Source regions of infragravity waves recorded at the bottom of the equatorial Atlantic Ocean, using OBS of the PI‐LAB experiment

Infragravity waves are generated along coasts, and some small fraction of their energy escapes to the open oceans and propagates with little attenuation. Due to the scarcity of deep‐ocean observations of these waves, the mechanism and the extent of the infragravity waves energy leakage from the coasts remains poorly understood. Understanding the generation and pathways of infragravity wave energy is important among others for understanding the breakup of ice‐shelves and the contamination of high‐resolution satellite radar altimetry measurements of sea level. We examine data from 37 differential pressure gauges of Ocean Bottom Seismometers (OBS) near the equatorial mid‐Atlantic ridge, deployed during the Passive Imaging of the Lithosphere‐Asthenosphere Boundary (PI‐LAB) experiment. We use the beamforming technique to investigate the incoming directions of infragravity waves. Next, we develop a graph‐theory‐based global back‐projection method of noise cross‐correlation function envelopes, which minimizes the effects of array geometry using an adaptive weighting scheme. This approach allows us to locate the sources of the infragravity energy. We assess our observations by comparing to a global model of infragravity wave heights. Our results reveal strong coherent energy from sources and/or reflected phases at the west coast of Africa and some sources from South America. These energy sources are in good agreement with the global infragravity wave model. In addition, we also observe infragravity waves arriving from North America during specific events that mostly occur during October–February 2016. Finally, we find indications of waves that propagate with little attenuation, long distances through sea ice, reflecting off Antarctica

Local seismicity around the Chain Transform Fault at the Mid-Atlantic Ridge from OBS observations

Summary Seismicity along transform faults provides important constraints for our understanding of the factors that control earthquake ruptures. Oceanic transform faults are particularly informative due to their relatively simple structure in comparison to their continental counterparts. The seismicity of several fast-moving transform faults has been investigated by local networks, but as of today there been few studies of transform faults in slow spreading ridges. Here we present the first local seismicity catalogue based on event data recorded by a temporary broadband network of 39 ocean bottom seismometers located around the slow-moving Chain Transform Fault (CTF) along the Mid-Atlantic Ridge (MAR) from March 2016 to March 2017. We locate 972 events in the area by simultaneously inverting for a 1-D velocity model informed by the event P- and S-arrival times. We refine the depths and focal mechanisms of the larger events using deviatoric moment tensor inversion. Most of the earthquakes are located along the CTF (700) and Romanche transform fault (94) and the MAR (155); a smaller number (23) can be observed on the continuing fracture zones or in intraplate locations. The ridge events are characterised by normal faulting and most of the transform events are characterised by strike slip faulting, but with several reverse mechanisms that are likely related to transpressional stresses in the region. CTF events range in magnitude from 1.1 to 5.6 with a magnitude of completeness around 2.3. Along the CTF we calculate a b-value of 0.81 ± 0.09. The event depths are mostly shallower than 15 km below sea level (523), but a small number of high-quality earthquakes (16) are located deeper, with some (8) located deeper than the brittle-ductile transition as predicted by the 600˚C-isotherm from a simple thermal model. The deeper events could be explained by the control of seawater infiltration on the brittle failure limit

The Dulmage-Mendelsohn permutation in seismic tomography

Seismic tomography inverse problems are among the largest high-dimensional parameter estimation tasks in Earth science. Although iterative algorithms can be used to efficiently solve these problems, their size gives rise to several issues such as the intractability of the computation of the model resolution and the model posterior covariance matrices that provide the means of assessing the robustness of the solution. In this work, we utilize methods from combinatorics and graph theory to study the structure of typical regional seismic body-wave tomography problems, and to effectively decompose them into subsets that can be solved efficiently by means of the least squares method. In combination with recent high performance direct sparse algorithms, this reduction in dimensionality allows for an efficient computation of the model resolution and covariance matrices using limited resources. We apply this methodology to a moderate size imaging of the structure of the crust and the upper mantle beneath Japan using deep local earthquakes recorded by the High Sensitivity Seismograph Network stations. Among the prominent features that are being imaged is a strong low-velocity region beneath the subducting Pacific slab along the entire Japan trench

