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Natural Hazards - In the analysis of seismic hazards of a particular region, the site response functions play a significant role. Site response functions for the central seismic gap become more... 相似文献
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Coda Q Estimates in the Koyna Region, India 总被引：1，自引：0，他引：1
—The coda Q, Q c ?, have been estimated for the Koyna region of India. The coda waves of 76 seismograms from thirteen local earthquakes, recorded digitally in the region during July–August, 1996, have been analyzed for this purpose at nine central frequencies viz., 1.5, 2.0, 3.0, 4.0, 6.0, 8.0, 12.0, 16.0 and 24.0 Hz using a single backscattering model. All events with magnitude less than 3 fall in the epicentral distances less than 60 km and have focal depths which range from 0.86 to 9.43 km. For the 30 sec coda window length the estimated Q c values vary from 81 to 261 at 1.5 Hz and 2088 to 3234 at 24 Hz, whereas the mean values of Q c with the standard error vary from 148 ± 13.5 at 1.5 Hz to 2703 ± 38.8 at 24 Hz. Both the estimated Q c values and their mean values exhibit the clear dependence on frequency in the region and a frequency dependence average attenuation relationship, Q c = 96f 1.09, has been obtained for the region, covering an approximate area of 11500 km2 with the surfacial extent of about 120 km and depth of 60 km.¶Lapse time dependence of Q c has also been studied for the region, with the coda waves analyzed at five lapse time windows from 20 to 60 sec duration with the difference of 10 sec. The frequency dependence average Q c relationships obtained at these window lengths Q c = 66f 1.16 (20 sec), Q c = 96f 1.09 (30 sec), Q c =131f 1.04 (40 sec), Q c = 148f 1.04 (50 sec), Q c = 182f 1.02 (60 sec) show that the frequency dependence (exponentn) remains mostly stationary at all the lapse time window lengths, while the change in Q 0 value is significant. Lapse time dependence of Q c in the region is also interpreted as the function of depth. 相似文献
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Babita Sharma Dinesh Kumar S. S. Teotia B. K. Rastogi Arun K. Gupta Srichand Prajapati 《Pure and Applied Geophysics》2012,169(1-2):89-100
The attenuation characteristics based on coda waves of two areas—Jamnagar and Junagarh of Saurashtra, Gujarat (India)—have been investigated in the present study. The frequency dependent relationships have been developed for both the areas using single back scattering model. The broadband waveforms of the vertical components of 33 earthquakes (Mw 1.5–3.5) recorded at six stations of the Jamnagar area, and broadband waveforms of 68 earthquakes (Mw 1.6–5) recorded at five stations of the Junagarh area have been used for the analysis. The estimated relations for the Junagarh area are: Q c?=?(158?±?5)f(0.99±0.04) (lapse time : 20?s), Q c?=?(170?±?4.4)f(0.97±0.02) (lapse time : 30?s) and Q c?=?(229?±?6.6)f(0.94±0.03) (lapse time : 40?s) and for the Jamnagar area are: Q c?=?(178?±?3)f(0.95±0.05) (lapse time : 20?s), Q c?=?(224?±?6)f(0.98±0.06) (lapse time : 30?s) and Q c?=?(282?±?7)f(0.91±0.03) (lapse time : 40?s). These are the first estimates for the areas under consideration. The Junagarh area appears to be more attenuative as compared to the Jamnagar area. The increase in Q c values with lapse time found here for both the areas show the depth dependence of Q c as longer lapse time windows will sample larger area. The rate of decay of attenuation (Q ?1) with frequency for the relations obtained here is found to be comparable with those of other regions of the world though the absolute values differ. A comparison of the coda-Q estimated for the Saurashtra region with those of the nearby Kachchh region shows that the Saurashtra region is less heterogeneous. The obtained relations are expected to be useful for the estimation of source parameters of the earthquakes in the Saurashtra region of Gujarat where no such relations were available earlier. These relations are also important for the simulation of earthquake strong ground motions in the region. 相似文献
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Attenuation of P,S, and coda waves in Koyna region,India 总被引：1，自引：0，他引：1
The attenuation properties of the crust in the Koyna region of the Indian shield have been investigated using 164 seismograms
from 37 local earthquakes that occurred in the region. The extended coda normalization method has been used to estimate the
quality factors for P waves and S waves , and the single back-scattering model has been used to determine the quality factor for coda waves (Q
c). The earthquakes used in the present study have the focal depth in the range of 1–9 km, and the epicentral distance vary
