Seismic imaging of the lithosphere-asthenosphere boundary with a dense broadband array in central California

Lithospheric attachment to the high-velocity Isabella anomaly (IA) in central California was tested by mapping the lateral extent of interruption of the lithosphere-asthenosphere boundary (LAB). The study area spans the location of two plausible origins for the anomaly, the Monterey microplate (Wang et al., 2013) and Sierra Nevada batholith (Ducea & Saleeby, 1998). Results include 918 binned receiver functions that were made using multi-channel spectral deconvolution and an array-based spectral source estimation on the event data from an 18- month deployment of a high density array from the coast to the Sierra Nevada crossing the lateral location of the Isabella anomaly. Common conversion point (CCP) scattered wave imaging shows a strong negative velocity gradient (NVG) west of the San Andreas Fault (SAF) and a gap in a NVG horizon east of the SAF. This is interpreted as prominent arrivals at the base of a partially subducted microplate that become undetectable as the plate dips too steeply east of the SAF for reliable recovery of Sp converted phases. The gap in consistent NVG arrivals would indicate a local disruption of the LAB along the lateral extent of the anomaly. Although the LAB disruption cannot constrain the tectonic origin of the Isabella anomaly, it does indicate the anomaly is adjacent or attached to North American lithosphere. S-to-P (Sp) conversions west of the SAF are consistent with a sharp LAB contrast, but whether the mantle section of the lithosphere is composed of Monterey microplate mantle or North America forearc mantle is unknown. Imaging also shows a weak east-dipping NVG in the eastern half of the Great Valley. This suggests a continental LAB at the western edge of the Sierra Nevada foothills down to 100km depth. Sharp changes in Sp arrivals near and shallower than the Moho are consistent with a previously imaged west dipping sub-crustal shear zone extending down to about 30 km depth beneath the Great Valley. The Moho detected 45 km below the Sierra Nevada foothills shows that at least locally there is not a Moho hole as suggested by prior studies and any interruption of the Moho at this latitude is constrained farther to the west beneath the Great Valley. The results and interpretations in this study are in agreement with prior studies (Wang et al., 2013; Pikser et al., 2012) that suggest the IA is a steeply dipping continuation of the partially subducted Monterey microplate

Observations on the Stress Related Variations of Soil Radon Concentration in the Gulf of Corinth, Greece

Our observations indicate a characteristic pattern in the long-term variation of soil radon concentrations, which seems to be consistent with the expected variation of regional stress in relation to seismicity. However, it seems that the major changes in radon level begin before the rock rapture, i.e. before the earthquake occurs. These conclusions have emerged after long-term observations with continuous and thorough real-time gamma-radiation monitoring in the seismically active area of the Gulf of Corinth, Greece. The recordings acquired close to a hot spring were of very high quality, implying that the deep hydraulic flow can possibly play a key role in the pre-earthquake variation of radon level. We were able to observe outstanding examples of radon level variations before significant seismic events in the Gulf of Corinth that cannot be attributed to other external factors such as atmospheric phenomena

A noninvasive method for measuring the velocity of diffuse hydrothermal flow by tracking moving refractive index anomalies

Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/95358/1/ggge1802.pd

Dynamics of a Small Tidal Estuarine Plume

Merged with duplicate record 10026.1/844 on 13.03.2017 by CS (TIS)Small-scale estuarine plume discharges into adjacent seas are common inshore features responsible for the transportation and dispersion of brackish water in the coastal zone. However, the physics that govern small-scale mixing in the frontal regions of river plumes are still poorly understood. The current study quantified and compared the observed hydrodynamic properties present inside a radially spreading river plume discharge from the River Teign, Teignmouth, Devon, UK, to those predicted by a generic plume model. Numerical simulations designed to replicate time dependent radial plume spreading from a constant source predicted the development of an internal interfacial bore that lagged the surface plume front through radial distance and time from initial plume release. The model was scaled from time lapse X-band radar imagery that recorded several plume discharge events. Scaled model output predicted the internal bore to form approximately 180 m behind the leading surface front. Subsequent field studies employed instrumentation capable of recording high-resolution measurements of temperature, salinity and velocity, spatially and vertically throughout the plume's buoyant layer over two ebb tidal cycles. Results suggested the plume advanced at a rate dependent on a super-critical interfacial Froude number of 0(1.3) and was a region of intense mixing and downward mass entrainment. Temperature contours recorded through the stratified plume gave no indication of an internal bore in its predicted position but did show an abrupt shallowing of the interfacial region some 40 to 60 m behind the surface plimie front. Super-critical interfacial Froude and critical Gradient Richardson numbers present in this region of the plume implied that this was the position of the predicted bore. The form of the bore often appeared as an ensemble of undular internal hydraulic jumps rather than a singular discontinuity as predicted by the model. Bulk mixing analysis inside the leading front based on established gravity current theory suggested that the extent of turbulent exchange in the model frontal boundary condition P, was underestimated by about a factor of 2. With the required increase in p, model simulations showed a decrease in the lag distance of internal bore formation to one where critical Froude numbers were detected inside the actual plume. Throughout both surveys, the gravity head remained a reasonably constant size due to any increase in across frontal velocity over the ebb tidal cycle being matched by an increase in entrainment and mixing. The subsequent conclusions from the study show the outflow and mixing dynamics are controlled by the estuary's tidal modulation of estuarine brackish water outflow / plume inflow rate behind the leading plume frontal discontinuity

The impact of assimilating along-track SLA data on simulated Eddy characteristics in the Agulhas system

The Agulhas Current System is a vital element of the global ocean-climate system by virtue of its role in the transfer of energy, nutrients and organic material. In the context of working towards better climate change projections, it is necessary to develop a robust understanding of the complex dynamical mechanisms which facilitate this transfer. Mesoscale cyclonic and anticyclonic eddies transport heat, salt, organic matter and nutrients from the Indian Ocean into the South Atlantic Ocean. In so doing, they are key drivers of the Atlantic Meridional Overturning Circulation (AMOC). As such, it is important that they are adequately simulated by numerical models in order to advance the accuracy of climate prediction. In the absence of spatially and temporally coherent observing systems, numerical models provide the capacity to describe the oceanographic conditions of the region. Given the complexity of the regional dynamics, and the challenges it presents to free-running numerical models, data assimilation is a valuable tool in improving simulation quality. An important step in this continuing process is the objective, quantitative evaluation of model configurations, such that they can be continuously refined. In this study, the impact of assimilating along-track sea level anomaly (SLA) data is investigated with regard to the simulation of mesoscale eddies in the Agulhas System. Two configurations of a Hybrid Coordinate Ocean Model (HYCOM) configuration are analysed; one free run (hereafter 'Free') and one with along-track SLA data from satellite altimetry assimilated (hereafter 'Assim.') via an Ensemble Optimal Interpolation (EnOI) data assimilation scheme. The results of these two configurations are compared with each other, and against a set of corresponding observational data from satellite altimetry (hereafter 'Aviso'). To this end, an automatic eddy detection and tracking algorithm is implemented, in order to quantify eddy characteristics in a coherent and consistent manner

The evolution of mountain permafrost in the context of climate change:: towards a comprehensive analysis of permafrost monitoring data from the Swiss Alps

