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We propose a new quantitative determination of shear wave velocities for distinct geological units in the Bohemian Massif,
Czech Republic (Central Europe). The phase velocities of fundamental Love wave modes are measured along two long profiles
(~200 km) crossing three major geological units and one rift-like structure of the studied region. We have developed a modified
version of the classical multiple filtering technique for the frequency-time analysis and we apply it to two-station phase
velocity estimation. Tests of both the analysis and inversion are provided. Seismograms of three Aegean Sea earthquakes are
analyzed. One of the two profiles is further divided into four shorter sub-profiles. The long profiles yield smooth dispersion
curves; while the curves of the sub-profiles have complicated shapes. Dispersion curve undulations are interpreted as period-dependent
apparent velocity anomalies caused both by different backazimuths of surface wave propagation and by surface wave mode coupling.
An appropriate backazimuth of propagation is found for each period, and the dispersion curves are corrected for this true
propagation direction. Both the curves for the long and short profiles are inverted for a 1D shear wave velocity model of
the crust. Subsurface shear wave velocities are found to be around 2.9 km/s for all four studied sub-profiles. Two of the
profiles crossing the older Moldanubian and Teplá-Barrandian units are characterized by higher velocities of 3.8 km/s in the
upper crust while for the Saxothuringian unit we find the velocity slightly lower, around 3.6 km/s at the same depths. We
obtain an indication of a shear wave low velocity zone above Moho in the Moldanubian and Teplá-Barrandian units. The area
of the Eger Rift (Teplá-Barrandian–Saxothuringian unit contact) is significantly different from all other three units. Low
upper crust velocities suggest sedimentary and volcanic filling of the rift as well as fluid activity causing the earthquake
swarms. Higher velocities in the lower crust together with weak or even missing Moho implies the upper mantle updoming. 相似文献
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Probabilistic aftershock hazard assessment I: numerical testing of methodological features 总被引：1，自引：1，他引：0
Probabilistic aftershock hazard assessment (PAHA, Wiemer, Geoph Res Lett 27:3405–3408, 2000), provided for California within the frame of the STEP project, is based on a methodology, two features of which are addressed
in detail: (1) the parameters of Omori’s law and (2) application of attenuation relations in evaluating peak ground velocity
exceedence probability. Concerning the first point, we perform a simple parametric study. We assume the generalized Omori’s
law by Shcherbakov et al. (Geoph Res Lett 31:L11613, 2004), in which characteristic time c scales with aftershock magnitude. The study shows that, among all the parameters, the hazard is most sensitive to the choice
of m* (controlling the overall aftershock productivity) and least sensitive to the scaling of c. We also conclude that the hazard is mainly due to very early (less than 1 day) aftershocks. As regards the second point,
we employ various attenuation relations from different tectonic areas to study their effect on the hazard analysis. We conclude
that the resulting variations are relatively large, comparable to those obtained for varying m*. 相似文献
4.
Probabilistic aftershock hazard assessment II: application of strong ground motion simulations 总被引：1，自引：1，他引：0
Probabilistic aftershock hazard assessment (PAHA) has been introduced by Wiemer (Geophys Res Lett 27:3405–3408, 2000). The method, in its original form, utilizes attenuation relations in evaluating peak ground velocity (PGV) exceedence probability.
