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991.
Ionospheric precursor of a destructive earthquake that occurred on April 6, 2009 at L’Aquila (Italy)
On the basis of the 15-min data from a series of ground-based vertical ionospheric sounding stations, a study of variations
of the foF2 critical frequency before the strong earthquake (M = 6.3) that occurred on April 6, 2009 at L’Aquila (Italy) was carried out. The earthquake epicenter was located 85 km north-eastward
from Rome. Approximately 20 h prior to the earthquake, a well-pronounced statistically significant effect of foF2 increase relative to the average background for magnetically quiet days was observed for almost 1.5 h at the Rome ionospheric
station. In this case, at control stations distanced from the earthquake epicenter, no statistically significant deviations
of foF2 from the background values were detected during the same observations period. This fact provides grounds for consideration
of the foF2 increase observed at Rome station as a possible ionospheric precursor of this earthquake. 相似文献
992.
Ivanka Charvátová Jaroslav Klokočník Josef Kolmaš Jan Kostelecký 《Studia Geophysica et Geodaetica》2011,55(1):159-174
Extant written records indicate that knowledge of an ancient type of compass in China is very old — dating back to before
the Han dynasty (206 BC-220 AD) to at least the 4th century BC. Geomancy (feng shui) was practised for a long time (for millenia)
and had a profound influence on the face of China’s landscape and city plans. The tombs (pyramids) near the former Chinese
capital cities of Xi’an and Luoyang (together with their suburban fields and roads) show strong spatial orientations, sometimes
along a basic South-North axis (relative to the geographic pole), but usually with deviations of several degrees to the East
or West. The use of the compass means that the needle was directed towards the actual magnetic pole at the time of construction,
or last reconstruction, of the respective tomb. However the magnetic pole, relative to the nearly ‘fixed’ geographic pole,
shifts significantly over time. By matching paleomagnetic observations with modeled paleomagnetic history we have identified
the date of pyramid construction in central China with the orientation relative to the magnetic pole positions at the respective
time of construction. As in Mesoamerica, where according to the Fuson hypothesis the Olmécs and Maya oriented their ceremonial
buildings and pyramids using a compass even before the Chinese, here in central China the same technique may have been used.
We found a good agreement of trends between the paleodeclinations observed from tomb aligments and the available global geomagnetic
field model CALS7K.2. 相似文献
993.
J. Laštovička J. Baše F. Hruška J. Chum T. Šindelářová J. Horálek J. Zedník V. Krasnov 《Journal of Atmospheric and Solar》2010,72(16):1231-1240
Various pre-seismic and co-seismic effects have been reported in the literature in the solid Earth, hydrosphere, atmosphere, electric/magnetic field and in the ionosphere. Some of the effects observed above the surface, particularly some of the pre-seismic effects, are still a matter of debate. Here we analyze the co-seismic effects of a relatively weak earthquake of 28 October 2008, which was a part of an earthquake swarm in the westernmost region of the Czech Republic. Special attention is paid to unique measurements of infrasonic phenomena. As far as we know, these have been the first infrasonic measurements during earthquake in the epicentre zone. Infrasonic oscillations (~1–12 Hz) in the epicentre region appear to be excited essentially by the vertical seismic oscillations. The observed oscillations are real epicentral infrasound not caused by seismic shaking of the instruments or by meteorological phenomena. Seismo-infrasonic oscillations observed 155 km apart from the epicentre were excited in situ by seismic waves. No earthquake-related infrasonic effects have been observed in the ionosphere. Necessity to make vibration tests of instruments is pointed out in order to be sure that observed effects are not effects of mechanical shaking of the instrument. 相似文献
994.
Microtremor study of site effects and soil-structure resonance in the city of Ljubljana (central Slovenia) 总被引:1,自引:0,他引:1
Andrej Gosar Janez Rošer Barbara Šket Motnikar Polona Zupančič 《Bulletin of Earthquake Engineering》2010,8(3):571-592
The city of Ljubljana is located in one of the three areas with the highest seismic hazard in Slovenia, and it is also the
most densely populated. Site effects due to Quaternary sediments, which fill the up to 200 m-deep basin, are characteristic
of the whole city area, but they can be especially strong in the southern part of Ljubljana, which is built on very soft lacustrine
deposits. Existing microzonation studies of the city are inadequate, since there is a lack of borehole, geophysical and earthquake
data. The microtremor horizontal-to-vertical spectral ratio (HVSR) method was therefore applied to a 200 m dense grid of free-field
measurements over an area of 45 km2 (1,223 measuring points) in order to assess the fundamental frequency of the sediments. The main difficulties in microtremor
measurement arose from high levels of traffic and industrial noise, and from underground structures. Experimental conditions
which can influence data quality, such as strong wind and water saturation of soil, were analysed. Very clear HVSR peaks were
obtained in the entire southern part of the city, whereas in the northern part the site response is in general lower due to
lower impedance contrast of gravel with the bedrock. The iso-frequency map of sediments shows a distribution in the range
of 0.9–10 Hz. In the southern part of Ljubljana, sediment frequency correlates well with the thickness of soft sediments known
from geophysical investigations and sparse drilling. Average amplitude of the HVSR peaks is considerably higher in the southern
part (6.7 ± 2.4) than in the northern part (4.0 ± 2.0) of the city, indicating a high impedance contrast of lacustrine sediments
with the bedrock. Microtremor measurements were also performed inside 122 buildings of various heights. We focused on important
public buildings and selected blocks of flats and houses. To assess the longitudinal and transverse fundamental frequencies
of each building, amplitude spectra and the spectral ratio between the upper floor and the basement were analysed for both
directions. When one of these frequencies is close to a nearby free-field fundamental frequency, a potential soil-structure
resonance is present. This was found in 12 of the measured buildings. Three of them are tall residential buildings (from 10
to 15 floors) with a fundamental frequency of 2–3 Hz, and nine of them are low-rise buildings (from 3 to 5 floors) with a
fundamental frequency ranging from 3 to 4.5 Hz. Using the relationship between fundamental frequency and height, the typical
height of buildings that might cause soil-structure resonance can be estimated at a given sediment frequency obtained from
free-field measurements. 相似文献
995.
