A comparison of the sedimentary records of the 1960 and 2010 great Chilean earthquakes in 17 lakes: Implications for quantitative lacustrine palaeoseismology

Seismically‐induced event deposits embedded in the sedimentary infill of lacustrine basins are highly useful for palaeoseismic reconstructions. Recent, well‐documented, great megathrust earthquakes provide an ideal opportunity to calibrate seismically‐induced event deposits for lakes with different characteristics and located in different settings. This study used 107 short sediment cores to investigate the sedimentary impact of the 1960 M_w 9·5 Valdivia and the 2010 M_w 8·8 Maule earthquakes in 17 lakes in South‐Central Chile (i.e. lakes Negra, Lo Encañado, Aculeo, Vichuquén, Laja, Villarrica, Calafquén, Pullinque, Pellaifa, Panguipulli, Neltume, Riñihue, Ranco, Maihue, Puyehue, Rupanco and Llanquihue). A combination of image analysis, magnetic susceptibility and grain‐size analysis allows identification of five types of seismically‐induced event deposits: (i) mass‐transport deposits; (ii) in situ deformations; (iii) lacustrine turbidites with a composition similar to the hemipelagic background sediments (lacustrine turbidites type 1); (iv) lacustrine turbidites with a composition different from the background sediments (lacustrine turbidites type 2) and (v) megaturbidites. These seismically‐induced event deposits were compared to local seismic intensities of the causative earthquakes, eyewitness reports, post‐earthquake observations, and vegetation and geomorphology of the catchment and the lake. Megaturbidites occur where lake seiches took place. Lacustrine turbidites type 2 can be the result of: (i) local near‐shore mass wasting; (ii) delta collapse; (iii) onshore landslides; (iv) debris flows or mudflows; or (v) fluvial reworking of landslide debris. On the contrary, lacustrine turbidites type 1 are the result of shallow mass wasting on sublacustrine slopes covered by hemipelagic sediments. Due to their more constrained origin, lacustrine turbidites type 1 are the most reliable type of seismically‐induced event deposits in quantitative palaeoseismology, because they are almost exclusively triggered by earthquake shaking. Moreover, they most sensitively record varying seismic shaking intensities. The number of lacustrine turbidites type 1 linearly increases with increasing seismic intensity, starting with no lacustrine turbidites type 1 at intensities between V½ and VI and reaching 100% when intensities are higher than VII½. Combining different types of seismically‐induced event deposits allows the reconstruction of the complete impact of an earthquake.     

Effective stresses and shear failure pressure from in situ Biot's coefficient,Hejre Field,North Sea

We propose a combination of Biot's equations for effective stress and the expression for shear failure in a rock to obtain an expression for minimum pore pressure in a stable vertical well bore. We show that a Biot's coefficient calculated from logging data in the Hejre Field, North Sea, is significantly different from 1. The log‐derived Biot's coefficient is above 0.8 in the Shetland Chalk Group and in the Tyne Group, and 0.6–0.8 in the Heno Sandstone Formation. We show that the effective vertical and horizontal stresses obtained using the log‐derived Biot's coefficient result in a drilling window for a vertical well larger than if approximating Biot's coefficient by 1. The estimation of the Biot's coefficient is straightforward in formations with a stiff frame, whereas in formations such as shales, caution has to be taken. We discuss the consequence of assumptions made on the mineral composition of shales as unphysical results could be obtained when choosing inappropriate mineral moduli.     

Sm–Nd isotopic mapping of lithospheric growth and stabilization in the eastern Kaapvaal craton

Whole-rock Sm–Nd isotope systematics of 79 Archean granitoids from the eastern Kaapvaal craton, southern Africa, are used to delineate lithospheric boundaries and to constrain the timescale of crustal growth, assembly and geochemical differentiation c. 3.66–2.70 Ga. Offsets in ε_Nd values for 3.2–3.3 Ga granitoids across the Barberton greenstone belt (BGB) are consistent with existing models for c. 3.23 Ga accretion of newly formed lithosphere north of the BGB onto pre-existing c. 3.66 Ga lithosphere south of the BGB along a doubly verging subduction margin. The Nd isotopic signature of c. 3.3–3.2 Ga magmatic rocks show that significant crustal growth occurred during subduction–accretion. After c. 3.2 Ga, however, the Nd signature of intrusive rocks c. 3.1 and 2.7 Ga is dominated by intracrustal recycling rather than by new additions from the mantle, signalling cratonic stability.     

