the main feature controlling the seismic activity and the offshore extent of large megathrust earthquakes in the region, IGEPN, IRD, INSU‐CNRS, ANR, NERC, IRIS PASSCAL and NSF RAPID Program Award, ANID under Programa Formación de Capital Humano Avanzado, Becas Chile, UCA/JEDI project