B. Růžek P. Hrubcová M. Novotný A. Špičák O. Karousová 《Studia Geophysica et Geodaetica》2007,51(1):141-164
A series of kinematic inversions based on robust non-linear optimization approach were performed using travel time data from
a series of seismic refraction experiments: CELEBRATION 2000, ALP 2002 and SUDETES 2003. These experiments were performed
in Central Europe from 2000 to 2003. Data from 8 profiles (CEL09, CEL10, Alp01, S01, S02, S03, S04 and S05) were processed
in this study. The goal of this work was to find seismic velocity models yielding travel times consistent with observed data.
Optimum 2D inhomogeneous isotropic P-wave velocity models were computed. We have developed and used a specialized two-step
inverse procedure. In the first “parametric” step, the velocity model contains interfaces whose shapes are defined by a number
of parameters. The velocity along each interface is supposed to be constant but may be different along the upper and lower
side of the interface. Linear vertical interpolation is used for points in between interfaces. All parameters are searched
for using robust non-linear optimization (Differential Evolution algorithm). Rays are continuously traced by the bending technique.
In the second “tomographic” step, small-scale velocity perturbations are introduced in a dense grid covering the currently
obtained velocity model. Rays are fixed in this step. Final velocity models yield travel time residuals comparable to typical
picking errors (RMS ∼ 0.1 s).
As a result, depth-velocity cross-sections of P waves along all processed profiles are obtained. The depth range of the models
is 35–50 km, the velocity varies in the range 3.5–8.2 km/s. Lowest velocities are detected in near-surface depth sections
crossing sedimentary formations. The middle crust is generally more homogeneous and has typical P wave velocity around 6 km/s.
Surprisingly the lower crust is less homogeneous and the computed velocity is in the range 6.5–7.5 km/s. The MOHO is detected
in the depth ≈30–45 km. 相似文献
996.
P. Šauli Z. Mošna J. Boška D. Kouba J. Laštovička D. Altadill 《Studia Geophysica et Geodaetica》2007,51(3):449-459
The changing state of the ionosphere is generally monitored by networks of vertical ionosondes that provide us with regular
ionospheric sounding. Many ionospheric applications require determination of the true-height electron density profiles. Therefore,
ionograms must be further inverted into real-height electron density profiles. The paper presents the comparison study of
the true-height electron density profiles inverted from ionograms using two different methods POLAN (Titheridge, 1985) and
NHPC (Huang and Reinish, 1996; Reinish et al., 2005), widely used by the ionospheric research community. Our results show
significant systematic differences between electron density profiles calculated by these two inversion methods.
pkn@ufa.cas.cz 相似文献
997.
Seismic response analysis using characteristic ground motion records for risk‐based decision‐making (3R method) 下载免费PDF全文
The concept of intensity‐based assessment for risk‐based decision‐making is introduced. It is realized by means of the so‐called 3R method (response analysis, record selection and risk‐based decision‐making), which can be used to check the adequacy of design of a new building or of the strengthening of an existing building by performing conventional pushover analysis and dynamic analysis for only a few ground motions, which are termed characteristic ground motions. Because the objective of the method is not a precise assessment of the seismic risk, a simple decision model for risk acceptability can be introduced. The engineer can decide that the reliability of a no‐collapse requirement is sufficient when collapse is observed in the case of less than half of, for example, seven characteristic ground motions. From the theoretical point of view, it is shown that the accuracy of the method is acceptable if the non‐linear response history analyses are performed at a low percentile of limit‐state intensity, which is also proven by means of several examples of multi‐storey reinforced concrete frame buildings. The 3R method represents a compromise between the exclusive use of either pushover analysis or dynamic analysis and can be easily introduced into building codes provided that its applicability is further investigated (e.g. asymmetric structures and other performance objectives) and that the procedure for the selection of characteristic ground motions is automated and readily available to engineers (www.smartengineering.si). 相似文献
998.
999.
Transport and accumulation of non-cohesive and suspended load in Klaipeda Strait are analyzed. The budget of non-cohesive and suspended load is calculated for the Klaipeda Seaport area under different hydrological conditions. A method is proposed for the forecasting of sedimentation dynamics under anthropogenic changes in the hydrological and morphometric characteristics of the strait caused by the expansion of the Klaipeda Seaport. 相似文献
1000.
A truncated convolution integral often has to be used as an approximation of a complete convolution over the sphere in many Earth science or related studies, such as geodesy, geophysics and meteorology. The truncated integration is necessary because detailed input data are not usually available over the entire Earth. In this contribution, a symmetrical mathematical apparatus is presented with which to treat the truncation problem elegantly. Some important aspects are mentioned and one practical example is shown for regional gravimetric geoid determination of Canada. 相似文献