from 11 to 55 km. The values of
and Q
c show a dependence on frequency in the Koyna region. The average frequency dependent relationships (Q = Q
0
f
n) estimated for the region are , and . The ratio is found to be greater than one for the frequency range considered here (1.5–18 Hz). This ratio, along with the frequency
dependence of quality factors, indicates that scattering is an important factor contributing to the attenuation of body waves
in the region. A comparison of Q
c and in the present study shows that for frequencies below 4 Hz and for the frequencies greater than 4 Hz. This may be due to the multiple scattering effect of the medium. The outcome of this
study is expected to be useful for the estimation of source parameters and near-source simulation of earthquake ground motion,
which in turn are required in the seismic hazard assessment of a region. 相似文献
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We present a simple and efficient hybrid technique for simulating earthquake strong ground motion. This procedure is the combination
of the techniques of envelope function (Midorikawa
et al. Tectonophysics 218:287–295, 1993) and composite source model (Zeng
et al. Geophys Res Lett 21:725–728, 1994). The first step of the technique is based on the construction of the envelope function of the large earthquake by superposition
of envelope functions for smaller earthquakes. The smaller earthquakes (sub-events) of varying sizes are distributed randomly,
instead of uniform distribution of same size sub-events, on the fault plane. The accelerogram of large event is then obtained
by combining the envelope function with a band-limited white noise. The low-cut frequency of the band-limited white noise
is chosen to correspond to the corner frequency for the target earthquake magnitude and the high-cut to the Boore’s f
max or a desired frequency for the simulation. Below the low-cut frequency, the fall-off slope is 2 in accordance with the ω2 earthquake source model. The technique requires the parameters such as fault area, orientation of the fault, hypocenter,
size of the sub-events, stress drop, rupture velocity, duration, source–site distance and attenuation parameter. The fidelity
of the technique has been demonstrated by successful modeling of the 1991 Uttarkashi, Himalaya earthquake (Ms 7). The acceptable
locations of the sub-events on the fault plane have been determined using a genetic algorithm. The main characteristics of
the simulated accelerograms, comprised of the duration of strong ground shaking, peak ground acceleration and Fourier and
response spectra, are, in general, in good agreement with those observed at most of the sites. At some of the sites the simulated
accelerograms differ from observed ones by a factor of 2–3. The local site geology and topography may cause such a difference,
as these effects have not been considered in the present technique. The advantage of the technique lies in the fact that detailed
parameters such as velocity-Q structures and empirical Green’s functions are not required or the records of the actual time
history from the past earthquakes are not available. This method may find its application in preparing a wide range of scenarios
based on simulation. This provides information that is complementary to the information available in probabilistic hazard
maps. 相似文献
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Source parameters and scaling relations for small earthquakes in Kumaon Himalaya, India 总被引：1，自引：0，他引：1
K. Sivaram Dinesh Kumar S. S. Teotia S. S. Rai K. S. Prakasam 《Journal of Seismology》2013,17(2):579-592
We investigate the scaling relationships among earthquake source parameters using more than 300 good quality broad band seismograms from 30 small earthquakes in the Kumaon Himalaya from the spectral analysis of P and S waves. The average ratio of P/S wave corner frequency is found to be 1.13, which is suggestive of shift in the corner frequency. The estimated seismic moment range from 1.6?×?1013–5.8?×?1015 N?m, while the stress drop varies from 0.6 to 16 bars with 80 % of the events below 10 bars. An analysis of stress drop and apparent stress drop indicates the partial stress drop mechanism in the region. The source radii are between 0.17 and 0.88 km. The total seismic energy varies from 1.79?×?108 to 7.30?×?1011 J. We also observe the variation in seismic energy for a given seismic moment. The scaling relation between the seismic moment and stress drop suggests the breakdown of constant stress drop scaling for the range of seismic moments obtained here for the region. This shows the anomalous behavior of small earthquakes in the region. The study indicates that the stress drop is the dominant scaling factor for the moments studied here. 相似文献
7.
