An efficient method for estimating the collapse risk of structures in seismic regions

Assessing the probability of collapse is a computationally demanding component of performance‐based earthquake engineering. This paper examines various aspects involved in the computation of the mean annual frequency of collapse (λ_c) and proposes an efficient method for estimating the sidesway collapse risk of structures in seismic regions. By deaggregating the mean annual frequency of collapse, it is shown that the mean annual frequency of collapse is typically dominated by earthquake ground motion intensities corresponding to the lower half of the collapse fragility curve. Uncertainty in the collapse fragility curve and mean annual frequency of collapse as a function of the number of ground motions used in calculations is also quantified, and it is shown that using a small number of ground motions can lead to unreliable estimates of a structure's collapse risk. The proposed method is shown to significantly reduce the computational effort and uncertainty in the estimate. Copyright © 2012 John Wiley & Sons, Ltd.     

Particle path characteristics at the large gravel‐bed river Danube: results from a tracer study and numerical modelling

A tracer study performed on a 3 km long reach of the Danube River in Austria is presented. Forty artificial stones of three different sizes (intermediate b‐axis: 25 mm, 40 mm, 70 mm) were produced and a coded radio acoustic transmitter was implanted. The measurement system had to be improved to be applicable to large rivers with water depths up to 12 m. The positions of the stones were observed approximately once a week, depending on hydrology, over a period of at least one year by radio‐tracking from a boat, including a 15 year flood event. Transport paths and velocities, as well as the incipient motion of bedload transport, could be monitored for the first time on a large gravel‐bed river. The particle paths were found to be mostly bankline‐parallel, even though the stones passed a 30° river bend. The median of the transverse particle displacement was found to be 4% of the longitudinal displacement. Calculations considering both transverse slope and transverse flow velocities showed transverse transport to be 6·6% of the longitudinal transport indicating that marginal lateral transport is mainly influenced by morphology. A three‐dimensional (3D) numerical model using a stochastic particle tracing approach was validated with the data, indicating that the observed positions are well reproduced by the model. Within the observation period, 74% of all stones passed the reach. With more than 1000 detections, particle transport could be characterized by a mean travel velocity of about 10 m per day (variable for the different grain sizes); single tracer stones were transported up to 1000 m during a single flood event. Size‐selective behaviour could be shown and the incipient motion of the large 70 mm gravel was detected at lower discharges than predicted by commonly used uniform bedload transport formulae. Copyright © 2012 John Wiley & Sons, Ltd.     

Palaeogeographic Changes at Lake Chokrak on the Kerch Peninsula,Ukraine, during the Mid‐ and Late‐Holocene

This project has reconstructed the palaeogeographic and environmental evolution of Lake Chokrak on the Kerch Peninsula, Ukraine, during the mid‐ and late‐Holocene. This record has been evaluated in association with a regional archaeological data set to explore human–environment interactions over this period. The results show major changes in the palaeogeographic setting of Lake Chokrak since the 3rd millennium B.C., when the postglacial marine transgression had started to fill the study area. Microfaunal analyses reveal the long persistence of an open marine embayment that only became separated from the Sea of Azov when a sand barrier developed during the late 2nd millennium B.C. When colonizing the Black Sea region after the 8th century B.C., the Greek settlers erected a fortification with a small settlement on a promontory that was by then a peninsula‐like headland extending into the lake. The colonists abandoned their settlement at the end of the 1st millennium B.C. when the depth of the surrounding lake decreased from 1.5 m to less than 1 m. Today, Lake Chokrak dries up completely during summer. A detailed relative sea level (RSL) curve for the northern coast of Kerch has been established. Sea level reached its highest position at the present day. Since the 3rd millennium B.C., sea level continuously rose, without any of the previously postulated regression/transgression cycles. The RSL curve indicates differential subsidence rates within short distances in relatively stable areas, exceeding 40 cm per 1000 years. © 2012 Wiley Periodicals, Inc.     

A Mathematical View of Weights-of-Evidence,Conditional Independence,and Logistic Regression in Terms of Markov Random Fields

New light is shed on mathematical methods of potential modeling from the point of view of Markov random fields. In particular, weights-of-evidence and logistic regression models are discussed in terms of graphical models possessing Markov properties, where the notion of conditional independence is essential, and will be related to log-linear models. While weights-of-evidence with respect to indicator predictor variables and logistic regression with unrestricted predictor variables model conditional probabilities of an indicator random target variable, the subject of log-linear models is the joint probability of random variables. The relationship to log-linear models leads to a likelihood ratio test of conditional independence, rendering an omnibus test of conditional independence restricted by a normality assumption obsolete. Moreover, it reveals a hierarchy of methods comprising weights-of-evidence, logistic regression without interaction terms, and logistic regression including interaction terms, where each former method is a special case of the consecutive latter method. The assumptions of conditional independence of all predictor variables given the target variable lead to logistic regression without interaction terms. Violations of conditional independence are compensated exactly by corresponding interaction terms, no cumbersome approximate corrections are needed. Thus, including interaction terms into logistic regression models is an appropriate means to account for lacking conditional independence. Logistic regression exempts from the burden to worry about lack of conditional independence. Eventually, the relationship to log-linear models renders logistic regression with indicator predictor variables optimum for discrete predictor variables. Weights-of-evidence applies for indicator predictor variables only, logistic regression applies without restrictions of the type of predictor variables and approximates the proper distribution in the general case.     

