Seismology: neotectonics and structure of the Baltic Shield

Recent Danish seismological projects involving neotectonic investigations and structural studies have determined the edge of the Baltic Shield underlying Denmark. The most active earthquake zones in Denmark are located in northwestern Jylland and adjoining offshore areas, and in the region around Kattegat, Øresund and north-east Sjælland (Fig. 1). This pattern was originally recognised by Lehmann (1956) and has been confirmed by several later studies, e.g. Gregersen et al. (1998). Recent, more detailed investigations have documented that changes in the pattern of earthquake activity have occurred within a short time span. The most pronounced example of change – possibly related to exploitation of hydrocarbons – is an activity recorded in the Central Graben area of the North Sea that was first documented by Gregersen et al. (1998). The south-western margin of the Precambrian Baltic Shield separates areas of different earthquake activity (Fig. 1; Gregersen et al. 1991). Although lithospheric stresses are more or less uniform in northern Europe, there are pronounced differences in the behaviour of the lithosphere across Denmark. The north-eastern area underlain by the Baltic Shield experiences brittle failure as recorded by common earthquakes, whereas earthquakes are virtually absent in the region southwest of the shield (Fig. 1). The margin of the Baltic Shield as defined by earthquake activity is not identical with that distinguished structurally in sedimentary studies (EUGENO-S Working Group 1988; Vejbæk & Britze 1994), in crustal studies (Abramovitz & Thybo 2000), or by recent studies of the structure of the subcrustal lithosphere (Gregersen et al. 2002; Shomali et al. 2002). The physical edge of the Baltic Shield cannot be uniquely determined on the basis of seismological studies. The earthquakes recorded, although of low magnitude, do give information about the released stresses. The earthquakes seem to be a response to a dominant NW–SE compression, also apparent elsewhere in Scandinavia and northern Europe (Slunga et al. 1984; Slunga 1989; Gregersen 1992; Müller et al. 1992). These stresses are part of the large-scale stress systems associated with continued plate motion pattern (Gregersen & Basham 1989; Zoback et al. 1989). In contrast to present low-magnitude earthquakes, postglacial sediments in northern Scandinavia have preserved features interpreted as caused by earthquakes of magnitudes around 7; these major, c. 9000 years old earthquakes are believed to be related to the post-glacial uplift of Scandinavia (e.g. Arvidsson et al. 1991; Gregersen 2002). Earthquakes are always related to fault activity, but attempts to link recent earthquakes occurring in and around Denmark to geologically known faults have only been partly successful (Gregersen et al. 1996). The most significant fault zone in Denmark, the Sorgenfrei–Tornquist Zone, is only locally active. Recent geodetic and seismic investigations demonstrate that the two sides of the Sorgenfrei–Tornquist Zone are characterised by different patterns of deformation, but the zone itself is not defined by a present-day seismicity trend crossing the central parts of Denmark (Fig. 1)

Transit times and mean ages for nonautonomous and autonomous compartmental systems

We develop a theory for transit times and mean ages for nonautonomous compartmental systems. Using the McKendrick-von F\"orster equation, we show that the mean ages of mass in a compartmental system satisfy a linear nonautonomous ordinary differential equation that is exponentially stable. We then define a nonautonomous version of transit time as the mean age of mass leaving the compartmental system at a particular time and show that our nonautonomous theory generalises the autonomous case. We apply these results to study a nine-dimensional nonautonomous compartmental system modeling the terrestrial carbon cycle, which is a modification of the Carnegie-Ames-Stanford approach (CASA) model, and we demonstrate that the nonautonomous versions of transit time and mean age differ significantly from the autonomous quantities when calculated for that model

