228 research outputs found
Extending shear-wave tomography for the lower mantle using S and SKS arrival-time data
Seismic tomography using S wave travel times faces the difficulty imposed by the interference between S and SKS phases near 83° epicentral distance, as the SKS phase overtakes the S waves in the mantle. If the cross-over is avoided completely by excluding S data beyond 82° then no resolution is available below 2200 km in the lower mantle. A partial solution is to try to pick up the S phase beyond the cross-over which improves coverage and resolution in depth. However, a much larger improvement can be made by following the first arrival with S character and including SKS information with S.
Arrival times for both S and SKS phases and the event hypocentres have been taken from the reprocessing of data reported to international agencies. Each event has been relocated, including depth phase information, and later phases re-associated using the improved locations to provide a set of travel times whose variance is significantly reduced compared with the original data catalogues.
S travel-time tomography including SKS information out to 105°, provides tomographic images with improved rendition of heterogeneity in the lower mantle. The three-dimensional models of SV wavespeed relative to the ak135 reference velocity model show a significant increase in heterogeneity at the base of the mantle which matches the behaviour seen in results derived from waveform inversion.
For most of the mantle there is a considerable similarity between the patterns of heterogeneity in the S wave images and recent P wave tomographic results, but greater differences develop in the lowermost mantle. In the D″ region the SV wavespeed patterns also show some differences from recent SH wavespeed results which mostly correlate with regions of recognised structural complexity
Comparing P and S wave heterogeneity in the mantle
From the reprocessed data set of Engdahl and
co-workers we have carefully selected matching P and S data
for tomographic imaging. We assess data and model error
and conclude that our S model uncertainty is twice that of
the P model. We account for this in our comparison of the
perturbations in P and S-wavespeed. In accord with previous
studies we find that P and S perturbations are positively
correlated at all depths. However, in the deep mantle systematic
di fferences occur between regions that have undergone
subduction in the last 120 million years and those that
have not. In particular, below 1500 km depth ∂ ln Vs/∂ ln Vp
is signifi cantly larger in mantle regions away from subduction
than in mantle beneath convergent margins. This inference
is substantiated by wavespeed analyses with random realizations
of the slab/non-slab distribution. Through much
of the mantle there is no signi ficant correlation between bulk
sound and S-wave perturbations, but they appear to be negatively
correlated between 1700 and 2100 km depth, which
is also where the largest di erences in ∂ ln Vs/∂ ln Vp occur.
This finding supports convection models with compositional
heterogeneity in the lowermost mantle
Compositional heterogeneity near the base of the mantle transition zone beneath Hawaii
Global seismic discontinuities near 410 and 660 km depth in Earth’s mantle are expressions of solid-state phase transitions. These transitions modulate thermal and material fluxes across the mantle and variations in their depth are often attributed to temperature anomalies. Here we use novel seismic array analysis of SS waves reflecting off the 410 and 660 below the Hawaiian hotspot. We find amplitude–distance trends in reflectivity that imply lateral variations in wavespeed and density contrasts across 660 for which thermodynamic modeling precludes a thermal origin. No such variations are found along the 410. The inferred 660 contrasts can be explained by mantle composition varying from average (pyrolitic) mantle beneath Hawaii to a mixture with more melt-depleted harzburgite southeast of the hotspot. Such compositional segregation was predicted, from petrological and numerical convection studies, to occur near hot deep mantle upwellings like the one often invoked to cause volcanic activity on Hawaii
Imaging along-strike variations in mechanical properties of the Gofar transform fault, East Pacific Rise
Author Posting. © American Geophysical Union, 2014. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Journal of Geophysical Research: Solid Earth 119 (2014): 7175–7194, doi:10.1002/2014JB011270.A large part of global plate motion on mid-ocean ridge transform faults (RTFs) is not accommodated as major earthquakes. When large earthquakes do occur, they often repeat quasiperiodically. We focus here on the high slip rate (∼14 cm/yr) Gofar transform fault on the equatorial East Pacific Rise. This fault is subdivided into patches that slip during Mw 5.5–6 earthquakes every 5 to 6 years. These patches are separated by rupture barriers that accommodate slip through swarms of smaller events and/or aseismic creep. We performed an imaging study to investigate which spatiotemporal variations of the fault zone properties control this segmentation in mechanical behavior and could explain the specific behavior of RTFs at the global scale. We adopt a double-difference approach in a joint inversion of active air gun shots and microseismicity recorded for 1 year. This data set includes the 2008 Mw 6 Gofar earthquake. The along-strike P wave velocity structure reveals an abrupt transition between the barrier area, characterized by a damaged fault zone of 10–20% reduced Vp and a nearly intact fault zone in the asperity area. The importance of the strength of the damage zone on the mechanical behavior is supported by the temporal S wave velocity changes which suggest increased damage within the barrier area, during the week preceding the Mw 6 earthquake. Our results support the conclusion that extended highly damaged zones are the key factor in limiting the role of major earthquakes to accommodate plate motion along RTFs.The material presented here is based on work supported by the National Science Foundation grants 1232725 and 0242117.2015-03-2
