Source mechanism of the 1906 San Francisco earthquake

Surface-wave amplitudes in the period range 50–100 s at eight European and North American stations, horizontal slip profiles along the rupture zone and the timing of certain events along the fault during rupture time are all engaged in unison to reconstruct the motion at the source. A modified source model is used to accommodate a moving rupture with variable dislocation in the direction of propagation.It is inferred that the rupture started at about 13 h 11 m 55 s GMT near San Juan Bautista and propagated unilaterally northwestward along N35°W over 400 km with an average rupture velocity of 3.5 km/s. At 13 h 12 m 12 s, the dynamic shear front, moving with the rupture speed, hit the Lick Observatory. Then, at 13 h 12 m 18 s, the rupture arrived to the vicinity of the epicenter in the Santa Cruz Mountains given by B. Bolt. There the slip changed sharply from an average of 0.5 m to a high value of 3 m causing extensive landslides and avalanches. At 13 h 12 m 32.5 s two railroad clocks at San Rafael were stopped. Finally, at 13 h 12 m 36 s the offset front hit the Naval Observatory at Mare Island and stopped the astronomical clocks there. Conspicuous surface waves, visible on Wiechert seismograms in Europe in the period range 55–65 s, reflect the true rupture time.The seismic data inversion yields an effective radiation source some 240 km long with an average vertical extent of some 34 km over a total fault length of 400 km ($\text{U}\text{d}\text{S ? 29,000}\text{m km}{}^2\text{or}\text{μ}\text{U}\text{d}\text{S ? 9 · 10}{}^{27}\text{dyn cm}$). It began at the Santa Cruz Mountains and ended some 20 km off coast Point Arena. Thus, due to the nonuniform slip profile, only $\text{3}\text{5}$ of the total fracture length contributed to the far radiation field.Although the product of the average source displacement (over the entire fault) and the vertical extent appears to be fairly well determined from the surface-wave spectrums, the separate values of these entities cannot be uniquely determined. If the average surface displacements (～ 3.2 m) are diagnostic of the entire fault, a vertical extent of H = 34 km is required.Finally, a new analysis of surface waves from the Alaska earthquake of July 10, 1958, the Queen Charlotte Islands earthquake of August 22, 1949 and the Kern County shock of July 21, 1952, enables us to draw parallels between the three biggest major events which occurred along the NE Pacific coast during 1906–1958. A common feature of all of these earthquakes is that vertical failure extents of 30–40 km are implied.     

Evolution and degradation of flat‐top mesas in the hyper‐arid Negev,Israel revealed from 10Be cosmogenic nuclides

Mesas are ubiquitous landforms in arid and semiarid regions and are often characterized by horizontal stratified erodible rocks capped by more resistant strata. The accepted conceptual model for mesa evolution and degradation considers reduction in the width of the mesa flat‐top plateau due to cliff retreat but ignores possible denudation of the mesa flat‐top and the rates and mechanism of erosion. In this study we examine mesas in the northeastern hyperarid Negev Desert where they appear in various sizes and morphologies and represent different stages of mesa evolution. The variety of mesas within a single climatic zone allows examination of the process of mesa evolution through time. Two of the four sites examined are characterized by a relatively wide (200–230 m) flat‐top and a thick caprock whereas the other two are characterized by a much narrower remnant flat‐top (several meters) and thinner caprock. We use the concentration of the cosmogenic nuclide ¹⁰Be for: (a) determining the chronology of the various geomorphic features associated with the mesa; and (b) understanding geomorphic processes forming the mesa. The ¹⁰Be data, combined with field observations, suggest a correlation between the width of flat‐top mesa and the denudation and cliff retreat rates. Our results demonstrate that: (a) cliff retreat rates decrease with decreasing width of the flat‐top mesa; (b) vertical denudation rates increase with decreasing width of the flat‐top mesa below a critical value (~60 m, for the Negev Desert); (c) the reduction in the width of the flat‐top mesa is driven mainly by cliff retreat accompanied by extremely slow vertical denudation rate which can persist for a very long time (>10⁶ Ma); and (d) when the width of the mesa decreases below a certain threshold, its rate of denudation increases dramatically and mesa degradation is completed in a short time. Copyright © 2014 John Wiley & Sons, Ltd.     

