A framework for coupling shoals and shallow embayments with main channels in numerical modeling of coastal plain estuaries

A simple computational framework is developed to include shoals and shallow embayments (SSE) and their interaction with main channels in estuarine modeling. The scheme, treating SSE as temporary storage areas, accounts for the water and material exchanges between SSE and main channels as the tide rises and falls, and for the biogeochemical processes affecting nonconservative substances such as water-quality parameters in SSE. The scheme, because of its simple nature, can be easily incorporated into one-, two- or three-dimensional models of estuaries with substantial SSE. The concept and the model implementation of the scheme are explained using a vertical two-dimensional model of estuarine hydrodynamics and water quality. The model application to the tidal Rappahannock River, a western shore tributary of Chesapeake Bay, demonstrates the scheme is simple and physically reasonable, and the importance of SSE in estuarine modeling. The inclusion of SSE in a water-quality model not only provides a framework, for computing water-quality conditions therein but also accounts for the interaction between SSE and the main channel. It is shown that significant errors may result if the effects of SSE are not properly accounted for in modeling of an estuary with extensive SSE.     

Tectonic evolution at an early proterozoic continental margin: The Svecokarelides of eastern Finland

The early Proterozoic history of the Baltic Shield in Fennoscandia provides evidence of fragmentation of a late Archaean craton, continental sedimentation and then back-arc spreading, interpreted as being associated with the uprise of a mantle diapir. Basin subsidence and infilling with flysch debris is explained on the basis of thermal decay, locking of a subduction zone and erosion of an uplifted arc. Compressive tectonism in the Karelian part of the Svecokarelian orogen resulted in obduction of the contents of the back-arc basin on to the continental foreland. Further compression caused further thickening of the supracrustal pile and interdigitation of tectonic slices of basement and cover due to thrusting. Subsequently movement was resolved along major NW-trending wrench-faults that generally follow the margin of teh craton and which represent zones of reactivation of planar features developed in late Archaean times. As the crustal pile was warped and uplifted in subsequent deformational phases, the deep levels of the wrench-faults acted as sites of granitoid emplacement.The tectonic activity in the Karelian part of the Svecokarelides is interpreted as the response to the northward movement of oceanic lithosphere in the Svecofennian part of the orogen where successively-formed early Proterozoic island arcs moved northwards and were intensely deformed, associated with the development and emplacement of large masses of igneous material. The resultant orogen, consisting of a stable craton, obducted nappes, exotic terrane, transcurrent faults and island arcs that moved obliquely to the margin of the craton, shows many similarities in development to that of the western Cordillera of North America.     

Inverse groundwater modelling in the Willunga Basin,South Australia

A new inverse technique for modelling groundwater flow, based on a functional minimization technique, has been used to calibrate a groundwater flow model of a subregion of the Port Willunga aquifer within the Willunga Basin in South Australia. The Willunga Basin is the location of extensive viticulture, irrigated primarily by groundwater, the levels and quality of which have declined significantly over the last 40 years. The new method is able to generate estimates of transmissivity, storativity and groundwater recharge over the whole subregion as a time-varying continuous surface; previous methods estimate local discrete parameter values at specific times. The new method has also been shown to produce accurate head values for the subregion and very good estimates of groundwater recharge. Its ultimate goal will be to provide a new and invaluable tool for significantly improved groundwater resource management. Supported in part by US National Science Foundation grants, DMS-0107492 and DMS-0079478.     

