Slab pileup in the mantle transition zone and the 30 May 2015 Chichi‐jima earthquake

The 30 May 2015 Chichi-jima M8 earthquake is one of the largest deep focus earthquakes ever recorded and its depth of 682km puts it near the base of the mantle transition zone. Before source mechanisms and slip models of this earthquake can be reliably assessed, a better understanding of the tectonic setting and structures of the region near the origin is required. Here we present evidence from receiver functions, a method of isolating subsurface material contrast with converted seismic waves, that the earthquake initiated within the upper mantle transition zone, above a significantly depressed 660km phase boundary. Additionally, we observe multiple conversions within and below the transition zone, which we associate with seismic waves passing into and out of segments of the subducting Pacific plate. From this, we infer slab material is piling up at the base of the transition zone and segments are penetrating into the lower mantle.NSF-EAR Postdoctoral Fellowship [1249776]; Kobe University Research Center for Urban Safety and Security6 month embargo; first published: 05 May 2016This item from the UA Faculty Publications collection is made available by the University of Arizona with support from the University of Arizona Libraries. If you have questions, please contact us at [email protected]

Multiscale crustal architecture of Alaska inferred from P reciever functions

The geologic mosaic of continental and oceanic terranes, displaced and deformed by multiple plate reorganization episodes, rapid lateral topographic variations, and heterogeneous distribution of strain throughout Alaska, all predict strong variability of crustal architecture. We present the first wide-scale model of crustal thickness based on broadband seismic data across the region that is constrained where seismic instrumentation has been deployed; dense coverage in the south-central region and more sparse coverage in the western and Arctic regions as the USArray Transportable Array (TA) is installed. Analyses of P receiver functions (PRFs) provide the first detailed look at crustal structure across all of Alaska. The variable thickness reflects inherited structure from Mesozoic to early Cenozoic convergent and extension events that in some regions is being extensively modified by ongoing convergence and collision, particularly along the active southern margin. Beneath the southern Alaska forearc to the central Alaska Range, the Yakutat slab Moho is also observed, illustrating the most recent ongoing accretionary event resulting from the collision of the Yakutat microplate. Combining three different receiver function methodologies, i.e., common conversion point stacking, receiver function stacks, and receiver gathers, for viewing and imaging P receiver functions allows for an interpretation of Alaskan crustal structure that spans multiple scales. The four-dimensional interpretation of the Alaskan crust will continue to evolve as the full TA is deployed and geologic studies are combined with the interpretations from this extensive seismic experiment.Funding for this work was provided through National Science Foundation CAREER award EAR-1054638 to Miller

Seismic imaging of the Alaska subduction zone: implications for slab geometry and volcanism

Alaska has been a site of subduction and terrane accretion since the mid‐Jurassic. The area features abundant seismicity, active volcanism, rapid uplift, and broad intraplate deformation, all associated with subduction of the Pacific plate beneath North America. The juxtaposition of a slab edge with subducted, overthickened crust of the Yakutat terrane beneath central Alaska is associated with many enigmatic volcanic features. The causes of the Denali Volcanic Gap, a 400‐km‐long zone of volcanic quiescence west of the slab edge, are debated. Furthermore, the Wrangell Volcanic Field, southeast of the volcanic gap, also has an unexplained relationship with subduction. To address these issues, we present a joint ambient noise, earthquake‐based surface wave, and P‐S receiver function tomography model of Alaska, along with a teleseismic S wave velocity model. We compare the crust and mantle structure between the volcanic and nonvolcanic regions, across the eastern edge of the slab and between models. Low crustal velocities correspond to sedimentary basins, and several terrane boundaries are marked by changes in Moho depth. The continental lithosphere directly beneath the Denali Volcanic Gap is thicker than in the adjacent volcanic region. We suggest that shallow subduction here has cooled the mantle wedge, allowing the formation of thick lithosphere by the prevention of hot asthenosphere from reaching depths where it can interact with fluids released from the slab and promote volcanism. There is no evidence for subducted material east of the edge of the Yakutat terrane, implying the Wrangell Volcanic Field formed directly above a slab edge

Global patient outcomes after elective surgery: prospective cohort study in 27 low-, middle- and high-income countries.