Impacts of climate change on the risk of snow-induced forest damage in Finland

This work studied the temporal and spatial variability of the risk of snow-induced forest damage in Finland under current and changing climatic conditions until the end of this century. The study was based on a snow accumulation model in which cumulative precipitation, air temperature and wind speed were used as input variables. The risk was analyzed in terms of the number of days per year when the accumulated amount of snow exceeded 20 kg m^???2. Based on the risk, the forest area and mean carbon stock of seedling, young thinning and advanced thinning stands at risk were calculated. Furthermore, the number of 5-day periods, when the accumulated amount of snow exceeded a risk limit, was calculated for the current and changing climatic conditions in order to study the frequency of damaging snowfalls. Compared to the baseline period 1961–1990, the risk of snow-induced forest damage and the amount of damaging snowfalls were predicted to decrease from the first 30-year period (1991–2020) onwards. Over the whole country, the mean annual number of risk days decreased by 11%, 23% and 56% in the first, second and third 30-year period, respectively, compared to the baseline period. In the most hazardous areas in north-western and north-eastern Finland, the number of risk days decreased from the baseline period of over 30 days to about 8 days per year at the end of the century. Correspondingly, the shares of the forest area at risk were 1.9%, 2.0% and 1.0% in the first, second and third 30-year period, respectively. The highest mean annual carbon stocks of young stands at risk were found in central, north-eastern and north-western Finland in the first and second 30-year period, varying between 0.6 and 1.2 Mg C ha^???1 year^???1, meaning at highest 3% of the mean carbon stock (Mg C stem wood ha^???1) of those areas. This study showed that although the risk of snow-induced forest damage was mainly affected by changes in critical weather events, the development of growing stock under the changing climatic conditions also had an effect on the risk assessment. However, timely management of forest stands in the areas with a high risk of snow-induced damage contributes to the trees’ increased resistance to the damage.     

Latent Heat Flux from Small Sheltered Lakes

The dependency of the latent heat flux on the over-water fetch on lakes surrounded by tall, dense forest was studied by making use of measurements made on two different-sized lakes. The measurements were made during the NOPEX (Northern Hemisphere Climate-Processes Land Surface Experiment) field campaign. It was found that, in the case of a typical Scandinavian lake with a size of less than 10 km², the latent heat flux will increase as a function of over-water fetch due to the increase of wind speed and in spite of the increased air humidity. This also has implications on area-averaged fluxes: when two lakes having similar shorelines, lake water temperatures and solar radiation conditions are compared, then the evaporation per unit area is smaller from the smaller lake. When the lakes are large, with fetches of several kilometres, then the significance of sheltering is small. If point measurements are used for the estimation of area-averaged latent heat fluxes from lakes with short fetches and forested shorelines then the distance of the measuring site from the shoreline should be taken into account, otherwise errors of tens of percent may occur.     

Displacement barrier heights from experimental nucleation rate data

Nucleation phenomena have a great importance in many areas of science. However, the main theoretical tool to analyse these phenomena, the classical nucleation theory (CNT), has known weaknesses. A decade ago a nucleation theorem based correction to CNT was developed [McGraw, R., Laaksonen, A., 1996. Scaling properties of the critical nucleus in classical and molecular-based theories of vapor–liquid nucleation. Phys. Rev. Lett. 76, 2754–2757]. We have analysed experimental nucleation rate data of two homologous series of molecular fluids in terms of this scaling relation. Our first results suggest possible universal functional form for correction to the temperature dependence of CNT.     

Cost–benefit analysis of climate change dynamics: uncertainties and the value of information