Fast numerical solution of the linearized Molodensky problem

 When standard boundary element methods (BEM) are used in order to solve the linearized vector Molodensky problem we are confronted with two problems: (1) the absence of O(|x|⁻²) terms in the decay condition is not taken into account, since the single-layer ansatz, which is commonly used as representation of the disturbing potential, is of the order O(|x|⁻¹) as x→∞. This implies that the standard theory of Galerkin BEM is not applicable since the injectivity of the integral operator fails; (2) the N×N stiffness matrix is dense, with N typically of the order 10⁵. Without fast algorithms, which provide suitable approximations to the stiffness matrix by a sparse one with O(N(logN) ^s ), s≥0, non-zero elements, high-resolution global gravity field recovery is not feasible. Solutions to both problems are proposed. (1) A proper variational formulation taking the decay condition into account is based on some closed subspace of co-dimension 3 of the space of square integrable functions on the boundary surface. Instead of imposing the constraints directly on the boundary element trial space, they are incorporated into a variational formulation by penalization with a Lagrange multiplier. The conforming discretization yields an augmented linear system of equations of dimension N+3×N+3. The penalty term guarantees the well-posedness of the problem, and gives precise information about the incompatibility of the data. (2) Since the upper left submatrix of dimension N×N of the augmented system is the stiffness matrix of the standard BEM, the approach allows all techniques to be used to generate sparse approximations to the stiffness matrix, such as wavelets, fast multipole methods, panel clustering etc., without any modification. A combination of panel clustering and fast multipole method is used in order to solve the augmented linear system of equations in O(N) operations. The method is based on an approximation of the kernel function of the integral operator by a degenerate kernel in the far field, which is provided by a multipole expansion of the kernel function. Numerical experiments show that the fast algorithm is superior to the standard BEM algorithm in terms of CPU time by about three orders of magnitude for N=65 538 unknowns. Similar holds for the storage requirements. About 30 iterations are necessary in order to solve the linear system of equations using the generalized minimum residual method (GMRES). The number of iterations is almost independent of the number of unknowns, which indicates good conditioning of the system matrix. Received: 16 October 1999 / Accepted: 28 February 2001     

Problems in predicting the mobility of slow-moving landslides

This paper discusses problems in predicting the mobility of slow-moving landslides. Three case studies are presented here where research has been carried out by the Utrecht University: The La Valette landslide complex in the French Alps, the La Mure landslide in the French pre-Alps near Grenoble and the Hau landslide in Switzerland.To predict field velocities of these slow-moving landslides the viscosity parameters of the material of these landslides were determined by strain-controlled tests in a ring shear apparatus based on Bishop's design at Utrecht University. The viscosity parameters from the laboratory proved to be 10 to 1000 times lower than viscosities obtained from back analyses on the observed velocities in the field. This discrepancy may be explained by the development of negative pore pressures when the plastic material slides over a rigid, wavy slip surface and/or by convergent flow effects. The associated gain in strength results in a higher apparent viscosity.A more detailed analysis is made for the movements of the La Valette landslide. Observed velocities at the La Valette landslide are difficult to describe by one parameter set as the response to a change in groundwater level is not the same during a rise or fall in the piezometric level. This deviation may be explained by rapid changes in total stresses and consequently changes in pore pressure under (partly) undrained conditions. The emerging hysteresis with local and temporal variations in pore pressure makes it difficult to predict in detail the moving pattern of landslides.     

Low-Temperature Oxidation of Magnetite in Loess-Paleosol Sequences: a Correction of Rock Magnetic Parameters

Low-temperature oxidation under atmospheric conditions affects the magnetic properties of magnetite in natural rocks: the coercivities of magnetite grains increase and other parameters change accordingly. It was recently shown that heating to 150°C largely removes the effects of low-temperature oxidation (van Velzen and Zijderveld, 1995). Heating may therefore serve as a detection tool for the presence of the effect of low-temperature oxidation. In the present study, a collection of loess and paleosol samples from various loess regions of the world is examined for the influence of low-temperature oxidation. In all samples of the collection a decrease of coercivities was found after heating to 150°C. Generally loess samples were affected to a larger extent than paleosol samples. The original range of remanent coercivities(B _cr)of 21-58 mT changed to 20-42 mT after heating. The IRM capacity of the samples decreased from 0 up to 25%. ARM showed changes between a decrease of 10% and an increase of 15%. The grain-size indicative parameter IRM/ARM is considerably influenced by the heating and therefore by low-temperature oxidation. The changes in susceptibility are limited and will not influence the interpretation of large-scale features of the susceptibility record as a paleoclimate proxy. Small variations, however, may be obscured by the varying influence of oxidation in the outcrop, which can significantly modify the rock-magnetic record. Rock-magnetic parameters used to determine magnetic mineral content and grain sizes should be corrected for the effect of low-temperature oxidation. To this end heating to 150°C is recommended. The occurrence of the changes is in itself already an indication for the presence of magnetite. Low-temperature oxidation will not only be due to recent weathering in the outcrop, but also to earlier oxidation processes in the source area, during transport and deposition of the loess and during pedogenesis. Truly fresh sediment samples are only influenced by this earlier oxidation. In that case heating will reveal the degree of ancient low-temperature oxidation, which may be related to climate at the time of deposition and pedogenesis.     