2-D analytical P-to-S and S-to-P scattered wave finite frequency kernels

Scattered wave imaging provides a powerful tool for understanding Earth's structure. The development of finite frequency kernels for scattered waves has the potential for improving the resolution of both the structure and magnitude of discontinuities in S-wave velocity. Here we present a 2-D analytical expression for teleseismic P-to-S and S-to-P scattered wave finite-frequency kernels for a homogeneous medium. We verify the accuracy of the kernels by comparing to a spectral element method kernel calculated using SPECFEM2D. Finally, we demonstrate the ability of the kernels to recover seismic velocity discontinuities with a variety of shapes including a flat discontinuity, a discontinuity with a sharp step, a discontinuity with a smooth bump, and an undulating discontinuity. We compare the recovery using the kernel approach to expected recovery assuming the classical common conversion point (CCP) stacking approach. We find that the P-to-S kernel increases recovery of all discontinuity structures in comparison to CCP stacking especially for the shallowest discontinuity in the model. The S-to-P kernel is less successful but can be useful for recovering the curvature of shallow discontinuity undulations. Finally, although we observe some variability in the amplitude of the kernels along the discontinuities, the kernels show some potential for recovering the magnitude of the velocity contrast across a discontinuity.</p

Towards the use of artificial intelligence deep learning networks for detection of archaeological sites

Funder: Daphne Jackson Trust; doi: https://doi.org/10.13039/501100000643Funder: National Environment Research CouncilAbstract While remote sensing data have long been widely used in archaeological prospection over large areas, the task of examining such data is time consuming and requires experienced and specialist analysts. However, recent technological advances in the field of artificial intelligence (AI), and in particular deep learning methods, open possibilities for the automated analysis of large areas of remote sensing data. This paper examines the applicability and potential of supervised deep learning methods for the detection and mapping of different kinds of archaeological sites comprising features such as walls and linear or curvilinear structures of different dimensions, spectral and geometrical properties. Our work deliberately uses open-source imagery to demonstrate the accessibility of these tools. One of the main challenges facing AI approaches has been that they require large amounts of labeled data to achieve high levels of accuracy so that the training stage requires significant computational resources. Our results show, however, that even with relatively limited amounts of data, simple eight-layer, fully convolutional network can be trained efficiently using minimal computational resources, to identify and classify archaeological sites and successfully distinguish them from features with similar characteristics. By increasing the number of training sets and switching to the use of high-performance computing the accuracy of the identified areas increases. We conclude by discussing the future directions and potential of such methods in archaeological research.</jats:p

Lupus vulgaris - Identification of myobacterial DNA with polymerase chain reaction LUPUS VULGARIS - NACHWEIS MYKOBAKTERIELLER DNA MITTELS POLYMERASEKETTENREAKTION

During the last decades the prevalence of skin tuberculosis has decreased in the industrialized countries, due to the modern antitubercular drugs and the improvement of hygienic and social standards. Although lupus vulgaris, the most frequent form of skin tuberculosis in our regions, is rarely seen nowadays, it should always be considered in the differential diagnosis of chronic granulomatous skin lesions. The detection of Mycobacterium tuberculosis in histopathologic sections or even in culture media is difficult, and in immunocompetent patients negative results are frequent. We present two cases of lupus vulgaris in immunocompetent patients, showing the typical clinical features of plaque-type lupus vulgaris. Histopathology revealed epithelioid tubercles in the dermis, surrounded by a dense cuff of lymphocytes. Mycobacteria could neither be detected by special staining methods nor by culture. In both cases, however, the tuberculous nature of the lesions has been proven in formalin-fixed and paraffin-embedded specimen by identification of mycobacterial DNA with polymerase chain reaction

Source regions of infragravity waves recorded at the bottom of the equatorial Atlantic Ocean, using OBS of the PI‐LAB experiment

