Seismic imaging of the Northern Andean subduction zone from teleseismic tomography: a torn and fragmented Nazca slab

The Nazca-South America subduction zone in Ecuador is characterized by a complicated along-strike geometry as the slab transitions from flat slab subduction in the south, with the Peruvian flat slab, to what has been characterized as ‘normal’ dipping subduction beneath central Ecuador. Plate convergence additionally changes south to north as the trench takes on a convex shape. Highly heterogeneous bathymetry at the trench, including the aseismic oceanic Carnegie Ridge (CR), and sparse intermediate-depth seismicity has led many to speculate about the behaviour of the downgoing plate at depth. In this study, we present a finite-frequency teleseismic P-wave tomography model of the northern Andes beneath Ecuador and Colombia from 90 to 1200 km depth. Our model builds on prior tomography models in South America by adding relative traveltime residuals recorded at stations in Ecuador. The complete data set is comprised of 114 096 relative traveltime residuals from 1133 stations across South America, with the added data serving to refine the morphology of the Nazca slab in the mantle beneath the northern Andes. Our tomography model shows a Nazca slab with a fragmented along-strike geometry and the first teleseismic images of several proposed slab tears in this region. At the northern edge of the Peruvian flat slab in southern Ecuador, we image a shallow tear at 95–200 km depth that appears to connect mantle flow from beneath the flat slab to the Ecuadorian Arc. Beneath central Ecuador at the latitudes of the CR, the Nazca slab is continuous into the lower mantle. Beneath southern Colombia, the Malpelo Tear breaks the Nazca slab below ∼200 km depth

Crustal structure beneath the Strait of Juan de Fuca and southern Vancouver Island from seismic and gravity analyses

Wide-angle and vertical incidence seismic data from Seismic Hazards Investigations in Puget Sound (SHIPS), gravity modeling, and seismicity are used to derive two-dimensional crustal models beneath the Strait of Juan de Fuca. The Eocene volcanic Crescent-Siletz terrane is significantly thicker than previously recognized and extends from near the surface to depths of 22 km or greater. For the northern strait, a weak midcrustal reflector, dipping east from 12- to 22-km depth, is inferred from wide-angle reflections. A stronger deeper reflector, dipping eastward from 23- to 36-km depth, is associated with the top of ‘‘reflector band E,’’ a zone of high reflectivity on coincident Multichannel Seismic (MCS) data, interpreted as a shear zone. A high-velocity zone (7.60 ± 0.2 km s⁻¹) between these reflectors is interpreted as a localized slice of mantle accreted with the overlying Crescent-Siletz terrane. For the southern strait, no deep high-velocity layer is observed and the E-band reflectivity is weaker than to the north. A strong deep reflector, interpreted as the oceanic Moho dips eastward from 35 to 42 km. Seismicity within the subducting slab occurs mainly above the inferred oceanic Moho. Gravity modeling, constrained by the wide-angle seismic models and seismicity, is consistent with the inferred large thickness of Crescent-Siletz and high-density rocks (3030 kg m⁻³) in the lower crust.Keywords: crustal structure, wide-angle seismic, deep crustal reflectivity, Cascadia subduction zone, Vancouver Island, Juan de Fuca Strait, Crescent-Siletz terran

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

Megathrust 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 M$_{w}$ 7.8 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

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

Status and Recent Results of the Acoustic Neutrino Detection Test System AMADEUS

The AMADEUS system is an integral part of the ANTARES neutrino telescope in the Mediterranean Sea. The project aims at the investigation of techniques for acoustic neutrino detection in the deep sea. Installed at a depth of more than 2000m, the acoustic sensors of AMADEUS are based on piezo-ceramics elements for the broad-band recording of signals with frequencies ranging up to 125kHz. AMADEUS was completed in May 2008 and comprises six "acoustic clusters", each one holding six acoustic sensors that are arranged at distances of roughly 1m from each other. The clusters are installed with inter-spacings ranging from 15m to 340m. Acoustic data are continuously acquired and processed at a computer cluster where online filter algorithms are applied to select a high-purity sample of neutrino-like signals. 1.6 TB of data were recorded in 2008 and 3.2 TB in 2009. In order to assess the background of neutrino-like signals in the deep sea, the characteristics of ambient noise and transient signals have been investigated. In this article, the AMADEUS system will be described and recent results will be presented.Comment: 7 pages, 8 figures. Proceedings of ARENA 2010, the 4th International Workshop on Acoustic and Radio EeV Neutrino Detection Activitie

Performance of the First ANTARES Detector Line

In this paper we report on the data recorded with the first Antares detector line. The line was deployed on the 14th of February 2006 and was connected to the readout two weeks later. Environmental data for one and a half years of running are shown. Measurements of atmospheric muons from data taken from selected runs during the first six months of operation are presented. Performance figures in terms of time residuals and angular resolution are given. Finally the angular distribution of atmospheric muons is presented and from this the depth profile of the muon intensity is derived.Comment: 14 pages, 9 figure

Structure of the Lesser Antilles subduction forearc and backstop from 3D seismic tomography

