Sedimentology of a subtidal,tide-dominated sand body in the Yellow Sea,southwest Korea

Odanam Satoe, a subtidal, tide-dominated sand body in the Yellow Sea, Korea, is linear in plan and asymmetrical in cross-section. It consists of fine- to medium-grained, well-sorted subangular sand. Bedforms consist of high-amplitude (1–2 m) sandwaves on the lower flanks of the gentler-sloping bar surface, and medium-amplitude (0.5-1 m) sandwaves on the sand body trough adjoining the steeper face, the bar crest and shallower parts of the gently sloping bar surface. Bedforms are absent on the relatively steeper bar surface, which is characterized by 2° slopes. Bedform orientation on the gentler slope is oblique by 30° to the bar crest, parallel to the sand-body crest on the crest itself, and opposite to the steeper sand-body face in the trough below the steeper slope of the bar.Bottom current velocity data show that tidal currents are semi-rotary with a flood time—velocity asymmetry over the gentler slope, and ebb time—velocity asymmetry over the steeper slope during most of the tidal cycle. Tidal-current flow parallels bar elongation over the steeper slope, whereas over the gentler slope, tidal-current flow is directed at 30° to the bar crest and changes to normal to the crest one hour prior to low tide. Bedform orientation mapped with side-scan sonar shows agreement with these flow directions.Sand dispersal around the sand body is controlled by time—velocity asymmetry and partial rotary flow directions of tidal currents. This circulation causes not only a trapezoidal mode of grain dispersal, but also westerly migration of the sand body documented from comparative bathymetric surveys in 1964 and 1980.     

Fully nonlinear numerical wave tank (NWT) simulations and wave run-up prediction around 3-D structures

A finite-difference scheme and a modified marker-and-cell (MAC) algorithm have been developed to investigate the interactions of fully nonlinear waves with two- or three-dimensional structures of arbitrary shape. The Navier–Stokes (NS) and continuity equations are solved in the computational domain and the boundary values are updated at each time step by the finite-difference time-marching scheme in the framework of a rectangular coordinate system. The fully nonlinear kinematic free-surface condition is implemented by the marker-density function (MDF) technique developed for two fluid layers.To demonstrate the capability and accuracy of the present method, the numerical simulation of backstep flows with free-surface, and the numerical tests of the MDF technique with limit functions are conducted. The 3D program was then applied to nonlinear wave interactions with conical gravity platforms of circular and octagonal cross-sections. The numerical prediction of maximum wave run-up on arctic structures is compared with the prediction of the Shore Protection Manual (SPM) method and those of linear and second-order diffraction analyses based on potential theory and boundary element method (BEM). Through this comparison, the effects of non-linearity and viscosity on wave loading and run-up are discussed.     

Fine resolution 3D temperature fields off Kerguelen from instrumented penguins

The use of diving animals as autonomous vectors of oceanographic instruments is rapidly increasing, because this approach yields cost-efficient new information and can be used in previously poorly sampled areas. However, methods for analyzing the collected data are still under development. In particular, difficulties may arise from the heterogeneous data distribution linked to animals’ behavior. Here we show how raw temperature data collected by penguin-borne loggers were transformed to a regular gridded dataset that provided new information on the local circulation off Kerguelen. A total of 16 king penguins (Aptenodytes patagonicus) were equipped with satellite-positioning transmitters and with temperature–time–depth recorders (TTDRs) to record dive depth and sea temperature. The penguins’ foraging trips recorded during five summers ranged from 140 to 600 km from the colony and 11,000 dives >100 m were recorded. Temperature measurements recorded during diving were used to produce detailed 3D temperature fields of the area (0–200 m). The data treatment included dive location, determination of the vertical profile for each dive, averaging and gridding of those profiles onto 0.1°×0.1° cells, and optimal interpolation in both the horizontal and vertical using an objective analysis. Horizontal fields of temperature at the surface and 100 m are presented, as well as a vertical section along the main foraging direction of the penguins. Compared to conventional temperature databases (Levitus World Ocean Atlas and historical stations available in the area), the 3D temperature fields collected from penguins are extremely finely resolved, by one order finer. Although TTDRs were less accurate than conventional instruments, such a high spatial resolution of penguin-derived data provided unprecedented detailed information on the upper level circulation pattern east of Kerguelen, as well as the iron-enrichment mechanism leading to a high primary production over the Kerguelen Plateau.     

