Constitutive modeling of earthquake-induced slides on clays along slip surfaces

The paper first proposes and validates a constitutive model simulating the change of resistance along a slip surface of clay for both the undrained and drained cases. The proposed model is based on (a) the critical state theory and (b) the assumption that the critical state changes once failure is reached, in terms of the further shear displacement. Under undrained conditions, the proposed model simulates the excess pore pressure generation and, subsequently, the continuous change of resistance along the slip surface from its initial value to the peak strength and then, at large displacement, the residual value. The latter can be measured in constant-volume ring shear tests. Then, the developed constitutive model is implemented in the multi-block sliding system model for the prediction of the triggering and deformation of slides. The improved model is applied at the well-documented Fourth Avenue landslide of the 1964 Alaska earthquake.     

Floodplain evolution in a small,tectonically active basin of northern California

Sediment cores were collected along ?oodplains in the Navarro River basin of coastal northern California to examine the controls on ?oodplain evolution in a tectonically active setting. Sedimentary strata were subsampled for organic content, bulk density, and grain size measurements. Organic samples were analysed for ¹⁴C age, which yielded net‐averaged sedimentation rates for all cores. Overbank deposition rates decreased at all study sites through time and declined in the downstream direction. The ability of intermediary‐order streams to store sediment in ?oodplains decreased the ability of highest‐order streams to record sediment‐pulse events. The effects of anthropogenic disturbance, primarily logging, on long‐term overbank deposition rates were minimal. Climatic variability, by affecting sediment loading in the channel network, is the principal control on ?oodplain evolution through the Holocene. A hypothetical model is proposed to explain overbank deposition rates in the Navarro basin, which may be extrapolated to the northern‐coastal California region during the late Pleistocene and Holocene. The complexities observed in sediment storage and routing in this study imply that caution should be made when extrapolating sediment‐yield measurements obtained at river mouths or coastal shelves to geomorphic events within small, tectonically active basins. Copyright © 2003 John Wiley & Sons, Ltd.     

Mitigation of the seismic motion near the edge of cliff-type topographies using anchors and piles

Concentration of damage of buildings near the edge of cliff-type topographies has been observed during a number of recent earthquakes and interpreted by numerical dynamic analyses that illustrate the amplification of the horizontal acceleration and the generation of parasitic vertical acceleration near the tip of slopes. The paper performs a detailed parametric numerical analysis to investigate the ability of mitigating this topographic effect using anchors and piles. A typical field case, the Aegion slope of Greece, is considered. Different input motions are applied. The results illustrated that anchors and piles can be effective in mitigating the topographic effect. The main issue is that if the part of the slope in which topographic amplification occurs is connected to that at larger depths, in which the acceleration is smaller, then the accelerations at the top of the slope have to become more uniform and smaller. For typical diameter and material properties of anchors/piles the effectiveness of the mitigation depends on the length, inclination, location and number of anchors/piles. An optimum configuration of anchors/piles mitigating the topographic effect is proposed.     

Examining the physical meaning of the bank erosion coefficient used in meander migration modeling

Widely used models of meander evolution relate migration rate to vertically averaged near-bank velocity through the use of a coefficient of bank erosion (E). In applications to floodplain management problems, E is typically determined through calibration to historical planform changes, and thus its physical meaning remains unclear. This study attempts to clarify the extent to which E depends on measurable physical characteristics of the channel boundary materials using data from the Sacramento River, California, USA. Bend-average values of E were calculated from measured long-term migration rates and computed near-bank velocities. In the field, unvegetated bank material resistance to fluvial shear (k) was measured for four cohesive and noncohesive bank types using a jet-test device. At a small set of bends for which both E and k were obtained, we discovered that variability in k explains much of the variability in E. The form of this relationship suggests that when modeling long-term meander migration of large rivers, E depends largely on bank material properties. This finding opens up the possibility that E may be estimated directly from field data, enabling prediction of meander migration rates for systems where historical data are unavailable or controlling conditions have changed. Another implication is that vegetation plays a limited role in affecting long-term meander migration rates of large rivers like the Sacramento River. These hypotheses require further testing with data sets from other large rivers.     

Logging effects on sediment flux observed in a pollen‐based record of overbank deposition in a northern California catchment

A palynological approach was used to estimate overbank deposition rates in a forested catchment affected by logging. The palynological approach uses downcore variations in total fossil pollen and fossil pollen assemblage to calculate rates of overbank deposition and has a distinct advantage over radioisotopic approaches in that it is not limited by radioactive decay. Using this approach, we determined that overbank deposition rates increased over 400 per cent within years of logging events and that the increased rates persisted for less than 4 years. After logging‐induced deposition peaked, overbank deposition decreased over 60 per cent relative to the pre‐logging background values. The decreased deposition rates persisted for over 40 years. The immediate effect of logging in this catchment was to induce mass‐wasting events in hollows that produced rapidly travelling sediment pulses. In the subsequent recovery period, reduced sediment loading occurred as a result of a reduction in the volume of sediment available for transport. The reduction in sediment load led to a reduction in overbank deposition rates until subsequent logging disturbances destabilized and emptied other hollows. Copyright © 2005 John Wiley & Sons, Ltd.     

A forecast of solar activity for the 21st solar cycle

Predicted values of some main indices of solar activity for the 21st solar cycle are given. The epoch of maximum of solar activity has been placed in 1980.8±0.1. The predicted peak values of the relative sunspot numbers published by other authors are also given.     

Holocene Treeline History and Climate Change Across Northern Eurasia

Radiocarbon-dated macrofossils are used to document Holocene treeline history across northern Russia (including Siberia). Boreal forest development in this region commenced by 10,000 yr B.P. Over most of Russia, forest advanced to or near the current arctic coastline between 9000 and 7000 yr B.P. and retreated to its present position by between 4000 and 3000 yr B.P. Forest establishment and retreat was roughly synchronous across most of northern Russia. Treeline advance on the Kola Peninsula, however, appears to have occurred later than in other regions. During the period of maximum forest extension, the mean July temperatures along the northern coastline of Russia may have been 2.5° to 7.0°C warmer than modern. The development of forest and expansion of treeline likely reflects a number of complimentary environmental conditions, including heightened summer insolation, the demise of Eurasian ice sheets, reduced sea-ice cover, greater continentality with eustatically lower sea level, and extreme Arctic penetration of warm North Atlantic waters. The late Holocene retreat of Eurasian treeline coincides with declining summer insolation, cooling arctic waters, and neoglaciation.