Large aperture seismic imaging at a convergent margin: Techniques and results from the Costa Rica seismogenic zone

In March and April 1995 a cooperative German, Costa Rican, and United States research team recorded onshore-offshore seismic data sets along the Pacific margin of Costa Rica using the R/V Ewing. Off the Nicoya Peninsula we used a linear array of ocean bottom seismometers and hydrophones (OBS/H) with onshore seismometers extending across much of the isthmus. In the central area we deployed an OBS/H areal array consisting of 30 instruments over a 9 km by 35-km area and had land stations on the Nicoya Peninsula adjacent to this marine array and also extending northeast on the main Costa Rican landmass. Our goal in these experiments was to determine the crustal velocity structure along different portions of this convergent margin and to use the dense instrument deployments to create migrated reflection images of the plate boundary zone and the subducting Cocos Plate. Our specific goal in the central area was to determine whether a subducted seamount is present at the location of the 1990, M 7 earthquake off the Nicoya Peninsula and can thus be linked to its nucleation. Subsequently we have processed the data to improve reflection signals, used the data to calculate crustal velocity models, and developed several wide-aperture migration techniques, based on a Kirchhoff algorithm, to produce reflection images. Along the northern transect we used the ocean bottom data to construct a detailed crustal velocity model, but reflections from the plate boundary and top and bottom of the subducting Cocos plate are difficult to identify and have so far produced poor images. In contrast, the land stations along this same transect recorded clear reflections from the top of the subducting plate or plate boundary, within the seismogenic zone, and we have constructed a clear image from this reflector beneath the Nicoya shelf. Data from the 3-D seismic experiment suffer from high-amplitude, coherent noise (arrivals other than reflections), and we have tried many techniques to enhance the signal to noise ratio of reflected arrivals. Due to the noise, an apparent lack of strong reflections from the plate boundary zone, and probable structural complexity, the resulting 3-D images only poorly resolve the top of the subducting Cocos Plate. The images are not able to provide compelling evidence of whether there is a subducting seamount at the 1990 earthquake hypocenter. Our results do show that OBS surveys are capable of creating images of the plate boundary zone and the subducting plate well into the seismogenic zone if coherent reflections are recorded at 1.8 km instrument spacing (2-D) and 5 km inline by 1 km crossline spacing for 3-D acquisition. However, due to typical high amplitude coherent noise, imaging results may be poorer than expected, especially in unfavorable geologic settings such as our 3-D survey area. More effective noise reduction in acquisition, possibly with the use of vertical hydrophone arrays, and in processing, with advanced multiple removal and possibly depth filtering, is required to achieve the desired detailed images of the seismogenic plate boundary zone.     

Water wave interaction with a floating porous cylinder

The interaction of water waves with a freely floating circular cylinder possessing a side-wall that is porous over a portion of its draft is investigated theoretically. The porous side-wall region is bounded top and bottom by impermeable end caps thereby resulting in an enclosed fluid region within the structure. The problem is formulated based on potential flow and linear wave theory and assuming small-amplitude structural oscillations. An eigenfunction expansion approach is then used to obtain semi-analytical expressions for the hydrodynamic excitation and reaction loads on the structure. Numerical results are presented which illustrate the effects of the various wave and structural parameters on these quantities. It is found that the permeability, size and location of the porous region may have a significant influence on the horizontal components of the hydrodynamic excitation and reaction loads, while its influence on the vertical components in most cases is relatively minor.     

Plane state of stress and transverse oscillations of a simply supported rectangular plate

This paper deals with a general discussion of the subject matter. Recent literature is analyzed including some incorrect treatments of particular cases. Finally, a variational solution is obtained for the case where a linear distribution of stresses is applied to the plate boundaries. The lower natural frequencies are then tabulated as a function of the governing geometric and mechanical parameters.     

Bifurcation and chaos of an articulated loading platform with piecewise non-linear stiffness using the incremental harmonic balance method

The bifurcation behavior of an articulated loading platform subjected to harmonic excitation is investigated by the incremental harmonic balance (IHB) method. The platform is modeled as a single-degree-of-freedom (SDOF) non-linear system with piecewise non-linear restoring force characteristics. The elements of the Jacobian matrix and the residue vector arising in the IHB formulations are derived in closed form. The path-following procedure using the arc length continuation method is used to trace the response curves and bifurcation diagrams. The periodic solutions and the subharmonic solutions obtained by the IHB method compare very well with the numerically integrated solutions. The bifurcation points also compare well with the numerically obtained results. The system exhibits chaotic motion through a sequence of period doubling bifurcations. Isolated period 3 solutions are also present. The Lyapunov exponents are computed and the initial condition map corresponding to coexistent attractors are obtained by the interpolated cell mapping (ICM) method.     

Demersal fish and megafaunal assemblages on the Cretan continental shelf and slope (NE Mediterranean): seasonal variation in species density, biomass and diversity

The demersal fish and megafaunal assemblages in the area of Heraklion Bay, Crete, were sampled during four research cruises from September 1994 to September 1995 within the framework of the CINCS project, using a stratified trawl survey at depths ranging from 50 to 1000 m. The sampling at 1000 m depth was the deepest ever made in Greek waters, using a bottom otter trawl. Species density and biomass were found to vary between depths and seasons, with higher values occurring at the shallow stations. Species distribution and communities are also reported. A total of 127 species of osteichthyes, chondrichthyes, crustaceans, and cephalopods were recorded, four of which were found for the first time in Greek waters. Percent similarity index (PSI) was used to assess the differences in the relative abundance of species pairs between depth zones and seasons. Principal component analysis (PCA) was applied to examine station variability statistically.