Sediment non-linearity and attenuation of seismic waves: a study of accelerograms from Lefkas, western Greece

We investigate the dependence of the S-wave high-frequency spectral-decay parameter, κ (“kappa”) — a measure of wave attenuation — on ground-motion amplitude. 21 three-component accelerograms from two adjacent sediment sites in the town of Lefkas, western Greece, are used, representing 17 earthquakes with magnitudes M_w 4.7–7.0 and hypocentral distances 12–93 km. Recorded peak horizontal ground accelerations (PGA) and velocities (PGV) are 22–540 cm/s² and 1.3–54.5 cm/s.Fourier amplitude spectra are computed for S-wave windows, and the frequency range is visually determined where the high-frequency spectral decay can be approximated by a straight line on the linear-log plot; its slope (and hence κ) is computed by linear regression. κ is found to depend on hypocentral distance as κ=0.108+0.058R (r=0.518).As PGV increases from 1.3 to 54.5 cm/s, κ₀ (κ at 0 km, characterising inelastic attenuation in the site's subsurface geology) varies between 0.060 and 0.160 s. κ₀ is found to correlate very strongly with log MGA (r=0.645) (MGA — mean horizontal acceleration in the S-wave window) but also with log PGA (r=0.447) and log PGV (r=0.627). We attribute this behaviour to sediment non-linearity (shear-modulus degradation), resulting in the decrease of the site's dominant-resonance frequency (from about 3.5 to 2.4 Hz) and leading to the increase of κ₀. Our results imply that at sediment sites, an important contribution to κ comes from wave attenuation (damping) in the softest sediments and show that κ₀ is amplitude dependent, thus being a measure of sediment non-linearity.     

Monitoring viscosity changes from time‐lapse seismic attenuation: case study from a heavy oil reservoir

Heating heavy oil reservoirs is a common method for reducing the high viscosity of heavy oil and thus increasing the recovery factor. Monitoring of these viscosity changes in the reservoir is essential for delineating the heated region and controlling production. In this study, we present an approach for estimating viscosity changes in a heavy oil reservoir. The approach consists of three steps: measuring seismic wave attenuation between reflections from above and below the reservoir, constructing time‐lapse Q and Q^?1 factor maps, and interpreting these maps using Kelvin–Voigt and Maxwell viscoelastic models. We use a 4D relative spectrum method to measure changes in attenuation. The method is tested with synthetic seismic data that are noise free and data with additive Gaussian noise to show the robustness and the accuracy of the estimates of the Q‐factor. The results of the application of the method to a field data set exhibit alignment of high attenuation zones along the steam‐injection wells, and indicate that temperature dependent viscosity changes in the heavy oil reservoir can be explained by the Kelvin–Voigt model.     

Sensitivity of ground motion coherency to the choice of time windows from a dense seismic array in Argostoli,Greece

The stochastic estimation of coherency is perceived, in some studies, to be significantly influenced by the time window length under consideration. For the engineering purposes, usually, coherency is estimated from the strong motion, i.e., the shear (S-) wave segment of the recorded time series. However, the identification of a purely S-wave dominated window could be challenging in case of complex wave mixing on the seismograms. Moreover, there has been relatively little research on how much variation, in reality, is introduced in the coherency estimates owing to the choice of signal length. Therefore, the debate about the procedure for selecting a representative time window has remained inconclusive: different authors keep their different practices. The current article is an effort to shed light on the sensitivity of the coherency estimates to the choice of various time windows. The research draws on the dataset gathered from a dense seismic-array deployed at the small size, shallow alluvial valley of Koutavos-Argostoli, situated in Cephalonia Island, Greece. The lagged coherency over interstation distance ranges from 5 to 80 m has been estimated from a set of 46 earthquakes having magnitude 2–5 and epicentral distance 3–200 km, considering different lengths of time windows. The findings revealed that the statistics of coherency estimates derived from many events is only weakly sensitive to the selection of time window lengths provided that the segments include the same energetic pulse.     

A study on the seismic performance of steel‐reinforced concrete frame‐concrete core wall high‐rise mixed structure by large‐scale shaking table tests and numerical simulations

This paper reports a study for the seismic performance of one large‐scaled (1/15) model of 30‐story steel‐reinforced concrete frame‐concrete core wall mixed structure. The study was implemented by both shaking table tests, in which the similarity ratio for lateral and gravitational accelerations was kept to 1:1, and numerical nonlinear dynamic analysis. The test observations presented herein include story displacement, interstory drift, natural vibration periods, and final failure mode. The numerical analysis was performed to simulate the shaking table test procedure, and the numerically obtained responses were verified by the test results. On the basis of the numerical results, the progressions of structural stiffness, base shear, and overturning moment were investigated, and the distributions of base shear and overturning moment between frame and core wall were also discussed. The test demonstrates the seismic performance of the steel‐reinforced concrete frame‐core wall mixed structure and reveals the potential overturning failure mode for high rise structures. The nonlinear analysis results indicate that the peripheral frames could take more shear forces after core wall damaged under severe earthquakes. Copyright © 2013 John Wiley & Sons, Ltd.     