Solvable Critical Dense Polymers

A lattice model of critical dense polymers is solved exactly for finite strips. The model is the first member of the principal series of the recently introduced logarithmic minimal models. The key to the solution is a functional equation in the form of an inversion identity satisfied by the commuting double-row transfer matrices. This is established directly in the planar Temperley-Lieb algebra and holds independently of the space of link states on which the transfer matrices act. Different sectors are obtained by acting on link states with s-1 defects where s=1,2,3,... is an extended Kac label. The bulk and boundary free energies and finite-size corrections are obtained from the Euler-Maclaurin formula. The eigenvalues of the transfer matrix are classified by the physical combinatorics of the patterns of zeros in the complex spectral-parameter plane. This yields a selection rule for the physically relevant solutions to the inversion identity and explicit finitized characters for the associated quasi-rational representations. In particular, in the scaling limit, we confirm the central charge c=-2 and conformal weights Delta_s=((2-s)^2-1)/8 for s=1,2,3,.... We also discuss a diagrammatic implementation of fusion and show with examples how indecomposable representations arise. We examine the structure of these representations and present a conjecture for the general fusion rules within our framework.Comment: 35 pages, v2: comments and references adde

Comparison of stimulation patterns for FES-cycling using measures of oxygen cost and stimulation cost

<b>Aim</b><p></p> The energy efficiency of FES-cycling in spinal cord injured subjects is very much lower than that of normal cycling, and efficiency is dependent upon the parameters of muscle stimulation. We investigated measures which can be used to evaluate the effect on cycling performance of changes in stimulation parameters, and which might therefore be used to optimise them. We aimed to determine whether oxygen cost and stimulation cost measurements are sensitive enough to allow discrimination between the efficacy of different activation ranges for stimulation of each muscle group during constant-power cycling. <p></p> <b>Methods</b><p></p> We employed a custom FES-cycling ergometer system, with accurate control of cadence and stimulated exercise workrate. Two sets of muscle activation angles (“stimulation patterns”), denoted “P1” and “P2”, were applied repeatedly (eight times each) during constant-power cycling, in a repeated measures design with a single paraplegic subject. Pulmonary oxygen uptake was measured in real time and used to determine the oxygen cost of the exercise. A new measure of stimulation cost of the exercise is proposed, which represents the total rate of stimulation charge applied to the stimulated muscle groups during cycling. A number of energy-efficiency measures were also estimated. <p></p> <b>Results</b><p></p> Average oxygen cost and stimulation cost of P1 were found to be significantly lower than those for P2 (paired <i>t</i>-test, <i>p</i> < 0.05): oxygen costs were 0.56 ± 0.03 l min<sup>−1</sup> and 0.61 ± 0.04 l min<sup>−1</sup>(mean ± S.D.), respectively; stimulation costs were 74.91 ± 12.15 mC min<sup>−1</sup> and 100.30 ± 14.78 mC min<sup>−1</sup> (mean ± S.D.), respectively. Correspondingly, all efficiency estimates for P1 were greater than those for P2. <p></p> <b>Conclusion</b><p></p> Oxygen cost and stimulation cost measures both allow discrimination between the efficacy of different muscle activation patterns during constant-power FES-cycling. However, stimulation cost is more easily determined in real time, and responds more rapidly and with greatly improved signal-to-noise properties than the ventilatory oxygen uptake measurements required for estimation of oxygen cost. These measures may find utility in the adjustment of stimulation patterns for achievement of optimal cycling performance. <p></p&gt

Human population growth offsets climate-driven increase in woody vegetation in sub-Saharan Africa

The rapidly growing human population in sub-Saharan Africa generates increasing demand for agricultural land and forest products, which presumably leads to deforestation. Conversely, a greening of African drylands has been reported, but this has been difficult to associate with changes in woody vegetation. There is thus an incomplete understanding of how woody vegetation responds to socio-economic and environmental change. Here we used a passive microwave Earth observation data set to document two different trends in land area with woody cover for 1992-2011: 36% of the land area (6,870,000 km2) had an increase in woody cover largely in drylands, and 11% had a decrease (2,150,000 km2), mostly in humid zones. Increases in woody cover were associated with low population growth, and were driven by increases in CO2 in the humid zones and by increases in precipitation in drylands, whereas decreases in woody cover were associated with high population growth. The spatially distinct pattern of these opposing trends reflects, first, the natural response of vegetation to precipitation and atmospheric CO2, and second, deforestation in humid areas, minor in size but important for ecosystem services, such as biodiversity and carbon stocks. This nuanced picture of changes in woody cover challenges widely held views of a general and ongoing reduction of the woody vegetation in Africa