1.        Kaiser, D., S. J. Duda and D. K. Chowdhury (1996): P-wave magnitude spectra, stress drops, rupture complexities and other source parameters from broadband seismograms of three 1987 Southern California earthquakes. Geofizika, 13, 1-29.  

 

ABSTRACT:

 

       Three large earthquakes in Southern California – the Whittier Narrows earthquake of 1 October 1987 (WN), and the Elmore Ranch (ER) and Superstition Hills (SH) earthquakes of 24 November 1987 are analyzed using broadband recordings from the Graefenberg (GRF) array. The P-wave seismograms from all stations of the array are utilized to determine the magnitude spectra of the earthquakes. The magnitude spectrum represents the velocity amplitude density spectrum at the earthquake source, scaled in magnitude units. At 1 Hz the magnitude spectra show good agreement with the NEIC m_b (5.8, 5.7, and 6.0 respectively for events WN, ER, and SH).  The maximum magnitudes, however, occur at longer periods (16 s, 4.1 s, and 3.6 for WN, ER, and SH) and have larger values (6.3, 6.6, and 6.7 for WN, ER, and SH). Other source parameters determined from magnitude spectra are P-wave energy, seismic moment, fault length, average stress drop, and source complexity. The magnitude spectra and the source parameters show systematic variations across the stations of the GRF array. These variations are interpreted as the effect of changes in the local geological conditions underneath the array. The variations are smallest for the maximum spectral magnitude (with a standard deviation of less than 1%) and the largest for the stress drop (average standard deviation of 40%).

       Additional source parameters derived from the magnitude spectra are: asperity radius, displacement across the asperity, localized stress drop, and ambient faulting stress. Significant differences in the magnitude spectra and source parameters are observed between ER and SH on one side and WN on the other. The magnitude spectra of ER and SH are much simpler in shape, as compared to WN, which in turn is characterized by a high complexity and a low average stress drop (0.1 MPa). ER appears to be the result of a smooth and simple rupture with a homogeneous stress drop. SH reveals a moderate rupture complexity. 

 

Keywords: Magnitude spectra, earthquake source parameters, Southern California

 

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2.        Orlić, M. (1996): An elementary model of density distribution, thermohaline circulation and quasigeostrophic flow in land-locked seas. Geofizika, 13, 61-80.

 

ABSTRACT:

 

       Density distribution and currents generated by the surface and coastal buoyancy flux in a land-locked basin are considered. A simple conceptual model is developed for the case when the surface buoyancy loss (gain) is locally balanced by the coastal buoyancy gain (loss). The model predicts cross-shore density gradient, coast-to-surface directed hydraulic flow, and thermohaline circulation characterized by upwelling (downwelling) along the coasts and downwelling (upwelling) prevailing over the greater part of the basin. Due to deflecting influence of the Coriolis force, long-shore currents appear as well, both barotropic (related to the hydraulic effect) and baroclinic (connected with the thermohaline circulation). The model shows some similarity with the winter residual dynamics of the Adriatic Sea. In particular, it predicts surface cyclonic circulation under the surface buoyancy loss that balances coastal buoyancy gain, in fair agreement with the observations. It is stressed, however, that the primary purpose of the model is didactic, as it bridges the gap between qualitative interpretations of the buoyancy-driven processes in inland seas and complex models which allow for more sophisticated parameterizations of friction, nonlinear coupling of differently generated phenomena, joint effects of baroclinicity and relief, and/or spatial variability of buoyancy forcing.

 

Keywords: Seawater density, thermohaline circulation, quasigeostrophic flow, land-locked seas

 

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3.        Vrhovec, T. and A. Hrabar (1996): Numerical simulations of dissipation of dry temperature inversions in basins. Geofizika, 13, 81-96.   

 

ABSTRACT:

 

       A hydrostatic mesometeorological model APIKA (Vrhovec, 1991) was improved by a new turbulence parameterization using a complete prognostic equation for the turbulent kinetic energy with both horizontal and vertical advection of turbulent kinetic energy. Horizontal diffusion of momentum and heat by turbulence is explicitly included.

       Three different forcing of temperature inversion layer dissipation in basins were studied: dissipation due to diabatic (thermal) forcing, dissipation due to the advection of colder air and dissipation due to wind shear. We show that in winter time the dissipation due to the cold air advection and dissipation due to increasing wind speed aloft are efficient enough to cause mixing of deep cold air lakes while irradiation thermal forcing at 45°N in deep basins is often insufficient to dissipate the inversion in deep basins when ground is cowered with snow.

 

Keywords: Numerical simulation, dry temperature inversions

 

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4.        Herak, M., I. Allegretti, D. Herak and S. J. Duda (1996): Calibration of the Wiechert seismographs relative to a reference seismometer. Geofizika, 13, 31-59.

 

ABSTRACT:

 

       The horizontal 1000 kg Wiechert seismograph has been calibrated on the basis of a comparison of its records to the ones that were obtained by the reference seismometer. The resulting magnification curve shows notable deviations from the theoretical one. Its most prominent feature is a sudden magnification increase observed for frequencies above 5 Hz, which is attributed to frame oscillations excited by the seismic waves. Also, the observed average response curve exhibits larger magnification for frequencies below the resonance frequency. The results indicate a non-linear behavior of the Wiechert instrument related to the degree of impulsiveness of the excitation.

 

Keywords: Seismograph calibration, historical instrumentation, Wiechert seismograph

 

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