In the Swiss Alps, permafrost occurs discontinuously and commonly has a temperature close to 0 °C. A reduction of Alpine permafrost area and volume is expected in the course of atmospheric warming, but to date, limited evidence is available for Alpine permafrost degradation. Permafrost warming or thaw is accompanied by structural changes in the subsurface, which endanger infrastructure by increasing kinematic activity or slope instability. Changes in the permafrost impact sediment transport to the valley bottom as well as gravitational natural hazards such as rock falls, landslides or debris flows. For these reasons, the quantitative analysis of past and potential future changes in the Alpine permafrost is of great interest and importance. The objective of this PhD project was to investigate observational data from the Swiss Permafrost Monitoring Network PERMOS using an interdisciplinary approach and to develop new methods for the homogenisation and quantitative analysis of long-term monitoring data. The main focus was on assessing changes in the energy fluxes at the ground surface as a function of the snow cover, as well as on evaluating permafrost response to different meteorological conditions and events. This PhD project was part of the research project The Evolution of Mountain Permafrost in Switzerland (TEMPS, 2011-2015), which used combined observational and model-based approaches and aimed at improving the consistency and completeness of permafrost monitoring data. One achievement of this PhD thesis consists of the development of data processing algorithms for filling data gaps in temperature time series and the quantification of resulting uncertainties. Moreover, algorithms for the approximation of the thermal insulation effect of the snow cover based on ground surface temperature (GST) data were developed. This was of particular importance because snow information is usually not available for the points of interest. Furthermore, possibilities for estimating temperature variations at depth based on GST data were evaluated. The information obtained about the propagation of the thermal signal into the ground led to new insights into the temperature dependency of rock glacier creep, which were supported by observational data. Data from more than 20 study sites were made comparable in order to quantify differences at the site- and the regional scale. The GST variability proved to be almost as high at the site scale as at the regional scale. This was explained by heterogeneous topo-climatic conditions as well as by the variable snow cover in the geographic context of the Swiss Alps. The roughness of the terrain played a key role, since it modifies the thermal insulation effect of the snow. Coarse-blocky terrains require more snow to be thermally insulated from the atmosphere and freeze more rapidly compared to smooth ground surfaces. The seasonal GST pattern showed that differences among sites and years were large in early winter, whereas GST were less variable in the summer season. Many locations showed similar snow conditions and therefore similar seasonal and inter-annual GST variations, which could not be explained by variations in air temperature. Although no overall increase in GST was found, the data indicate persistent warm conditions at the ground surface since 2009. Ground temperatures (GT) experienced an overall warming trend down to several tens of m depth over the past 10-25 years. This warming was most distinct in relatively cold permafrost with temperatures below -1 °C. Since the GT at depths between 10-30 m influences the kinematic activity of rock glaciers, the surface deformation rates of the majority of the observed rock glaciers reached maxima between 2013 and 2015. Surface deformation rates quantified by photogrammetry for selected rock glaciers showed an increase in the order of 200-600 % compared to 1990-1995 and 400-800 % compared to 1960-1980. Long-lasting warm conditions at the ground surface were identified to be the cause of the rise in ground temperature and the increased kinematic activity of rock glaciers. Compared with air temperature, where direct effect on the ground is limited to the snow-free period, the snow cover and its onset in early winter had a much greater influence on the heat and energy exchange at the ground surface. After one or two snow-poor winters, permafrost was able to regenerate thermally. Strong ground cooling occurred between 2005 and 2007, which caused a temporary trend reversal in the warming ground temperatures, limiting the effect of the particularly warm air temperatures between June 2006 and May 2007. Since Alpine permafrost is not in equilibrium with the current climatic conditions, recovery periods of efficient winter cooling will probably play a key role for its future evolution and preservation. Overall, the results of this PhD project contribute to an improved process understanding and put observed ground thermal and kinematic phenomena in the context of past and potential future changes of permafrost in the Swiss Alps

Eddies in motion : visualizing boundary-layer turbulence above an open boreal peatland using UAS thermal videos

High-resolution thermal infrared (TIR) imaging is opening up new vistas in biosphere-atmosphere heat exchange studies. The rapidly developing unmanned aerial systems (UASs) and specially designed cameras offer opportunities for TIR survey with increasingly high resolution, reduced geometric and radiometric noise, and prolonged flight times. A state-of-the-art science platform is assembled using a Matrice 210 V2 drone equipped with a Zenmuse XT2 thermal camera and deployed over a pristine boreal peatland with the aim of testing its performance in a heterogeneous sedgefen ecosystem. The study utilizes the capability of the UAS platform to hover for prolonged times (about 20 min) at a height of 500ma.g.l. while recording high frame rate (30 Hz) TIR videos of an area of ca. 430 x 340 m. A methodology is developed to derive thermal signatures of near-ground coherent turbulent structures impinging on the land surface, surface temperature spectra, and heat fluxes from the retrieved videos. The size, orientation, and movement of the coherent structures are computed from the surface temperature maps, and their dependency on atmospheric conditions is examined. A range of spectral and wavelet-based approaches are used to infer the properties of the dominant turbulent scene structures. A ground-based eddy-covariance system and an in situ meteorological setup are used for reference.Peer reviewe

空間密度と発生間隔解析に基づく新たな地震群抽出アルゴリズム

Tohoku University遠田晋次課