Modeling of a large‐scale magneto‐rheological damper for seismic hazard mitigation. Part II: Semi‐active mode

A magneto‐rheological (MR) damper is a semi‐active device where the damper force capacity is controlled by varying the input current into the damper. In this paper, the dynamics of MR dampers associated with variable current input is studied. Electromagnetic theory is used to model the dynamics of an MR damper including the eddy current effect and the nonlinear hysteretic behavior of damper material magnetization. A nonlinear differential equation that relates the input current to the damper with a constant equivalent current is proposed. The nonlinear differential equation is combined with the Maxwell Nonlinear Slider (MNS) model to create the variable current MNS model to predict the damper force under variable input current and random damper displacement loading. The model is evaluated by comparing the predicted response of a large‐scale MR damper to the measured damper response from experiments. The experiments include a real‐time hybrid simulation of a 3‐story building structure with a large‐scale MR damper subjected to the design earthquake. The exceptional agreement observed between the predicted and experimental results illustrate the robustness and the accuracy of the variable current MNS model. Copyright © 2012 John Wiley & Sons, Ltd.     

Effects of forest roads on the hydrological response of a small‐scale mountain watershed in Greece

The effects of land use changes on the ecology and hydrology of natural watersheds have long been debated. However, less attention has been given to the hydrological effects of forest roads. Although less studied, several researchers have claimed that streamflow changes related to forest roads can cause a persistent and pervasive effect on hillslope hydrology and the functioning of the channel system. The main potential direct effects of forest roads on natural watersheds hydrologic response are runoff production on roads surfaces due to reduced infiltration rates, interruption of subsurface flow by road cutslopes and rapid transfer of the produced runoff to the stream network through roadside ditches. The aforementioned effects may significantly modify the total volume and timing of the hillslope flow to the stream network. This study uses detailed field data, spatial data, hydro‐meteorological records, as well as numerical simulation to investigate the effects of forest roads on the hydrological response of a small‐scale mountain experimental watershed, which is situated in the east side of Penteli Mountain, Attica, Greece. The results of this study highlight the possible effects of forest roads on the watersheds hydrological response that may significantly influence direct runoff depths and peak flow rates. It is demonstrated that these effects can be very important in permeable watersheds and that more emphasis should be given on the impact of roads on the watersheds hydrological response. Copyright © 2014 John Wiley & Sons, Ltd.     

Modeling of a large‐scale magneto‐rheological damper for seismic hazard mitigation. Part I: Passive mode

Magneto‐rheological (MR) dampers are a promising device for seismic hazard mitigation because their damping characteristics can be varied adaptively using an appropriate control law. During the last few decades researchers have investigated the behavior of MR dampers and semi‐active control laws associated with these types of dampers for earthquake hazard mitigation. A majority of this research has involved small‐scale MR dampers. To investigate the dynamic behavior of a large‐scale MR damper, characterization tests were conducted at the Lehigh Network for Earthquake Engineering Simulation equipment site on large‐scale MR dampers. A new MR damper model, called the Maxwell Nonlinear Slider (MNS) model, is developed based on the characterization tests and is reported in this paper. The MNS model can independently describe the pre‐yield and post‐yield behavior of an MR damper, which makes it easy to identify the model parameters. The MNS model utilizes Hershel–Bulkley visco‐plasticity to describe the post‐yield non‐Newtonian fluid behavior, that is, shear thinning and thickening behavior, of the MR fluid that occurs in the dampers. The predicted response of a large‐scale damper from the MNS model along with that from existing Bouc–Wen and hyperbolic tangent models, are compared with measured response from various experiments. The comparisons show that the MNS model achieves better accuracy than the existing models in predicting damper response under cyclic loading. Copyright © 2012 John Wiley & Sons, Ltd.     