Infragravity waves are generated along coasts, and some small fraction of their energy escapes to the open oceans and propagates with little attenuation. Due to the scarcity of deep‐ocean observations of these waves, the mechanism and the extent of the infragravity waves energy leakage from the coasts remains poorly understood. Understanding the generation and pathways of infragravity wave energy is important among others for understanding the breakup of ice‐shelves, and the contamination of high‐resolution satellite radar altimetry measurements of sea level. We examine data from 37 differential pressure gauges of Ocean Bottom Seismometers (OBS) near the equatorial mid‐Atlantic ridge, deployed during the Passive Imaging of the Lithosphere‐Asthenosphere Boundary (PI‐LAB) experiment. We use the beamforming technique to investigate the incoming directions of infragravity waves. Next, we develop a graph‐theory‐based global back‐projection method of noise cross‐correlation function envelopes, which minimizes the effects of array geometry using an adaptive weighting scheme. This approach allows us to locate the sources of the infragravity energy. We assess our observations by comparing to a global model of infragravity wave heights. Our results reveal strong coherent energy from sources and/or reflected phases at the west coast of Africa and some sources from South America. These energy sources are in good agreement with the global infragravity wave model. In addition, we also observe infragravity waves arriving from North America during specific events that mostly occur during Oct‐Feb 2016. Finally, we find indications of waves that propagate with little attenuation, long distances through sea ice, reflecting off Antarctica

Local seismicity around the Chain Transform Fault at the Mid-Atlantic Ridge from OBS observations

Seismicity along transform faults provides important constraints for our understanding of the factors that control earthquake ruptures. Oceanic transform faults are particularly informative due to their relatively simple structure in comparison to their continental counterparts. The seismicity of several fast-moving transform faults has been investigated by local networks, but as of today there been few studies of transform faults in slow spreading ridges. Here, we present the first local seismicity catalogue based on event data recorded by a temporary broad-band network of 39 ocean–bottom seismometers located around the slow-moving Chain Transform Fault (CTF) along the Mid-Atlantic Ridge (MAR) from 2016 to 2017 March. We locate 972 events in the area by simultaneously inverting for a 1-D velocity model informed by the event P- and S-arrival times. We refine the depths and focal mechanisms of the larger events using deviatoric moment tensor inversion. Most of the earthquakes are located along the CTF (700) and Romanche transform fault (94) and the MAR (155); a smaller number (23) can be observed on the continuing fracture zones or in intraplate locations. The ridge events are characterized by normal faulting and most of the transform events are characterized by strike-slip faulting, but with several reverse mechanisms that are likely related to transpressional stresses in the region. CTF events range in magnitude from 1.1 to 5.6 with a magnitude of completeness around 2.3. Along the CTF we calculate a b-value of 0.81 ± 0.09. The event depths are mostly shallower than 15 km below sea level (523), but a small number of high-quality earthquakes (16) are located deeper, with some (8) located deeper than the brittle-ductile transition as predicted by the 600 °C-isotherm from a simple thermal model. The deeper events could be explained by the control of sea water infiltration on the brittle failure limit

A dynamic lithosphere–asthenosphere boundary near the equatorial Mid-Atlantic Ridge

In plate tectonic theory a weak asthenosphere is required to facilitate the motions of the rigid plates. Partial melt could weaken the mantle, in turn impacting convection, but to date the existence of persistent melt has remained controversial. A wide range of scenarios has been reported in terms of the location, amount and pathways of melt. Here we use data collected by 39 ocean bottom seismometers deployed near the equatorial Mid-Atlantic Ridge on 0 to 80 Myr old seafloor. We calculate S-to-P (Sp) receiver functions and perform waveform modeling. We jointly interpret with shear-wave velocity tomography from surface waves and magnetotelluric (MT) imaging to take advantage of a range of resolutions and sensitivities and illuminate the structure of the oceanic lithosphere and the underlying asthenosphere. We image a tectonic plate thickness that increases with age in one location but undulates in another location. We infer thin and slightly thicker melt channels and punctuated regions of ascending partial melt several hundred kilometers off the ridge axis. This suggests melt persists over geologic timescales, although its character is dynamic, with implications for the lithosphere–asthenosphere boundary (LAB) and the driving forces of the plates. Ascending melt intermittently feeds melt channels at the base of the plate. The associated melt-enhanced buoyancy increases the influence of ridge-push in driving plate motions, whereas the channelized melt reduces the resistance of the plates to motion. Therefore, melt dynamics may play a larger role in controlling plate tectonics than previously thought