In 2007 the Sismantilles II experiment was conducted to constrain structure and seismicity in the central Lesser Antilles subduction zone. The seismic refraction data recorded by a network of 27 OBSs over an area of 65 km×95 km provide new insights on the crustal structure of the forearc offshore Martinique and Dominica islands. The tomographic inversion of first arrival travel times provides a 3D P-wave velocity model down to 15 km. Basement velocity gradients depict that the forearc is made up of two distinct units: A high velocity gradient domain named the inner forearc in comparison to a lower velocity gradient domain located further trenchward named the outer forearc. Whereas the inner forearc appears as a rigid block uplifted and possibly tilted as a whole to the south, short wavelength deformations of the outer forearc basement are observed, beneath a 3 to 6 km thick sedimentary pile, in relation with the subduction of the Tiburon Ridge and associated sea floor reliefs. North, offshore Dominica Island, the outer forearc is 70 km wide. It extends as far as 180 km to the east of the volcanic front where it acts as a backstop on which the accretionary wedge developed. Its width decreases strongly to the south to terminate offshore Martinique where the inner forearc acts as the backstop. The inner forearc is likely the extension at depth of the Mesozoic magmatic crust outcropping to the north in La Désirade Island and along the scarp of the Karukera Spur. The outer forearc could be either the eastern prolongation of the inner forearc, but the crust was thinned and fractured during the past tectonic history of the area or by recent subduction processes, or an oceanic terrane more recently accreted to the island arc.Peer Reviewe

The ANTARES Optical Beacon System

ANTARES is a neutrino telescope being deployed in the Mediterranean Sea. It consists of a three dimensional array of photomultiplier tubes that can detect the Cherenkov light induced by charged particles produced in the interactions of neutrinos with the surrounding medium. High angular resolution can be achieved, in particular when a muon is produced, provided that the Cherenkov photons are detected with sufficient timing precision. Considerations of the intrinsic time uncertainties stemming from the transit time spread in the photomultiplier tubes and the mechanism of transmission of light in sea water lead to the conclusion that a relative time accuracy of the order of 0.5 ns is desirable. Accordingly, different time calibration systems have been developed for the ANTARES telescope. In this article, a system based on Optical Beacons, a set of external and well-controlled pulsed light sources located throughout the detector, is described. This calibration system takes into account the optical properties of sea water, which is used as the detection volume of the ANTARES telescope. The design, tests, construction and first results of the two types of beacons, LED and laser-based, are presented.Comment: 21 pages, 18 figures, submitted to Nucl. Instr. and Meth. Phys. Res.

Seismic structure and activity of the north-central Lesser Antilles subduction zone from an integrated approach: similarities with the Tohoku forearc

The 300 km long north-central segment of the Lesser Antilles subduction zone, including Martinique and Guadeloupe islands has been the target of a specific approach to the seismic structure and activity by a cluster of active and passive offshore-onshore seismic experiments coordinated within the ¿Thales was right¿ proposal to the European Union action (Laigle et al., Tectonophys., in rev.) The top of the subducting plate can be followed under the wide accretionary wedge by a dense grid of dip- and strike-lines of multichannel reflection seismics. This reveals the hidden updip limit of the contact of the upper plate crustal backstop thrust onto the slab. Two OBS refraction seismic profiles constrained a 26 km large crustal thickness from the volcanic arc throughout the forearc domain (Kopp et al., EPSL, 2011). These new observations imply a three times larger width of the potential interplate seismogenic zone under the marine domain of the Caribbean plate with respect to a regular intra-oceanic subduction zone, in the common assumption that the upper plate Moho contact on the slab is a proxy of its downdip limit. Towards larger depth under the mantle corner, the top of the slab imaged from the conversions of teleseismic body-waves and the locations of earthquakes from the dense temporary array of 80 OBS and land seismometers appears with kinks which increase the dip from 10-20° under the forearc domain, to 60° on the segment from 70 km depth down to under the volcanic arc. There, at 140 km depth just north of Martinique the 2007 M 7.4 earthquake, largest for half a century, was accompanied by an increased seismic activity over the whole depth range, which provides a new focused image thanks to the OBS and land deployments. A double-planed dipping slab seismicity is thus now resolved, as originally discovered in Tohoku ( NE Japan) and since in some other subduction zones. Other types of seismic activity uniquely observed in Tohoku, are resolved now here, such as ¿supraslab¿ earthquakes with normal-faulting focal mechanisms reliably located in the mantle corner and ¿deep flat-thrust¿ earthquakes at 45 km depth on the interplate fault under the Caribbean plate forearc mantle. None such types of seismicity should occur under the paradigm of a regular peridotitic mantle of the upper plate which is serpentinized by the fluids provided from the dehydrating slab beneath, and which is commonly considered as limiting the downward extent of the interplate seismic coupling. If the upper plate here comprised lithospheric segments related to the earlier formation of the Caribbean oceanic plateau by the material advection from a mantle plume, it could then be underlain by a correspondingly modified, heterogeneous mantle, which may impose regions of stick-slip behaviour on the interplate under the mantle corner among stable-gliding areas. The Tohoku 2011 M9 earthquake was unexpected not only in its slip reaching to the trench, but also in its slip reaching far under the mantle corner against the serpentinization decoupling paradigm, and its structural setting may be revisited for resolving corresponding structural heterogeneityPeer Reviewe