We substitute the attenuation relations together with their uncertainties by strong ground motion simulations for a set of
scenarios. The main advantage of such an approach is that the simulations account for specific details of the aftershock source
effects (faulting style, slip distribution, position of the nucleation point, etc.). Mean PGVs and their standard deviations
are retrieved from the simulation results obtained by the new hybrid k-squared source model, and they are used for the PAHA analysis at a station under study. The model chosen for the testing
purposes is inspired by the Izmit A25 aftershock (M
w
= 5.8) that occurred 26 days after the mainshock. The PAHA maps are compared with (1) those obtained by the use of attenuation
relations and (2) the peak values of ten selected strong-motion recordings written by the aftershock at epicentral distances
<50 km. We conclude that, although the overall hazard decay with increasing fault distance is similar, the PAHA maps obtained
by the use of simulations exhibit remanent radiation pattern effect and prolongation in the strike direction due to the directivity
effect pronounced for some of the scenarios. As regard the comparison with real data, we conclude that the PAHA maps agree
with observed peak values due to appropriate attenuation model adopted in the analysis. 相似文献
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Pseudopterogorgia acerosa is the tallest gorgonian octocoral found in the Caribbean. In the Cartagena area, Colombia, P. acerosa is commonly harvested as raw material for handicrafts, rendering important the study of its growth and population dynamics. Growth measurements of branches from multiple colonies at two different reefs during 1 year revealed that the main growth was due to rapid elongation of the main (‘mother’) branch and production of daughter branches from the same (or just a few) mother branch(es). Some small colonies doubled their original size in a year. The branches’ absolute growth rate was weakly correlated with size, with no significant relationships between branch size and percent growth. While the pinnate branching pattern in P. acerosa is similar to its congeners Pseudopterogorgia elisabethae and Pseudopterogorgia bipinnata, it produces taller colonies due to the faster growth rate in the primordial mother branch. This growth mode seems advantageous as it augments exposure to light and higher fluxes of suspended food in the benthic boundary layer. Post‐harvesting, clipped colonies exhibited clear signs of regeneration, even in cases where only the holdfast remained. Pseudopterogorgia acerosa seems to have similar regeneration capabilities respect to other Pseudopterogorgia species, where regenerating colonies may grow faster than intact ones. 相似文献
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This study deals with the methodical aspects of k
–2(Bernard et al., 1996) kinematic strong motions modelling: (1) it is shown how to incorporate the k-dependent rise time for 2D fault geometry in the strong motion synthesis according to the representation theorem, (2) it is suggested how to produce realistic k
–2 slip models including asperity(ies), (3) modifications are introduced concerning the typeof used slip velocity function and the corner wave number in the slip distribution. High frequency effects of these generalized models are discussed.It is shown that, assuming the rise time proportional to the spatial slip wavelength at high wave numbers, the spectral decay of displacement at frequencies higher than the corner frequency is given just by the decay ofthe slip distribution spectrum, regardless of the type of slip velocity function. It is shown numerically that this model provides -squared source spectrum even in a vicinity of a 2D normal fault buried in 1D structure, which is an agreement with previous studies. 相似文献
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We present a detailed study of Western Bohemia Love waves generated by blasts with an intention to estimate the uppermost
crust structure for a more detailed layer distribution than previous studies have used. The use of short-period (4 s and shorter)
Love waves represents a new approach in the studied region. Properties of multiple filtering as a tool of frequency–time analysis
are discussed. A new method of selecting the dispersion ridges is introduced. Tests of filtering are provided by analyzing
signals with analytically known dispersion. The isometric algorithm for the inversion problem is applied, the problem of non-uniqueness
is discussed, and tests of reliability of the inversion are presented. During the inversion, the forward problem is solved
by use of the matrix method. Six records of blasts from the Western Bohemia region are analyzed to separate the fundamental
modes of Love wavegroups, and shear wave velocity distributions down to a depth of 3.0 km are inferred. Modal summation is
used to compute synthetic velocigrams, which are compared to measured ones. The lateral heterogeneity of the region is discussed
and the presented models are compared to those of previous studies and to the geological setting of the region. 相似文献
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Johana Brokešová 《Pure and Applied Geophysics》1996,148(3-4):503-538
The seismic wave field, in its high-frequency asymptotic approximation, can be interpolated from a low- to a high-resolution spatial grid of receivers and, possibly, point sources by interpolating the eikonal (travel time) and the amplitude. These quantities can be considered as functions of position only. The travel time and the amplitude are assumed to vary in space only slowly, otherwise the validity conditions of the theory behind would be violated. Relatively coarse spatial sampling is then usually sufficient to obtain their reasonable interpolation. The interpolation is performed in 2-D models of different complexity. The interpolation geometry is either 1-D, 2-D, or 3-D according to the source-receiver distribution. Several interpolation methods are applied: the Fourier interpolation based on the sampling theorem, the linear interpolation, and the interpolation by means of the paraxial approximation. These techniques, based on completely different concepts, are tested by comparing their results with a reference ray-theory solution computed for gathers and grids with fine sampling. The paraxial method holds up as the most efficient and accurate in evaluating travel times from all investigated techniques. However, it is not suitable for approximation of amplitudes, for which the linear interpolation has proved to be universal and accurate enough to provide results acceptable for many seismological applications. 相似文献