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1.
A formula to determine the local magnitude (ML) following Richters original definition was empirically derived for the Korean Peninsula. A total of 1,644 digital seismograms from 142 Korean earthquakes that occurred from 1997 to 2000 were corrected for instrument response and convolved with the nominal Wood-Anderson torsion seismograph response to be appropriate for the original definition of ML. Then, the zero-to-peak amplitude was measured in millimeters on the synthetic Wood-Anderson seismogram. Multiple regression analysis was conducted to determine distance and station correction terms for the measured peak amplitudes. The best-fit solution for ML yielded the following formula for the Korean Peninsula:where A() and S denote the peak amplitude on the synthetic Wood-Anderson seismogram at distance and the station correction term, respectively. Note that the second term, distance correction, was adjusted with Richters ML, taking into consideration attenuation differences between the Korean Peninsula and southern California, where Richter originally introduced ML. On average, the magnitudes determined in this study are nearly the same as those determined by the Korea Institute of Geoscience and Mineral Resources (KIGAM), but are larger than those of the Korea Meteorological Administration (KMA) by as much as 0.36.  相似文献   

2.
The possibility of intermediate-term earthquake prediction at Mt. Vesuvius by means of the CN algorithm is explored. CN was originally designed to identify the Times of Increased Probability (TIPs) for the occurrence of strong tectonic earthquakes, with magnitude M ≥ M0, within a region a priori delimited. Here the CN algorithm is applied, for the first time, to the analysis of volcanic seismicity. The earthquakes recorded at Mt. Vesuvius during the period from February 1972 to June 2004 are considered, and the magnitude threshold M0 selecting the events to be predicted is varied within the range: 3.0–3.3. Satisfactory prediction results are obtained, by retrospective analysis, when a time scaling is introduced. In particular, when the length of the time windows is reduced by a factor 2.5–3, with respect to the standard version of CN algorithm, more than 90% of the events with M ≥ M0 occur within the TIP intervals, with TIPs occupying about 30% of the total time considered. The control experiment ``Seismic History' demonstrates the stability of the obtained results and indicates that the CN algorithm can be applied to monitor the preparation of impending earthquakes with M ≥ 3.0 at Mt. Vesuvius.  相似文献   

3.
In the light of the single scattering model of coda originating from local earthquakes, and based on the aftershock coda registered respectively at the 4 short period stations installed near the foci shortly after theM7.6 Lancang andM7.2 Gengma earthquakes, this paper has tentatively calculated the rate of amplitude attenuation and theQ c-value of the coda in the Lancang and Gengma areas using a newly-founded synthetic determination method. Result of the study shows the rate of coda amplitude attenuation demonstrates remarkable regional differences respectively in the southern and northern areas. The southern area presents a faster attenuation (Q c=114), whereas the northern area shows a slower attenuation (Q c=231). The paper also discusses the reasons causing such differences. Result of the study also suggests a fairly good linear relation between the coda source factorA o(f) and the seismic moment and the magnitude. Using the earthquake scaling law, the following formulas can be derived: lgM 0=lgA 0(f)+17.6,M D=0.67lgA 0(f)+1.21 and logM 0=1.5M D+15.79. In addition, the rates of amplitude attenuationβ s andβ m are respectively calculated using the single scattering and multiple scattering models, and the ratioβ s/βm=1.20−1.50 is found for the results respectively from the two models. Finally, the mean free pathL of the S-wave scattering in the southern and northern areas are determined to be 54 km and 122 km respectively by the relations which can distinguish between the inherentQ i and scatteringQ s, testify to this areas having lowQ-values correspond to stronger scatterings. The Chinese version of this paper appeared in the Chinese edition ofActa Seismologica Sinica,14, 71–82, 1992. This study is partly supported by the Seismological Science Foundation of the State Seismological Bureau of China, and the present English version of the paper is translated from its Chinese original by Wenyi Xia, Seismological Bureau of Yunnan Province.  相似文献   