Focal mechanisms of recent earthquakes in the Southern Korean Peninsula

We evaluate the stress field in and around the southern Korean Peninsula with focal mechanism solutions, using the data collected from 71 earthquakes ( M_L = 1.9–5.2) between 1999 and 2004. For this, the hypocentres were relocated and well-constrained fault plane solutions were obtained from the data set of 1270 clear P -wave polarities and 46 SH / P amplitude ratios. The focal mechanism solutions indicate that the prevailing faulting types in South Korea are strike-slip-dominant-oblique-slip faultings with minor reverse-slip component. The maximum principal stresses (σ₁) estimated from fault-slip inversion analysis of the focal mechanism solutions show a similar orientation with E–W trend (269°–275°) and low-angle plunge (10°–25°) for all tectonic provinces in South Korea, consistent with the E–W trending maximum horizontal stress (σ_Hmax) of the Amurian microplate reported from in situ stress measurements and earthquake focal mechanisms. The directions of the intermediate (σ₂) and minimum (σ₃) principal stresses of the Gyeongsang Basin are, however, about 90 deg off from those of the other tectonic provinces on a common σ₂–σ₃ plane, suggesting a permutation of σ₂ and σ₃. Our results incorporated with those from the kinematic studies of the Quaternary faults imply that NNW- to NE-striking faults (dextral strike-slip or oblique-slip with a reverse-slip component) are highly likely to generate earthquakes in South Korea.     

Evaluation of density in layer compaction using SASW method

SASW test, which is non-intrusive and rapid in the field application, was used to evaluate the layer density in the roller compaction without performing the complicated inversion process. The concept of normalized shear wave velocity was introduced to minimize the effect of confinement in the density evaluation. SASW test was performed to determine the shear wave velocity of the layer, and the free–free resonant column (FF–RC) test was adopted to determine the correlation between the normalized shear wave velocity and density of the site, which is almost unique independent of confinement. Testing and data reduction procedures of both tests were briefly discussed and an evaluation procedure of the field density was proposed by effectively combining in-situ shear wave velocity determined by the SASW test with the correlation between the normalized shear wave velocity and the density determined by the FF–RC test. Finally, the feasibility of the proposed method was verified by performing a field case study at Hoengsung road construction site. Field densities determined by the proposed method matched well with those determined by sand cone tests, showing the potential of applying the proposed method in the field density evaluation.     

Precursors to earthquakes: Seismoelectromagnetic signals

Field measurements in the past several years have documented electromagnetic signals which are attributed to precursory stress and strain changes which were followed ultimately by earthquakes. Precursory electric field changes observed in Greece on multiple dipoles have been used to issue earthquake predictions. While the source of these signals is still unknown, a sufficient number of predictions has been issued to allow some, but not all, statistical analyses to show this method is better than randomly sampling the earthquake catalog. Ongoing efforts to identify the sources of both these signals and the magnetic field variations prior to the Loma Prieta earthquake are focusing on electrokinetic coupling of fluid flow and transient electric fields. A mechanism related to local fluid flow appears to be best suited at this time of explaining the variety of purported precursors. However, much more work is needed to improve the observations and refine the models of precursor generation. Efforts to monitor magnetotelluric transfer functions at longer periods (T>10s) have been hampered by variability of the functions. The use of modern noise reduction techniques such as remote referencing should reduce this variability, but may not reduce errors to a level needed for monitoring. Monitoring of high frequency (81 kHz) seismoelectric emissions may be promising, but lack of simultaneous observations on multiple instruments hinders the utility of this technique.     

Horizontal and vertical distribution of PCBs and chlorinated pesticides in sediments from Masan Bay,Korea

Horizontal and vertical distributions of organochlorine compounds (OCs) were determined in sediments from Masan Bay. The concentrations of polychlorinated biphenyls (PCBs), dichlorodiphenyltrichloroethanes (DDTs), HCB, hexachlorocyclohexanes (HCHs) and chlordane related compounds (CHLs) in sediments were in the range of 1.24-41.4, 0.28-89.2, 0.02-0.59, nd-1.03, and nd-2.56 ng/g, respectively. The spatial distribution of OCs showed a negative gradient from the inner of the bay to outer part of the bay, indicating that the source of OCs was probably located inside the bay. Compositional pattern of PCB congeners showed a relatively high concentration of high-chlorinated congeners in the inner part of the bay and a relatively low concentration of low-chlorinated congeners in the outer part. In sediment core from Masan Bay maximum concentrations of PCBs and DDTs are observed in the subsurface samples and correspond to an age of early 1980s and late 1960s. The concentration profiles of PCBs and DDTs in sediments of Masan Bay appear to correspond to use of PCBs and DDTs in Korea.