BACKGROUND: As global initiatives increase patient access to surgical treatments, there remains a need to understand the adverse effects of surgery and define appropriate levels of perioperative care. METHODS: We designed a prospective international 7-day cohort study of outcomes following elective adult inpatient surgery in 27 countries. The primary outcome was in-hospital complications. Secondary outcomes were death following a complication (failure to rescue) and death in hospital. Process measures were admission to critical care immediately after surgery or to treat a complication and duration of hospital stay. A single definition of critical care was used for all countries. RESULTS: A total of 474 hospitals in 19 high-, 7 middle- and 1 low-income country were included in the primary analysis. Data included 44 814 patients with a median hospital stay of 4 (range 2-7) days. A total of 7508 patients (16.8%) developed one or more postoperative complication and 207 died (0.5%). The overall mortality among patients who developed complications was 2.8%. Mortality following complications ranged from 2.4% for pulmonary embolism to 43.9% for cardiac arrest. A total of 4360 (9.7%) patients were admitted to a critical care unit as routine immediately after surgery, of whom 2198 (50.4%) developed a complication, with 105 (2.4%) deaths. A total of 1233 patients (16.4%) were admitted to a critical care unit to treat complications, with 119 (9.7%) deaths. Despite lower baseline risk, outcomes were similar in low- and middle-income compared with high-income countries. CONCLUSIONS: Poor patient outcomes are common after inpatient surgery. Global initiatives to increase access to surgical treatments should also address the need for safe perioperative care. STUDY REGISTRATION: ISRCTN5181700

Tracing the Farallon plate through seismic imaging with USArray

The Farallon plate system has been subducting off the western United States since at least the middle Mesozoic. This plate has undergone virtually every subduction process during this time including a long episode of flat-slab subduction, generation of microplates, and formation of oceanic plateaus. The shallow remains of this plate are two small microplates, the Gorda and Juan de Fuca, in the Pacific Northwest. The anomalous nature of these two small plates and the missing deeper evidence of subduction has motivated this study.The USArray seismic experiment has provided unprecedented spatial sampling of the seismic wavefield in the continuous United States. Utilizing this dataset, new imaging methods have been implemented and older imaging methods have been revitalized. This study first uses ambient seismic noise in the Pacific Northwest to extract short period Rayleigh waves which are sensitive to lithospheric scale structure. Phase velocities from this model are then combined with teleseismic delay times of body waves and surface waves to image the structure of the continuous United States from the surface through the mantle transition zone. The resolving power of this model allows tracing of the Farallon plate from the trench to the lower mantle.The seismic velocity structure of the continuous United States is broadly composed of a slow western half and fast eastern half separated by the Rocky Mountain Front. The low velocity of the western U.S. contains several high velocity anomalies. While previous work has focused on individual anomalies and suggested they represent lithospheric instabilities, a larger regional view indicates that these are the western remnants of the Farallon plate. Below the thick cratonic lithosphere of the eastern U.S., the Farallon plate contains significant topography due to a subducted heterogeneity of the oceanic plate and a viscosity contrast through the mantle transition zone. The velocity models presented herein provide a cohesive picture of Farallon subduction for the past 150 Ma

Tracing the Farallon plate through seismic imaging with USArray

The Farallon plate system has been subducting off the western United States since at least the middle Mesozoic. This plate has undergone virtually every subduction process during this time including a long episode of flat-slab subduction, generation of microplates, and formation of oceanic plateaus. The shallow remains of this plate are two small microplates, the Gorda and Juan de Fuca, in the Pacific Northwest. The anomalous nature of these two small plates and the missing deeper evidence of subduction has motivated this study.The USArray seismic experiment has provided unprecedented spatial sampling of the seismic wavefield in the continuous United States. Utilizing this dataset, new imaging methods have been implemented and older imaging methods have been revitalized. This study first uses ambient seismic noise in the Pacific Northwest to extract short period Rayleigh waves which are sensitive to lithospheric scale structure. Phase velocities from this model are then combined with teleseismic delay times of body waves and surface waves to image the structure of the continuous United States from the surface through the mantle transition zone. The resolving power of this model allows tracing of the Farallon plate from the trench to the lower mantle.The seismic velocity structure of the continuous United States is broadly composed of a slow western half and fast eastern half separated by the Rocky Mountain Front. The low velocity of the western U.S. contains several high velocity anomalies. While previous work has focused on individual anomalies and suggested they represent lithospheric instabilities, a larger regional view indicates that these are the western remnants of the Farallon plate. Below the thick cratonic lithosphere of the eastern U.S., the Farallon plate contains significant topography due to a subducted heterogeneity of the oceanic plate and a viscosity contrast through the mantle transition zone. The velocity models presented herein provide a cohesive picture of Farallon subduction for the past 150 Ma