4.
The seismic quality factor (Q c) and the attenuation coefficient (δ) in the earth’s crust in southwest (SW) Anatolia are estimated by using the coda wave method based on the decrease of coda wave amplitude by time on the seismogram. The quality factor Q o, the value of Q c at 1 Hz, and its frequency dependency η are determined from this method depending on the attenuation properties of scattered coda waves. δ is determined from the observations of amplitude variations of seismic waves. In applying the coda wave method, firstly, a type curve representing the average pattern of the individual coda decay curves for 0.75, 1.5, 3.0, 6.0, 12.0, and 24.0 Hz values was estimated. Secondly, lateral variation of coda Q and the attenuation coefficients for three main tectonic patterns are estimated. The shape of the type curve is controlled by the scattering and attenuation in the crustal volume sampled by the coda waves. The Q o and η values vary from 30 to 180 and from 0.55 to 1.25, respectively for SW Anatolia. In SW Anatolia, coda Q–f relation is described by and δ = 0.008 km−1. These results are expected to help in understanding the degree of tectonic complexity of the crust in SW Anatolia.  相似文献   

5.
The use of regional attenuation in computing the local magnitude, ML, from strong motion data gathered at distances less than 100 km may lead to systematic underestimates approaching 0·5 magnitude units (Trifunac & Herak, Soil Dynamics and Earthquake Engineering, 1992, 18, 229-41). The use of the attenuation law Att(Δ), for example, with synthetic estimates of Wood-Anderson seismometer response, during the Loma Preita earthquake, leads to estimates of ML which agree with the surface wave and moment magnitudes, and which are essentially distance-independent.  相似文献   

6.
An earthquake withM=6.5 happened on January 15, 2000 in Yao’an of Yunnan Province. After the earthquake, a temporary digital network with 6 detectors around the epicenter area was set up. 402 aftershocks were located more precisely. According to coda short recording observed, the coda averaging quality factor has been acquired via Sato’s single scattering model analyses,Q c(f)=49f 0.95,f=1.5～20.0 Hz, which has the attenuation characteristics of high structural active region.  相似文献   

7.
A method for the determination of consistent local magnitude M L values (Richter scale, or M WA) for earthquakes with epicentral distances ranging from 10 km through 1000 km is demonstrated. The raw data consists of nearly 1300 amplitude readings from a network of six digital seismographs in Baden–Württemberg (Southwestern Germany) during 26 months starting in 1995, later extended by another 1000 amplitude readings until 1999. Relying on most of the basics introduced by C.F. Richter a three-parameter attenuation curve (distance correction, magnitude-distance relation) for Baden–Württemberg and adjacent areas is presented. Station corrections are evaluated and the attenuation curve is calibrated with respect to other agencies for distances greater than 650 km. Reasonable parametrisations are discussed and meaningful error bars are attributed. Finally, a seventh station is incorporated by means of its station correction alone, without needing to update the attenuation curve.  相似文献   

8.
We analyse spectral ratio of the coda of doublets of microearthquakes. Our purpose is to find evidence for temporal changes of the attenuation in the crust before a large magnitude earthquake. A Moving Window Cross Spectral analysis of the coda of doublets gives a plot of the spectral ratio as a function of lapse time along the seismogram, for several frequency bands (SR(T, f) plot). From a certain pattern in theSR(T, f) plot, we should infer a temporal change in coda attenuation. Several doublets recorded in Central California by the USGS network are analysed.Using events very close in time from one another, we show that the radiation pattern can be different enough to induce important variations in the spectral ratio of the first arrivals and of the coda.Another doublet exhibits a strong variation of the low frequencies for stations in the region of Hollister (California), wherePhillips andAki (1986) have noted a strong amplification of low frequencies, that they attribute to site effects on unconsolidated sediments in the fault zone. These variations could be related to slight changes in local conditions (creep, or water table).On the other hand, some doublets, in the vicinity and close in time to the August 1979 Coyote Lake Earthquake (M=5.9), show no variation inSR(T, f) related to this earthquake: this proves that there was no major change in attenuation in the crust preceding this large shock. If a change occurred, it should have been confined to a very limited region, which was not sampled by the many paths we studied.The employed method probably provides today the most accurate estimation of spectral ratio in the coda of microearthquakes. It shows that there are numerous kinds of variations and that it is not straightforward to relate them to coda attenuation changes only. Also, we must be circumspect when dealing with the coda of microearthquakes in relation to forecasting earthquakes.  相似文献   

