The geological expression of subduction termination is poorly understood due to overprinting during the collisional stage of the Wilson Cycle. The Anatolian domain of the eastern Mediterranean represents a modern system where spatial variability can be interpreted in terms of the transition from subduction to collision. Convergence in the west is accommodated by the subduction of the last remnants of Neotethyan oceanic lithosphere, while in the east, the margin has transitioned to complete continent-continent collision. In central Anatolia, however, the expression of convergence is complicated by the underthrusting of small continental fragments and attenuated continental lithosphere. By investigating variations in the geological expression of convergence across this system, we can investigate the processes that accompany the transition from subduction to collision.Spatially variable tectonomagmatic and seismic characteristics along the Anatolian margin reflect this transition. Seismic images reveal a disjointed and disaggregating subducting slab beneath central Anatolia that interacts with, and in some cases induces, mantle flow. This spatially corresponds with Miocene-to-recent volcanism that is sourced from very shallow depths (

The high sensitivity of V-P/V-S to the presence of melt makes images of V-P/V-S structure particularly useful in magmatic systems, but detailed three-dimensional models of V-P/V-S structure in magmatic systems are often restricted to the upper crust where there is a concentration of seismic sources used for imaging. Ps-P tomography is a new technique that has been used to image three-dimensional crustal-scale variations in V-P/V-S in regions with limited seismic instrumentation. We apply the Ps-P tomography technique to a well-constrained, independently imaged magmatic setting, Mount St. Helens, to outline the efficacy and limitations of this imaging approach. Our Ps-P tomography model reveals previously imaged high V-P/V-S upper crustal magma reservoirs beneath active volcanic systems at Mount St. Helens, Mount Adams and the Indian Heaven Volcanic Field and low V-P/V-S anomalies associated with crystallized plutons. Our model also provides new V-P/V-S constraints in the lower crust that reveal a high V-P/V-S anomaly connecting the Mount St. Helens and Mount Adams reservoirs and a low V-P/V-S anomaly associated with lower crustal cumulates or mafic accreted terranes. Decimation tests further show that first order V-P/V-S structure is recoverable using as few as four recording seismometers. These images resemble those of independent, higher resolution images from traditional techniques, highlighting the utility of Ps-P tomography for imaging three-dimensional variations of V-P/V-S throughout the crust, including in data-poor settings or with arrays not designed for structural seismic investigations, such as many volcano monitoring networks.

SUMMARY 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.

The subducting African Plate in the easternmost Mediterranean is actively tearing and deforming beneath the Anatolian Plate as the margin transitions from long‐lived subduction to collision. In central Anatolia, the subducting slab is characterized by both lateral and vertical slab tears. We investigate patterns of mantle flow around the edges of a contorting and fragmenting African slab segment, called the Cyprean slab, using measurements of shear wave splitting. We observe three distinct regions of coherent shear wave splitting that correlate with the segmentation boundaries of the Cyprean slab. Regionally coherent mantle flow occurs near both the eastern and western the edges of the slab. These regions of coherent splitting are separated by an area of null splitting that encompasses the Central Anatolian Volcanic Province near the easternmost edge of the slab. The null measurements likely result from mantle upwelling due to the displacement of asthenosphere from the vertical Cyprean slab.

We present a new P‐wave seismic tomographic model for the region of the Paraná Basin and surroundings using a multiple‐frequency approach, providing better resolution than previous regional studies. We processed a total of 62,692 cross‐correlation delays for P, PKIKP, PcP, and PP phases distributed among 1,081 events using six different central frequencies (0.03, 0.06, 0.13, 0.25, 0.50, and 1 Hz). We merged our data with a previous multiple‐frequency study of the Amazonian Craton to cover regions outside of the study area, obtaining a total of 75,187 cross‐correlation delays. The data used are from the stations of the Brazilian Seismographic Network, and mainly from a temporary network (XC network) installed exclusively to study the region. The basement of the Paraná Basin is represented as a NE‐SW trending P‐wave high‐velocity anomaly, extending from the northern limit of the basin to the southwestern border of Brazil, consistent with previous reports. The limit between this block and the São Francisco Craton is characterized by decreased amplitude of the P‐wave high‐velocity anomaly. Synthetic tests show that a narrow boundary between these two blocks displays the same behavior. At the southeastern portion of this anomaly, decreasing amplitude is consistent with the limit of the Luiz Alves Craton, which was also corroborated by synthetic tests. The northern portion of the Rio Apa Block agrees with a previous tomographic model, confirming that it does not extend under the Pantanal Basin, however, in our model this structure does not extend as far south.