The National Capital Region (NCR) of India is exposed to high seismic hazard and risk due to a great earthquake in the central
seismic gap of Himalaya and/or due to moderate-size earthquake within NCR. The high population density, rapid growth of infrastructure,
and old engineering structures in the region make it more vulnerable to the human as well as economic loss due to moderate-size
earthquakes also. The evaluation of seismic hazard is the first step to prepare a proper mitigation plan for the region. The
aim of this paper is to evaluate the seismic hazard and risk due to moderate-size earthquakes in the vicinity of NCR. For
this purpose, a suit of accelerograms have been generated from hypothetical moderate-size earthquakes (M 5.5 and 6.0) in the region. A basic fault-plane solution is assumed for this purpose. The ranges of the different parameters
like depth of focus and stress drop values have been used in order to examine the effect of these parameters on hazard. The
accelerograms have been synthesized using two basic velocity models, one representing a hard site and the other a site with
a significant low-velocity cover. These two velocity models represent the ridge area and trans-Yamuna river area in the NCR.
The decay of peak ground acceleration (pga) values with distance is fast in trans-Yamuna region (with low-velocity surface
layer of 100 m) as compared to that of ridge area (with low-velocity surface layer of 1 m). Also, the decay of pga becomes
slower if we increase the depth of focus from 10 to 20 km. The response spectra (5% damping) of the synthetic accelerograms
for the three periods T = 0.4s, 0.75s, and 1.25s have been estimated and presented in the form of decay curves. The amplifications as a function
of epicentral distance and stress drop have also been estimated. We note that the amplifications in 100-m layer case do not
occur uniformly at all the distances, rather it is dependent on the angle of incidence of energy into the layers. The pga
values are generally amplified by more than twice with increase in stress drop from 100 to 400 bars. The seismic exposure
of the population in Delhi city has been presented. The results presented in this study may serve as an important input in
the planning of mitigation and disaster management programs in the National Capital Region. 相似文献
8.
Babita Sharma S. S. Teotia Dinesh Kumar P. S. Raju 《Pure and Applied Geophysics》2009,166(12):1949-1966
The attenuation properties of the crust in the Chamoli region of Himalaya have been examined by estimating the frequency-dependent relationships of quality factors for P waves (Qα) and for S waves (Qβ) in the frequency range 1.5–24 Hz. The extended coda normalization method has been applied on the waveforms of 25 aftershocks of the 1999 Chamoli earthquake (M 6.4) recorded at five stations. The average value of Qα is found to be varied from 68 at 1.5 Hz to 588 at 24 Hz while it varies from 126 at 1.5 Hz to 868 at 24 Hz for Qβ. The estimated frequency-dependent relations for quality factors are Qα = (44 ± 1)f(0.82±.04) and Qβ = (87 ± 3)f(0.71±.03). The rate of increase of Q(f) for P and S waves in the Chamoli region is comparable with the other regions of the world. The ratio Qβ/Qα is greater than one in the region which along with the frequency dependence of quality factors indicates that scattering is an important factor contributing to the attenuation of body waves in the region. A comparison of attenuation relation for S wave estimated here (Qβ = 87f0.71) with that of coda waves (Qc = 30f1.21) obtained by Mandal et al. (2001) for the same region shows that Qc > Qβ for higher frequencies (>8 Hz) in the region. This indicates a possible high frequency coda enrichment which suggests that the scattering attenuation significantly influences the attenuation of S waves at frequencies >8 Hz. This observation may be further investigated using multiple scattering models. The attenuation relations for quality factors obtained here may be used for the estimation of source parameters and near-source simulation of earthquake ground motion of the earthquakes, which in turn are required for the assessment of seismic hazard in the region. 相似文献
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