Reach‐scale contributions of road‐surface sediment to the Honna River,Haida Gwaii,BC

Gravel road surfaces can be a major source of fine sediment to streams, yet their contribution to channel reach sediment balances remains poorly documented. To quantify the input of road surface material and to compare this input with natural sediment sources at the reach scale, suspended sediment dynamics was examined and a 16‐month sediment balance was developed for a ~35 channel‐width (approx. 425 m) reach of the Honna River, a medium‐size, road‐affected stream located in coastal British Columbia. Of the 105 ± 33 t of suspended material passing through the reach, 18 ± 6% was attributed to the road surface. The high availability of sediment on the road surface appears to limit hysteresis in road run‐off. During rainstorms that increase streamflow, road surface material composed 0.5–15% of sediment inputs during relatively dry conditions from April to the end of September and 5–70% through wetter conditions from October to the end of March, but our data do not show evidence of major sediment accumulation on the riverbed in the reach. A comparison of modelled sediment production on the road surface with observed yields from drainage channels suggests that (1) during low intensity rainfall, ditches and drainage channels may trap sediment from road run‐off, which is subsequently released during events of greater intensity, and/or (2) production models do not effectively describe processes, such as deposition or erosion of sediment in ditches, which control sediment transport and delivery. Our findings further emphasize the risk of unpaved roads in polluting river systems and highlight the continued need for careful road design and location away from sensitive aquatic environments. Copyright © 2016 John Wiley & Sons, Ltd.     

Improving the predictions of a MIKE SHE catchment‐scale application by using a multi‐criteria approach

This paper suggests a multi‐criteria protocol for appropriately evaluating the predictions of hydrologic models during calibration and evaluation stages. The protocol includes different statistical, analytical and visual criteria such as analysis of peak and low flows, cumulative volumes, extreme value statistics, performance statistics, etc. Furthermore, the protocol assesses the physical consistency of model predictions by filtering the total observed hydrograph into different flow‐components (baseflow, interflow and overland flow) and using these filtered data in the calibration and evaluation processes. Based on the distributed modelling of a medium size catchment, it is shown that application of the suggested protocol, and in particular the use of the filtered flow‐components in model calibration, enhances the physical consistency of model predictions, adding considerable value to the calibration process. Copyright © 2007 John Wiley & Sons, Ltd.     

Developing a large‐scale water‐balance approach to seasonal forecasting: application to the 2012 drought in Britain

Seasonal hydrological forecasts, or outlooks, can potentially provide water managers with estimates of river flows and water resources for a lead time of several months ahead. An experimental modelling tool for national hydrological outlooks has been developed which combines a hydrological model estimate of sub‐surface water storage across Britain with a range of seasonal rainfall forecasts to provide estimates of area‐wide hydrological conditions up to a few months ahead. The link is made between a deficit in sub‐surface water storage and a requirement for additional rainfall over subsequent months to enable sub‐surface water storage and river flow to return to mean monthly values. The new scheme is assessed over a recent period which includes the termination of the drought that affected much of Britain in the first few months of 2012. An illustration is provided of its use to obtain return‐period estimates of the ‘rainfall required’ to ease drought conditions; these are well in excess of 200 years for several regions of the country, for termination within a month of 1 April 2012, and still exceed 40 years for termination within three months. National maps of sub‐surface water storage anomaly show for the first time the current spatial variability of drought severity. They can also be used to provide an indication of how a drought situation might develop in the next few months given a range of possible future rainfall scenarios. © 2013 CEH/Crown and John Wiley & Sons, Ltd.     

Decadal‐scale change of infiltration characteristics of a tephra‐mantled hillslope at Mount St Helens,Washington

The cataclysmic 1980 eruption of Mount St Helens radically reduced the infiltration characteristics of ∼60 000 ha of rugged terrain and dramatically altered landscape hydrology. Two decades of erosional, biogenic, cryogenic, and anthropogenic activity have modified the infiltration characteristics of much of that devastated landscape and modulated the hydrological impact of the eruption. We assessed infiltration and runoff characteristics of a segment of hillslope thickly mantled with tephra, but now revegetated primarily with grasses and other plants, to evaluate hydrological modifications due to erosion and natural turbation. Eruptive disturbance reduced infiltration capacity of the hillslope by as much as 50‐fold. Between 1980 and 2000, apparent infiltration capacities of plots on the hillslope increased as much as ten fold, but remain approximately three to five times less than the probable pre‐eruption capacities. Common regional rainfall intensities and snowmelt rates presently produce little surface runoff; however, high‐magnitude, low‐frequency storms and unusually rapid snowmelt can still induce broad infiltration‐excess overland flow. After 20 years, erosion and natural mechanical turbation have modulated, but not effaced, the hydrological perturbation caused by the cataclysmic eruption. Copyright © 2005 John Wiley & Sons, Ltd.     