Critical movement of large rocks in currents and waves

Cobbles, boulders, and rocks often are used in a bed protection layer near a structure to protect the underlying sand bed against erosion by combined current and waves. The design of a bed protection layer consisting of loose rocks (rubble mound) requires knowledge of the stability and movement (as bed load) of very coarse materials. If some movement (or damage) is acceptable, the rock diameter can be designed to be smaller. This paper addresses the stability and movement of very coarse materials (cobbles, boulders, and rocks) based on the concept of the critical Shields mobility number. It is shown that the bed load transport of large cobbles, boulders, and rocks can be described by the equations of Meyer-Peter and Mueller (MPM) and Cheng. Both are valid for relatively small Shields mobility numbers. New and general equations for the design of a bed protection layer (including some permissible damage) in conditions with a current with or without waves are proposed based on the Shields mobility parameter and the bed load transport equation of Cheng. Laboratory and field data of critical velocities for pebbles, cobbles, boulders, and rocks have been analyzed and compared to the computed results of the proposed equations. Practical applications are given to demonstrate the general applicability of the proposed equations.     

The Winchcombe meteorite—A regolith breccia from a rubble pile CM chondrite asteroid

The Winchcombe meteorite is a CM chondrite breccia composed of eight distinct lithological units plus a cataclastic matrix. The degree of aqueous alteration varies between intensely altered CM2.0 and moderately altered CM2.6. Although no lithology dominates, three heavily altered rock types (CM2.1–2.3) represent >70 area%. Tochilinite–cronstedtite intergrowths (TCIs) are common in several lithologies. Their compositions can vary significantly, even within a single lithology, which can prevent a clear assessment of alteration extent if only TCI composition is considered. We suggest that this is due to early alteration under localized geochemical microenvironments creating a diversity of compositions and because later reprocessing was incomplete, leaving a record of the parent body's fluid history. In Winchcombe, the fragments of primary accretionary rock are held within a cataclastic matrix (~15 area%). This material is impact-derived fallback debris. Its grain size and texture suggest that the disruption of the original parent asteroid responded by intergranular fracture at grain sizes <100 μm, while larger phases, such as whole chondrules, splintered apart. Re-accretion formed a poorly lithified body. During atmospheric entry, the Winchcombe meteoroid broke apart with new fractures preferentially cutting through the weaker cataclastic matrix and separating the breccia into its component clasts. The strength of the cataclastic matrix imparts a control on the survival of CM chondrite meteoroids. Winchcombe's unweathered state and diversity of lithologies make it an ideal sample for exploring the geological history of the CM chondrite group.     

Impacts of cloud and turbulence schemes on integrated water vapor: comparison between model predictions and GPS measurements

Summary Structures in atmospheric Integrated Water Vapor (IWV) have been studied for the three successive cyclones, Kerstin, Liane and Monika, which controlled the meteorological conditions in the Baltic Sea catchment region in the period from 28 August to 5 September 1995 (part of the PIDCAP observational campaign defined within BALTEX). Several model predictions of these cyclones have been performed with a regional atmospheric general circulation model (RACMO). The impact of two different versions of the model physics package (standard ECHAM4 and a revised version with modifications in the cloud and turbulence scheme) has been investigated. Model predicted IWV has been evaluated with GPS station data from several stations in Sweden and Finland. For the most strongly developed cyclone Monika, the revised scheme generates more pronounced IWV structures, with well defined bands of high and low values of IWV curving into the center of the cyclone. In particular, the shape of the minima are in better agreement with the GPS station data, and the consistency between two subsequent model forecasts is also larger with the revised physics package. For the weaker systems, Kerstin and Liane, results from both model versions are very similar. Received August 11, 2000 Revised February 13, 2001