SUMMARY The Andean Subduction Zone is one of the longest continuous subduction zones on Earth. The relative simplicity of the two-plate system has makes it an ideal natural laboratory to study the dynamics in subduction zones. We measure teleseismic S and SKS traveltime residuals at >1000 seismic stations that have been deployed across South America over the last 30 yr to produce a finite-frequency teleseismic S-wave tomography model of the mantle beneath the Andean Subduction Zone related to the Nazca Plate, spanning from ∼5°N to 45°S and from depths of ∼130 to 1200 km. Within our model, the subducted Nazca slab is imaged as a fast velocity seismic anomaly. The geometry and amplitude of the Nazca slab anomaly varies along the margin while the slab anomaly continues into the lower mantle along the entirety of the subduction margin. Beneath northern Brazil, the Nazca slab appears to stagnate at ∼1000 km depth and extend eastward subhorizontally for >2000 km. South of 25°S the slab anomaly in the lower mantle extends offshore of eastern Argentina, hence we do not image if a similar stagnation occurs. We image several distinct features surrounding the slab including two vertically oriented slow seismic velocity anomalies: one beneath the Peruvian flat slab and the other beneath the Paraná Basin of Brazil. The presence of the latter anomaly directly adjacent to the stagnant Nazca slab suggests that the plume, known as the Paraná Plume, may be a focused upwelling formed in response to slab stagnation in the lower mantle. Additionally, we image a high amplitude fast seismic velocity anomaly beneath the Chile trench at the latitude of the Sierras Pampeanas which extends from ∼400 to ∼1000 km depth. This anomaly may be the remnants of an older, detached slab, however its relationship with the Nazca–South America subduction zone remains enigmatic.

Seismic tomography of the crust is an essential tool for studying the three‐dimensional structure of magmatic plumbing systems feeding active volcanoes, but it is often limited in resolution by the absence of deep local seismicity. Teleseismic receiver functions can be used to illuminate local structural variations, but typically do not account for the effects of three‐dimensional velocity heterogeneities. Here we harness the complementary strengths of both techniques by processing Ps‐P delay times derived from teleseismic receiver functions in a tomographic S wave inversion. Using our inversion technique, we produce the first tomographic crustal velocity model beneath Cleveland Volcano, identifying a vertically extensive high VP/VS anomaly beneath the volcano that likely signifies a middle‐to‐lower crustal magma reservoir. The observation is the first of its kind in the central Aleutians, illustrating the potential of our technique to advance our understanding of crustal magmatic systems without broad seismic networks or distributed local seismicity.

The Andean subduction zone is an excellent place to study deformation within a subducting slab as a function of depth, owing to the varying and well‐resolved geometry of the subducting Nazca slab beneath South America. Here we combine the results of source‐side shear wave splitting with the latest regional tomography model to isolate intraslab raypaths and determine the spatial distribution of anisotropy within the Nazca slab. We observe that in the upper mantle, the intraslab anisotropy appears strongest where the slab is most contorted, suggesting a strong link between anisotropy and subduction‐related slab deformation. We identify a second source of anisotropy (δt∼ 1 s) within the subducting slab at lower mantle depths (660–800 km). The surrounding mantle and transition zone appear largely isotropic, with deep anisotropy concentrated within the slab as it deforms while entering the higher‐viscosity lower mantle.

Nazca subduction beneath South America is one of our best modern examples of long-lived ocean-continent subduction on the planet, serving as a foundation for our understanding of subduction processes. Within that framework, persistent heterogeneities at a range of scales in both the South America and Nazca plates is difficult to reconcile without detailed knowledge of the subducted Nazca slab structure. Here we use teleseismic travel time residuals from >1,000 broadband and short-period seismic stations across South America in a single tomographic inversion to produce the highest-resolution contiguous P wave tomography model of the subducting slab and surrounding mantle beneath South America to date. Our model reveals a continuous trench-parallel fast seismic velocity anomaly across the majority of South America that is consistent with the subducting Nazca slab. The imaged anomaly indicates a number of robust features of the subducted slab, including variable slab dip, extensive lower mantle penetration, slab stagnation in the lower mantle, and variable slab amplitude, that are incorporated into a new, comprehensive model of the geometry of the Nazca slab surface to similar to 1,100 km depth. Lower mantle slab penetration along the entire margin suggests that lower mantle slab anchoring is insufficient to explain along strike upper plate variability while slab stagnation in the lower mantle indicates that the 1,000 km discontinuity is dominant beneath South America.