Impact of model simplifications on soil erosion predictions: application of the GLUE methodology to a distributed event‐based model at the hillslope scale

In this paper, we analyse how the performance and calibration of a distributed event‐based soil erosion model at the hillslope scale is affected by different simplifications on the parameterizations used to compute the production of suspended sediment by rainfall and runoff. Six modelling scenarios of different complexity are used to evaluate the temporal variability of the sedimentograph at the outlet of a 60 m long cultivated hillslope. The six scenarios are calibrated within the generalized likelihood uncertainty estimation framework in order to account for parameter uncertainty, and their performance is evaluated against experimental data registered during five storm events. The Nash–Sutcliffe efficiency, percent bias and coverage performance ratios show that the sedimentary response of the hillslope in terms of mass flux of eroded soil can be efficiently captured by a model structure including only two soil erodibility parameters, which control the rainfall and runoff production of suspended sediment. Increasing the number of parameters makes the calibration process more complex without increasing in a noticeable manner the predictive capability of the model. Copyright © 2015 John Wiley & Sons, Ltd.     

High‐frequency precipitation and stream water quality time series from Plynlimon,Wales: an openly accessible data resource spanning the periodic table

This scientific briefing announces the availability of a new multi‐element high‐frequency water quality data set that is openly accessible to the research community. The data set comprises up to 2 years of 7‐hourly water quality data for two streams and one rainfall site in the Upper Severn catchment at Plynlimon in Mid‐Wales. The measurements cover 50 analytes ranging from H⁺ to U and spanning six orders of magnitude in concentration, including major, minor and trace elements as well as nutrients, and they complement decades of weekly measurements of the same analytes at the Upper Severn. Together, the weekly and 7‐hourly time series provide a unique data set for studying both long‐term trends and short‐term dynamics. The data show complex behaviour over a wide range of timescales, challenging our understanding of catchment processes and informing future modelling efforts. Copyright © 2013 John Wiley & Sons, Ltd.     

Integration of 3D large‐scale earthquake simulations into the assessment of the seismic risk of Bogota,Colombia

Bogotá, the capital city of Colombia, is mostly located on a lacustrine soil deposit surrounded by hills in a central plateau of the eastern cordillera of the Colombian Andes. This highly populated urban area is exposed to a significant seismic hazard from local and regional fault systems. In addition, the potential ground motion amplification during earthquakes due to the presence of soft soil deposits, along with the effects of the surface and subsurface topography, can strongly influence the seismic hazard and consequently the seismic risk to the city. This study aims to develop a physics‐based framework to generate synthetic ground records that can help better understand the seismic response of the basin and other amplification effects during strong earthquake shaking in the region, and to incorporate these effects into the estimation of seismic risk. To this end, a set of simulations were first conducted on Hercules, the wave propagation octree‐based finite element simulator developed by the Quake Group at Carnegie Mellon University, to identify the impacts of hypothetical strong earthquakes scenarios. Then, the results from these simulations were integrated with the exposure and vulnerability information previously developed for the main building constructions in the city to assess the seismic risk in the region under different conditions of analysis. Results from this more detailed model are compared with previously published results from simplified models. Sensitivity analyses help identify critical aspects that should be considered in the future to improve the seismic risk assessment of infrastructure.     

Torsion design implications from shake‐table responses of an RC low‐rise building model having irregularities at the ground story

A 1:5 scale five‐story RC building model having the irregularities of a soft/weak story and torsion at the ground story was subjected to a series of earthquake simulation tests. The test results reveal the following: The eccentricity varied from zero to infinity with the values of base shear and torque bounded by some limits. As the intensity of table excitations increased, representing earthquakes with return periods from 50 to 2500 years in Korea, the range of eccentricities at the peak values in the time histories of drift and base shear decreased from approximately ±30% to within ±10% of the transverse dimension of the model. The inertial torque was resisted by both longitudinal and transverse frames, in proportion to their instantaneous rigidity. Yielding of the longitudinal frames under severe table excitations caused a substantial loss in their instantaneous torsional resistance and thereby transferred most of the large torque to the transverse frames, resulting in a significantly degraded torsional stiffness with an enlarged torsional deformation despite almost zero eccentricity. From these observations, it is clear that the eccentricity in itself cannot represent the critical torsional behaviors. To overcome this problem, the demand in torque shall be determined in a direct relationship with the base or story shear, given as an ellipse constructed with the maximum points in its principal axes located by the two adjacent torsion‐dominant modal spectral values. This approach provides a simple but transparent design tool by enabling comparison between demand and supply in shear force–torque diagrams. Copyright © 2014 John Wiley & Sons, Ltd.     