Updates to FuncLab, a Matlab based GUI for handling receiver functions

Receiver functions are a versatile tool commonly used in seismic imaging. Depending on how they are processed,they can be used to image discontinuity structure within the crust or mantle or they can be inverted for seismicvelocity either directly or jointly with complementary datasets. However, modern studies generally require largedatasets which can be challenging to handle; therefore, FuncLab was originally written as an interactive MatlabGUI to assist in handling these large datasets. This software uses a project database to allow interactive traceediting, data visualization, H-κ stacking for crustal thickness and Vp/Vs ratio, and common conversion pointstacking while minimizing computational costs. Since its initial release, significant advances have been made inthe implementation of web services and changes in the underlying Matlab platform have necessitated a significantrevision to the software. Here, we present revisions to the software, including new features such as data down-loading via irisFetch.m, receiver function calculations via processRFmatlab, on-the-fly cross-section tools, inter-face picking, and more. In the descriptions of the tools, we present its application to a test dataset in Michigan,Wisconsin, and neighboring areas following the passage of USArray Transportable Array

Crustal structure across the eastern North American margin from ambient noise tomography

Passive tectonic margins, like the eastern North American margin (ENAM), represent the meeting of oceanic and continental material where no active deformation is occurring. The recent ENAM Community Seismic Experiment provides an opportunity to examine the crustal structure across the ENAM owing to the simultaneous deployment of offshore and onshore seismic instrumentation. Using Rayleigh wave phase and group velocities derived from ambient noise data, we invert for shear velocity across the ENAM. We observe a region of transitional crustal thicknesses that connects the oceanic and continental crusts. Associated with the transitional crust is a localized positive gravitational anomaly. Farther east, the East Coast magnetic anomaly (ECMA) is located at the intersection of the transitional and oceanic crusts. We propose that underplating of dense magmatic material along the bottom of the transitional crust is responsible for the gravitational anomaly and that the ECMA demarks the location of initial oceanic crustal formation.6 month embargo; published online: 3 July 2017This item from the UA Faculty Publications collection is made available by the University of Arizona with support from the University of Arizona Libraries. If you have questions, please contact us at [email protected]

Slab pileup in the mantle transition zone and the 30 May 2015 Chichi‐jima earthquake

The 30 May 2015 Chichi-jima M8 earthquake is one of the largest deep focus earthquakes ever recorded and its depth of 682km puts it near the base of the mantle transition zone. Before source mechanisms and slip models of this earthquake can be reliably assessed, a better understanding of the tectonic setting and structures of the region near the origin is required. Here we present evidence from receiver functions, a method of isolating subsurface material contrast with converted seismic waves, that the earthquake initiated within the upper mantle transition zone, above a significantly depressed 660km phase boundary. Additionally, we observe multiple conversions within and below the transition zone, which we associate with seismic waves passing into and out of segments of the subducting Pacific plate. From this, we infer slab material is piling up at the base of the transition zone and segments are penetrating into the lower mantle.NSF-EAR Postdoctoral Fellowship [1249776]; Kobe University Research Center for Urban Safety and Security6 month embargo; first published: 05 May 2016This item from the UA Faculty Publications collection is made available by the University of Arizona with support from the University of Arizona Libraries. If you have questions, please contact us at [email protected]

Tomographic Imaging of Slab Segmentation and Deformation in the Greater Antilles

We present a new tomographic P wave model of the upper mantle in the east Caribbean region. The model was built using 3-D finite frequency sensitivity kernels and ~20,000 teleseismic P and PP traveltime residuals from 535 events recorded across 130 broadband seismometers. We observe high-velocity features corresponding to a Caribbean beneath northern South America and an arcuate slab beneath the Lesser and Greater Antilles island arcs. The latter exhibits an along strike gradient in dip with steep edges and a reclined middle, consistent with ongoing slab rollback and collision. We divide the arcuate slab into three sections from two lateral discontinuities. The southern and northern Lesser Antilles sections are separated by a gap ~15°N down to ~200 km. Between Puerto Rico and Hispaniola, another gap down to ~300 km separates the northern Lesser Antilles slab from a narrow slab fragment further east. We relate these discontinuities to the subducted North American-South American plate boundary and a slab segmentation tear, respectively. The northern and southern ends of the Lesser Antilles trench are actively deforming from collision and differential rollback. However, these areas exhibit different styles of lithospheric tearing, as manifest in the morphology of the slab. We infer the contrast in tearing relates to the presence of microplates at the northern boundary of the Caribbean plate. Microplates facilitate block divergence and differential trench retreat/rollback, which drive slab segmentation. These results offer new insight into the tectonics of the Caribbean region and the factors driving lithospheric tearing in slabs generally