Reply to comment by Y. Park and J.-H. Ree on 'Chemical remagnetization of the Upper Carboniferous-Lower Triassic Pyeongan Supergroup in the Jeongseon area, Korea: fluid migration through the Ogcheon Fold Belt'

In their comment, Park & Ree have raised several points against the interpretation by Park et al. , and argued that the remagnetization in the Jeongseon area was caused by the thermal effects of a Late Cretaceous pluton and/or associated short-range hydrothermal fluids, rather than by long-range fluids advocated by us.
We disagree with most points raised by Park & Ree and we make a case that these are invalid because of what we believe is incorrect geologic evidence. Hence, our model—that the fluids causing the chemical remagnetization might migrate through the fault system within the Ogcheon Fold Belt—is the most plausible scenario. We recognize that our model needs to be tested in a future study and we welcome new interpretations for or against our model based on reliable geologic or geophysical data.     

Determining Q of near-surface materials from Rayleigh waves

High-frequency (≥2 Hz) Rayleigh wave phase velocities can be inverted to shear (S)-wave velocities for a layered earth model up to 30 m below the ground surface in many settings. Given S-wave velocity (V_S), compressional (P)-wave velocity (V_P), and Rayleigh wave phase velocities, it is feasible to solve for P-wave quality factor Q_P and S-wave quality factor Q_S in a layered earth model by inverting Rayleigh wave attenuation coefficients. Model results demonstrate the plausibility of inverting Q_S from Rayleigh wave attenuation coefficients. Contributions to the Rayleigh wave attenuation coefficients from Q_P cannot be ignored when Vs/V_P reaches 0.45, which is not uncommon in near-surface settings. It is possible to invert Q_P from Rayleigh wave attenuation coefficients in some geological setting, a concept that differs from the common perception that Rayleigh wave attenuation coefficients are always far less sensitive to Q_P than to Q_S. Sixty-channel surface wave data were acquired in an Arizona desert. For a 10-layer model with a thickness of over 20 m, the data were first inverted to obtain S-wave velocities by the multichannel analysis of surface waves (MASW) method and then quality factors were determined by inverting attenuation coefficients.     

Determination and application of the weights for landslide susceptibility mapping using an artificial neural network

The purpose of this study is the development, application, and assessment of probability and artificial neural network methods for assessing landslide susceptibility in a chosen study area. As the basic analysis tool, a Geographic Information System (GIS) was used for spatial data management and manipulation. Landslide locations and landslide-related factors such as slope, curvature, soil texture, soil drainage, effective thickness, wood type, and wood diameter were used for analyzing landslide susceptibility. A probability method was used for calculating the rating of the relative importance of each factor class to landslide occurrence. For calculating the weight of the relative importance of each factor to landslide occurrence, an artificial neural network method was developed. Using these methods, the landslide susceptibility index (LSI) was calculated using the rating and weight, and a landslide susceptibility map was produced using the index. The results of the landslide susceptibility analysis, with and without weights, were confirmed from comparison with the landslide location data. The comparison result with weighting was better than the results without weighting. The calculated weight and rating can be used to landslide susceptibility mapping.     

Modelling Korean extreme rainfall using a Kappa distribution and maximum likelihood estimate

Summary Attempts to use the 4-parameter Kappa distribution (K4D) with the maximum likelihood estimates (MLE) on the summer extreme daily rainfall data at 61 gauging stations over South Korea have been made to obtain reliable quantile estimates for several return periods. A numerical algorithm for searching MLE of K4D by minimizing the negative log-likelihood function with penalty method has been described. The isopluvial maps of estimated design values corresponding to selected return periods have been presented. The highest return values are centered at sites in the south-western part of the Korean peninsula. The distribution of return values for annual maxima of 2-day precipitation (AMP2) is more similar to the climatological features of annual total precipitation of Korea than that of annual maxima of daily precipitation (AMP1). Our results of return values delineate well the horizontal patterns of the heavy precipitation over the Korean peninsula. Received January 15, 2001 Revised October 8, 2001