The possible hydrologic effects of the proposed lignite open‐cast mining in Drama lignite field,Greece

The present study investigates the possible hydrologic effects of the proposed lignite open‐cast mining in Drama lignite field (north Greece). Recent years have seen a rapid increase in surface mining. This activity has generated a growing concern for the potential environmental impacts associated with large scale surface mining. In order to achieve a safe mine operation and allow extraction of lignite to considerable depths, extensive dewatering by pumping will be necessary, while at the same time it is desirable to avoid presence of overpumping conditions in the broader area. Based on stratigrafic, hydrologic and hydrogeologic data, a three‐dimensional finite difference model was developed in order to simulate the dewatering process of the western part of the lignite open‐cast mine in Drama and to predict both spatially and temporally the decline of ground water level down to the lignite surface. The dewatering of the part of the aquifer which underlies the mine area will influence the hydrological conditions of the broader region. The most important anticipated effects will be the abandonment of shallow wells as well as the decrease of ground water pumping rates of deep wells. Aquifer discharge towards the ditches of the study area will cease and there will be an inversion of ground water flow from the ditches towards the underlying aquifer. Dewatering activities will probably result in minor subsidence of the nearby peat deposits of Drama Philippi marshes. Moreover, sand pumping as well as the presence of gasses is likely to cause local subsidence phenomena, mainly in the pit slopes. Copyright © 2007 John Wiley & Sons, Ltd.     

The first post‐injection seismic monitor survey at the Ketzin pilot CO2 storage site: results from time‐lapse analysis

The injection of CO₂ at the Ketzin pilot CO₂ storage site started in June 2008 and ended in August 2013. During the 62 months of injection, a total amount of about 67 kt of CO₂ was injected into a saline aquifer. A third repeat three‐dimensional seismic survey, serving as the first post‐injection survey, was acquired in 2015, aiming to investigate the recent movement of the injected CO₂. Consistent with the previous two time‐lapse surveys, a predominantly west–northwest migration of the gaseous CO₂ plume in the up‐dip direction within the reservoir is inferred in this first post‐injection survey. No systematic anomalies are detected through the reservoir overburden. The extent of the CO₂ plume west of the injection site is almost identical to that found in the 2012 second repeat survey (after injection of 61 kt); however, there is a significant decrease in its size east of the injection site. Assessment of the CO₂ plume distribution suggests that the decrease in the size of the anomaly may be due to multiple factors, such as limited vertical resolution, CO₂ dissolution, and CO₂ migration into thin layers, in addition to the effects of ambient noise. Four‐dimensional seismic modelling based on dynamic flow simulations indicates that a dynamic balance between the newly injected CO₂ after the second repeat survey and the CO₂ migrating into thin layers and being dissolved was reached by the time of the first post‐injection survey. In view of the significant uncertainties in CO₂ mass estimation, both patchy and non‐patchy saturation models for the Ketzin site were taken into consideration.     

Spatio‐temporal evaluation of the groundwater quality in Kafr Al‐Zayat District,Egypt

Eighteen groundwater well sites located in Kafr Al‐Zayat (Egypt) were sampled monthly from January 2009 to November 2011 for microbial content, Mn⁺², Fe⁺², total dissolved solids (TDS), total hardness, NO₃^?, and turbidity. The data were analyzed combining the integrated use of factor and cluster analyses as well as the geostatistical semi‐variogram modeling. The prime objectives were to assess the groundwater suitability for drinking, to document the factors governing the spatio‐tempral variability, and to recognize distinctive groundwater quality patterns to help enable effective sustainability and proactive management of the limited resource. The groundwater microbial, Mn⁺², Fe⁺², TDS, and total hardness contents violated the drinking water local standards while the turbidity and the nitrate content complied with them. Factor analysis indicated that the microbial content is the most influential factor raising the variability potential followed, in decreasing order, by Mn²⁺, Fe²⁺, TDS, NO₃^?, turbidity, and finally the total hardness. Turbidity resulting from urban and agricultural runoff was strongly associated with most of the quality parameters. Quality parameters fluctuate sporadically without concrete pattern in space and time while their variability scores peak in November every year. Three spatially distinctive quality patterns were recognized that were consistent with and affected by the cumulative effects of the local topography, depth to water table, thickness of the silty clay (cap layer), surface water, and groundwater flow direction and hence the recharge from contaminated surface canals and agricultural drains. Copyright © 2013 John Wiley & Sons, Ltd.