We analyze climate change in a cost–benefit framework, using the emission and concentration profiles of Wigley et al. (Nature 379(6562):240–243, 1996). They present five scenarios that cover the period 1990–2300 and are designed to reach stabilized concentration levels of 350, 450, 550, 650 and 750 ppmv, respectively. We assume that the damage cost in each year t is proportional to the corresponding gross world product and the square of the atmospheric temperature increase (ΔT(t)). The latter is estimated with a simple two-box model (representing the atmosphere and deep ocean). Coupling the damage cost with the abatement cost, we interpolate between the five scenarios to find the one that is optimal in the sense of minimizing the sum of discounted annual (abatement plus damage) costs over a time horizon of N years. Our method is simpler than ‘traditional’ models with the same purpose, and thus allows for a more transparent sensitivity study with respect to the uncertainties of all parameters involved. We report our central result in terms of the stabilized emission level E _o and concentration level p _o (i.e. their values at t = 300 years) of the optimal scenario. For the central parameter values (that is, N = 150 years, a discount rate r _dis = 2%/year and a growth rate r _gro = 1%/year of gross world product) we find E _o  = 8.0 GtCO₂/year and p _o = 496 ppmv. Varying the parameters over a wide range, we find that the optimal emission level remains within a remarkably narrow range, from about 6.0 to 12 GtCO₂/year for all plausible parameter values. To assess the significance of the uncertainties we focus on the social cost penalty, defined as the extra cost incurred by society relative to the optimum if one makes the wrong choice of the emission level as a result of erroneous damage and abatement cost estimates. In relative terms the cost penalty turns out to be remarkably insensitive to errors. For example, if the true damage costs are three times larger or smaller than the estimate, the total social cost of global climate change increases by less than 20% above its minimum at the true optimal emission level. Because of the enormous magnitude of the total costs involved with climate change (mitigation), however, even a small relative error implies large additional expenses in absolute terms. To evaluate the benefit of reducing cost uncertainties, we plot the cost penalty as function of the uncertainty in relative damage and abatement costs, expressed as geometric standard deviation and standard deviation respectively. If continued externality analysis reduces the geometric standard deviation of relative damage cost estimates from 5 to 4, the benefit is 0.05% of the present value G _tot of total gross word product over 150 years (about $3.9 × 10¹⁵), and if further research reduces the standard deviation of relative abatement costs from 1 to 0.5, the benefit is 0.03% of G _tot .     

The surface energy balance and the mixing height in urban areas—activities and recommendations of COST-Action 715

The specific problems of determining and simulating the surface energy balance (SEB) and the mixing height (MH) over urban areas are examined. The SEB and MH are critical components of algorithms and numerical models for the urban boundary layer, though the constituent parts of the SEB and the MH are not routinely measured by national weather services. Parameterisations are thus needed in applications. In this investigation, several recently developed algorithms and models for estimating the SEB and MH were applied to new datasets and assessed. Results are discussed in terms of the need for spatial resolution and the parameters needed to describe the urban atmosphere. Limitations of models are identified and recommendations for further development and observations are given. Having identified gaps in knowledge, key findings from new urban experiments and numerical modelling for the SEB and MH are given. The diurnal cycle for the SEB is significantly different from rural conditions—urban heat storage is needed in urban parameterisations. The urban MH is increased over the rural MH, as shown by several numerical schemes and careful sodar analyses. This work has been carried out within the COST-715 Action “Meteorology applied to urban air pollution problems (1998–2004). COST 715 reached a consensus proposing representatively sited measurements of meteorological parameters and turbulent fluxes above roof-tops, and recognised that such data are needed to improve numerical models of the urban surface processes.     

Isotopic insights into smoothening of abandoned fan surfaces, Southern California

Cosmogenic nuclide concentrations measured on abandoned fan surfaces along the Mojave section of the San Andreas Fault suggest that sediment is generated, transported, and removed from the fans on the order of 30-40 kyr. We measured in situ produced cosmogenic ¹⁰Be, and in some cases ²⁶Al, in boulders (n = 15), surface sediment (n = 15), and one depth profile (n = 9). Nuclide concentrations in surface sediments and boulders underestimate fan ages, suggesting that ¹⁰Be accumulation is largely controlled by the geomorphic processes that operate on the surfaces of the fans and not by their ages.Field observations, grain-size distribution, and cosmogenic nuclide data suggest that over time, boulders weather into grus and the bar sediments diffuse into the adjacent swales. As fans grow older the relief between bars and swales decreases, the sediment transport rate from bars to swales decreases, and the surface processes that erode the fan become uniform over the entire fan surface. The nuclide data therefore suggest that, over time, the difference in ¹⁰Be concentration between bars and swales increases to a maximum until the topographic relief between bars and swales is minimized, resulting in a common surface lowering rate and common ¹⁰Be concentrations across the fan. During this phase, the entire fan is lowered homogeneously at a rate of 10-15 mm kyr⁻¹.