Mesozoic subducted slabs under Siberia
Recent results from seismic tomography demonstrate that subducted oceanic lithosphere can be observed globally as slabs of relatively high seismic velocity in the upper as well as lower mantle(1,2). The Asian mantle is no exception, with high-velocity slabs being observed downwards from the west Pacific subduction zones under the Kurile Islands, Japan and farther south(3-5), as well as under Asia's ancient Tethyan margin. Here we present evidence for the presence of slab remnants of Jurassic age that were subducted when the Mongol-Okhotsk and Kular-Nera oceans closed between Siberia, the combined Mongolia-North China blocks and the Omolon block(6-8). We identify these proposed slab remnants in the lower mantle west of Lake Baikal down to depths of at least 2,500 km, where they join what has been interpreted as a 'graveyard'(9) of subducted lithosphere at the bottom of the mantle. Our interpretation implies that slab remnants in the mantle can still be recognized some 150 million years or more after they have been subducted and that such structures may be useful in associating geodynamic to surface-tectonic processes.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/62524/1/397246a0.pd
Nummular keratopathy in a patient with Hyper-IgD Syndrome
<p>Abstract</p> <p>Purpose</p> <p>To report a case of recurrent nummular keratitis in a pediatric patient with Hyperimmunoglobulinemia D syndrome.</p> <p>Methods</p> <p>A retrospective chart review.</p> <p>Results</p> <p>A 14-year-old boy with Hyperimmunoglobulinemia D syndrome (HIDS) presented with photophobia and ocular irritation concomitant with disease exacerbation. He was found on exam to have significant nummular keratitis, which responded to a short course of topical steroids. Despite acute response to local immunosuppression, the patient had several recurrent attacks and eventually developed a large corneal scar and decreased vision. After initiation of infliximab therapy his ocular sequelae improved dramatically and his vision returned to 20/20.</p> <p>Conclusion</p> <p>One possible form of end-organ damage associated with HIDS is vision threatening nummular keratopathy.</p
Mid-mantle deformation inferred from seismic anisotropy
With time, convective processes in the Earth's mantle will tend to align crystals, grains and inclusions. This mantle fabric is detectable seismologically, as it produces an anisotropy in material properties—in particular, a directional dependence in seismic-wave velocity. This alignment is enhanced at the boundaries of the mantle where there are rapid changes in the direction and magnitude of mantle flow, and therefore most observations of anisotropy are confined to the uppermost mantle or lithosphere and the lowermost-mantle analogue of the lithosphere, the D" region. Here we present evidence from shear-wave splitting measurements for mid-mantle anisotropy in the vicinity of the 660-km discontinuity, the boundary between the upper and lower mantle. Deep-focus earthquakes in the Tonga–Kermadec and New Hebrides subduction zones recorded at Australian seismograph stations record some of the largest values of shear-wave splitting hitherto reported. The results suggest that, at least locally, there may exist a mid-mantle boundary layer, which could indicate the impediment of flow between the upper and lower mantle in this region
Early experience with robotic pancreatoduodenectomy versus open pancreatoduodenectomy:nationwide propensity-score-matched analysis
Background: Although robotic pancreatoduodenectomy has shown promising outcomes in experienced high-volume centres, it is unclear whether implementation on a nationwide scale is safe and beneficial. The aim of this study was to compare the outcomes of the early experience with robotic pancreatoduodenectomy versus open pancreatoduodenectomy in the Netherlands. Methods: This was a nationwide retrospective cohort study of all consecutive patients who underwent robotic pancreatoduodenectomy or open pancreatoduodenectomy who were registered in the mandatory Dutch Pancreatic Cancer Audit (18 centres, 2014-2021), starting from the first robotic pancreatoduodenectomy procedure per centre. The main endpoints were major complications (Clavien-Dindo grade greater than or equal to III) and in-hospital/30-day mortality. Propensity-score matching (1 : 1) was used to minimize selection bias. Results: Overall, 701 patients who underwent robotic pancreatoduodenectomy and 4447 patients who underwent open pancreatoduodenectomy were included. Among the eight centres that performed robotic pancreatoduodenectomy, the median robotic pancreatoduodenectomy experience was 86 (range 48-149), with a 7.3% conversion rate. After matching (698 robotic pancreatoduodenectomy patients versus 698 open pancreatoduodenectomy control patients), no significant differences were found in major complications (40.3% versus 36.2% respectively; P = 0.186), in-hospital/30-day mortality (4.0% versus 3.1% respectively; P = 0.326), and postoperative pancreatic fistula grade B/C (24.9% versus 23.5% respectively; P = 0.578). Robotic pancreatoduodenectomy was associated with a longer operating time (359 min versus 301 min; P < 0.001), less intraoperative blood loss (200 ml versus 500 ml; P < 0.001), fewer wound infections (7.4% versus 12.2%; P = 0.008), and a shorter hospital stay (11 days versus 12 days; P < 0.001). Centres performing greater than or equal to 20 robotic pancreatoduodenectomies annually had a lower mortality rate (2.9% versus 7.3%; P = 0.009) and a lower conversion rate (6.3% versus 11.2%; P = 0.032). Conclusion: This study indicates that robotic pancreatoduodenectomy was safely implemented nationwide, without significant differences in major morbidity and mortality compared with matched open pancreatoduodenectomy patients. Randomized trials should be carried out to verify these findings and confirm the observed benefits of robotic pancreatoduodenectomy versus open pancreatoduodenectomy.</p
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