Evaluation of the third- and fourth-generation GOCE Earth gravity field models with Australian terrestrial gravity data in spherical harmonics

In March 2013, the fourth generation of European Space Agency’s (ESA) global gravity field models, DIR4 (Bruinsma et al. in Proceedings of the ESA living planet symposium, 28 June–2 July, Bergen, ESA, Publication SP-686, 2010b) and TIM4 (Migliaccio et al. in Proceedings of the ESA living planet symposium, 28 June–2 July, Bergen, ESA, Publication SP-686, 2010), generated from the Gravity field and steady-state Ocean Circulation Explorer (GOCE) gravity observation satellite was released. We evaluate the models using an independent ground truth data set of gravity anomalies over Australia. Combined with Gravity Recovery and Climate Experiment (GRACE) satellite gravity, a new gravity model is obtained that is used to perform comparisons with GOCE models in spherical harmonics. Over Australia, the new gravity model proves to have significantly higher accuracy in the degrees below 120 as compared to EGM2008 and seems to be at least comparable to the accuracy of this model between degree 150 and degree 260. Comparisons in terms of residual quasi-geoid heights, gravity disturbances, and radial gravity gradients evaluated on the ellipsoid and at approximate GOCE mean satellite altitude ( $h=250$  km) show both fourth generation models to improve significantly w.r.t. their predecessors. Relatively, we find a root-mean-square improvement of 39 % for the DIR4 and 23 % for TIM4 over the respective third release models at a spatial scale of 100 km (degree 200). In terms of absolute errors, TIM4 is found to perform slightly better in the bands from degree 120 up to degree 160 and DIR4 is found to perform slightly better than TIM4 from degree 170 up to degree 250. Our analyses cannot confirm the DIR4 formal error of 1 cm geoid height (0.35 mGal in terms of gravity) at degree 200. The formal errors of TIM4, with 3.2 cm geoid height (0.9 mGal in terms of gravity) at degree 200, seem to be realistic. Due to combination with GRACE and SLR data, the DIR models, at satellite altitude, clearly show lower RMS values compared to TIM models in the long wavelength part of the spectrum (below degree and order 120). Our study shows different spectral sensitivity of different functionals at ground level and at GOCE satellite altitude and establishes the link among these findings and the Meissl scheme (Rummel and van Gelderen in Manusrcipta Geodaetica 20:379–385, 1995).     

Decametric modulation lanes as a probe for inner jovian magnetosphere

We use the specific scintillations of jovian decametric radio sources (modulation lanes), which are produced by plasma inhomogeneities in the vicinity of that planet, to probe the inner magnetosphere of Jupiter. The positions and frequency drift of 1762 lanes have been measured on the DAM spectra from archives. A special 3D algorithm is used for space localization of field-aligned magnetospheric inhomogeneities by the frequency drift of modulation lanes. As a result, the main regions of the lane formation are found: the Io plasma torus; the magnetic shell of the Gossamer Ring at Thebe and Amalthea orbits; and the region above the magnetic anomaly in the northern magnetosphere. It is shown that modulation lanes reveal the depleted magnetic tubes in practically unvisited, innermost regions of the jovian magnetosphere. The local and probably temporal plasma enhancement is found at the magnetic shell of Thebe satellite. Hence, the modulation lanes are a valuable instrument for remote sensing of those parts of jovian magnetosphere, which are not studied yet in situ.     

Intra‐event scale bar–bank interactions and their role in channel widening

Channel bars and banks strongly affect the morphology of both braided and meandering rivers. Accordingly, bar formation and bank erosion processes have been greatly explored. There is, however, a lack of investigations addressing the interactions between bed and bank morphodynamics, especially over short timescales. One major implication of this gap is that the processes leading to the repeated accretion of mid‐channel bars and associated widenings remain unsolved. In a restored section of the Drau River, a gravel‐bed river in Austria, mid‐channel bars have developed in a widening channel. During mean flow conditions, the bars divert the flow towards the banks. One channel section exhibited both an actively retreating bank and an expanding mid‐channel bar, and was selected to investigate the morphodynamic processes involved in bar accretion and channel widening at the intra‐event timescale. We repeatedly surveyed riverbed and riverbank topography, monitored riverbank hydrology and mounted a time‐lapse camera for continuous observation of riverbank erosion processes during four flow events. The mid‐channel bar was shown to accrete when it was submerged during flood events, which at the subsequent flow diversion during lower discharges narrowed the branch along the bank and increased the water surface elevation upstream from the riffle, which constituted the inlet into the branch. These changes of bed topography accelerated the flow along the bank and triggered bank failures up to 20 days after the flood events. Four analysed flow events exhibited a total bar expansion from initially 126 m² to 295 m², while bank retreat was 6 m at the apex of the branch. The results revealed the forcing role of bar accretion in channel widening and highlighted the importance of intra‐event scale bed morphodynamics for bank erosion, which were summarized in a conceptual model of the observed bar–bank interactions. Copyright © 2015 John Wiley & Sons, Ltd.     

Insights into Holocene relative sea-level changes in the southern North Sea using an improved microfauna-based transfer function

In light of global warming and rising relative sea level (RSL), detailed reconstructions of RSL histories and their controlling processes are essential in order to manage coastal-protection challenges. This study contributes to unravelling Holocene RSL change on the East Frisian North Sea coast in high resolution and with a new approach for the German Bight. For the first time, a transfer function (vertical error: 29.7 cm ? ~11% of the mean tidal range) for RSL change based on a combined training set of benthic foraminifers and ostracods from the back-barrier tidal basin of Spiekeroog is applied to the Holocene record of the back-barrier tidal basin of Norderney. The resulting RSL curve for the Norderney tidal basin is corrected for decompaction and shows a deceleration in RSL rise between 6000 and 5000 cal bp. The smallest possible error envelope (~1 m) results from the good suitability of salt-marsh layers between 5000 and 4000 cal bp. The RSL curve provides an approach towards the closure of the common data gap of peat-based curves for the southern North Sea related to a lack of basal peats in the youngest age range, and verifies regional differences in glacial isostatic adjustment.