9.
The paper presents the current state of magnitude estimation for Koyna earthquakes exceeding magnitude 3.0. We estimate coda duration magnitude from analogue seismograms recorded on the short period vertical (SPZ) seismometer at Hyderabad seismic observatory HYB and determine moment magnitude using very broad-band (VBB) data from the Geoscope station (HYB)and short period digital data from the local seismic network of NationalGeophysical Research Institute (NGRI) around the Koyna and Warna reservoirs.Firstly, the seismograms of 97 Koyna earthquakes exceeding magnitude 4.0 havebeen used to establish a new empirical coda duration magnitude scale which includes the higher order terms of log10, where is the coda length in seconds. Four background noise levels (1, 2, 6 and 10 mm) areconsidered to estimate the coda duration. We found that the duration magnitudes for 1 mm background level are more stable than those for 2, 6 and 10 mm. The new coda duration magnitude (Mdnew) scale, for 1 mmlevel, is:Mdnew = –0.594 + 2.04 log10 – 0.0435 (log10)2The estimated Mdnew are compatible with the reported MS values of the NGRI observatory and the mb values of the United States Geological Survey (USGS). These magnitudes can be obtained within the standard deviation of ± 0.26 units of MS (NGRI). A new relatively homogeneous catalog for Koyna earthquakes of Mdnew 4.0 is obtained. The momentmagnitudes for 34 Koyna-Warna events of Mdnew ranging from 3.0 to 5.4 have been estimated using two techniques. The first utilizes amplitudes of band-pass filtered (between 15 and 30 sec) velocity traces of moderate Koyna-Warna earthquakes of MW} 4.4 to 5.4, we abbreviate the magnitude using MA. The second is based on the S-wave spectrum of short period seismograms of local earthquakes (MW < 3.8). Moment magnitudes estimated by spectral analysis mainly depend on the estimation of event's long-period spectral level and appears to saturate for moderate Koyna-Warnaearthquakes (MW > 3.8). We recommend the use of both techniques whenever possible. The estimated moment magnitudes and Mdnew show an almost linear relationship with a standard deviation of ± 0.05.  相似文献   

10.
For short-period near-earthquake records in eastern China, from the empirical attenuation formula of coda ground motion amplitudeA with timeτ: lgA=G−2. 235 lgτ, using the single scattering theory modified with epicentral distance, we obtain the curve family of corrected coda amplitudeA c(r,t), andω/2Q c values for each time interval of coda. From this,Q c(f,h) values, which correspond to each observational average frequency and sampling depth, are calculated. The results substantially agree with those observationalQ c values in Yunnan, Beijing and central Asia.  相似文献   

11.
地方震尾波由地壳横向不均匀性而产生的反向散射波组成。从这一观点出发,根据尾波随掠过时间的衰减特性,结合地震矩对数和地方震里克特地震级的线性关系,导出利用任一掠过时间的震尾来计算的尾波震级Mc公式。它的简化形式可以和持续时间震级的表达式近似一致。尾波震级可作为持续时间震级的一种广义形式,它是直接从震源地震矩导出的震级标度,从而为解释持续时间震级物理基础提供了可能的途径。应用于丹江地震台的资料,得到丹江口及邻区的尾波品质因子和介质函数以及地震矩对数和震级的期望关系,同时得到实用于该台的持续时间震级和简化尾波震级公式。  相似文献   