Gaps within a subducting plate can alter the surrounding mantle flow field and the overall subduction zone dynamics by allowing hot sub-slab mantle to flow through the gaps and into the mantle wedge. This through-slab flow can produce melting of the slab gap edges as well as significant upwelling that can lead to anomalous alkaline volcanism and/or dynamic uplift in the overriding plate, while the altered mantle flow patterns affect the trench evolution. Numerous geodynamic models have investigated the processes that form slab gaps, but few studies have examined the dynamics of slab gap-altered mantle flow, its effects on trench morphology and kinematics, or the controlling parameters on these processes. Here, laboratory subduction models with a pre-cut gap in a subducting silicone plate are used to explore how slab gap size, and slab gap depth influence the surrounding mantle flow field and trench dynamics. Results suggest that both the vertical extent and the depth of the top (trailing edge) of the slab gap are crucial parameters for modulating overall subduction dynamics. They show that a slab gap, which occurs near the surface and initially comprises 30% of the subducting plate width, can extend enough vertically in the slab to produce significant vertical flow through the gap. Changes to the trench geometry and kinematics are also evident in the models, such that double- and triple-arc geometries are formed during subduction of a shallow slab gap. All of these results are consistent with observations of slab gaps and their induced surface expressions, or the lack thereof, in Eastern Anatolia, East Java, Italy, and Argentina.

The April 2016 Pedernales earthquake ruptured a 100 km by 40 km segment of the subduction zone along the coast of Ecuador in an M-w 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 Geofisico at the Escuela Politecnica Nacional in Quito deployed 55 seismometers and 10 ocean-bottom seismometers above the rupture zone and adjacent areas to record aftershocks. In this article, we describe the details of the U.S. portion of the rapid response and present an earthquake cata-log from May 2016 to 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 similar to 100 km to the northeast of the mainshock in the epicentral region of an M-w 7.8 earthquake in 1958. In December, an increase in seismicity occurred similar to 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 megathrust rupture, and the relationship between locked and creeping parts of the subduction interface.

Subduction zones are home to the most seismically active faults on the planet. The shallow megathrust interfaces of subduction zones host Earth's largest earthquakes and are likely the only faults capable of magnitude 9+ ruptures. Despite these facts, our knowledge of subduction zone geometry-which likely plays a key role in determining the spatial extent and ultimately the size of subduction zone earthquakes-is incomplete. We calculated the three-dimensional geometries of all seismically active global subduction zones. The resulting model, called Slab2, provides a uniform geometrical analysis of all currently subducting slabs.

Using finite-frequency teleseismic P-wave tomography, we developed a new three-dimensional (3-D) velocity model of the mantle beneath Anatolia down to 900 km depth that reveals the structure and behavior of the sub-ducting African lithosphere beneath three convergent domains of Anatolia: the Aegean, Cyprean, and Bitlis-Zagros domains. The Aegean slab has a relatively simple structure and extends into the lower mantle; the Cyprean slab has a more complex structure, with a western section that extends to the lower mantle with a consistent dip and an eastern section that is broken up into several pieces; and the Bitlis slab appears severely deformed, with only fragments visible in the mantle transition zone and uppermost lower mantle. In addition to the subducting slabs, high-amplitude slow velocity anomalies are imaged in the shallow mantle beneath recently active volcanic centers, and a prominent fast velocity anomaly dominates the shallow mantle beneath northern Anatolia and the southern Black Sea. As a whole, our model confirms the presence of well-established slow and fast velocity anomalies in the upper mantle beneath Anatolia and motivates two major findings about Eastern Mediterranean subduction: (1) Each of the slabs penetrates into the lower mantle, making the Eastern Mediterranean unique within the Mediterranean system, and (2) the distinct character of each slab segment represents different stages of subduction termination through progressive slab deformation. Our findings on the destructive processes of subduction termination and slab detachment have significant implications for understanding of the post-detachment-behavior of subducted lithosphere.