12.
Quality factor Q, which describes the attenuation of seismic waves with distance, was determined for South Africa using data recorded by the South African National Seismograph Network. Because of an objective paucity of seismicity in South Africa and modernisation of the seismograph network only in 2007, I carried out a coda wave decay analysis on only 13 tectonic earthquakes and 7 mine-related events for the magnitude range 3.6?≤?M L ?≤?4.4. Up to five seismograph stations were utilised to determine Q c for frequencies at 2, 4, 8 and 16 Hz resulting in 84 individual measurements. The constants Q 0 and α were determined for the attenuation relation Q c(f)?=?Q 0 f α . The result was Q 0?=?396?±?29 and α?=?0.72?±?0.04 for a lapse time of 1.9*(t s???t 0) (time from origin time t 0 to the start of coda analysis window is 1.9 times the S-travel time, t s) and a coda window length of 80 s. This lapse time and coda window length were found to fit the most individual frequencies for a signal-to-noise ratio of at least 3 and a minimum absolute correlation coefficient for the envelope of 0.5. For a positive correlation coefficient, the envelope amplitude increases with time and Q c was not calculated. The derived Q c was verified using the spectral ratio method on a smaller data set consisting of nine earthquakes and one mine-related event recorded by up to four seismograph stations. Since the spectral ratio method requires absolute amplitudes in its calculations, site response tests were performed to select four appropriate stations without soil amplification and/or signal distortion. The result obtained for Q S was Q 0?=?391?±?130 and α?=?0.60?±?0.16, which agrees well with the coda Q c result.  相似文献   

13.
Characterization of the microseismic activity (M L <2.0) has been performed at Mt. Melbourne since 1990. We recorded a group of low frequency events with common morphological characteristics, i.e., an emerging onset, an unclear second phase and a sharply dropping coda. Spectral analysis of events recorded at more than one station indicates that the seismogram characteristics and spectral content are largely due to source effects. A polarization filter applied to a set of three component data revealed a first phase made up ofP waves followed (after about 0.9–1.4 sec.) by a second phase probably composed ofSH-type waves. Particle motion analysis detected a seismic ray angle direction mainly between N70°E and N110°E and apparent angle of incidence between 35° and 48° for the first phase. The studied seismicity was localized in an area on the eastern slope of Mt. Melbourne Volcano which presents a surface temperature anomaly (Mazzarini andSalvini, 1994). We formulate two hypotheses for the type of earthquakes recorded: 1) long-period events involving active presence of magmatic fluids in the source processes; 2) or the result of fracturing processes (shear?) in a medium characterized by transition between brittle and plastic behaviors. In the latter hypothesis the superficial thermal anomaly may be a symptom of this behavior at depth and is confirmed by the lown values observed for the exponential fit in the codaQ analysis.  相似文献   

14.
本文根据Aki等人提出的尾波理论,导出了地方震尾波水平分量与垂直分量的持续时间比的具体表达式: τ_H/τ_V=I_H/I_V(Q_H/Q_V)~(1/4)·B_H/B_V。该式表明,τ_H/τ_V的变化主要反映了震源体一定范围内,由于介质的各向异性而引起的尾波在不同方向上的激发及衰减能力的差异。本文还讨论了地震前τ_H/τ_V短临异常的物理机制,认为异常的产生与孕震期间介质内裂隙的出现和闭合有关。1986年门源6.4级地震和1975年海城7.3级地震前,τ_H/τ_V都有不同程度的短期低值异常及临震高值突跳。门源地震前后门源台记录的直达S波的最大振幅比A_(mH)/A_(mv)也有与尾波持续时间比类似的异常。最后通过对一些震例的分析,初步得到震级与异常时间的关系为M=0.657lnT+3.44。  相似文献   

15.
Coda wave data from California microearthquakes were studied in order to delineate regional fluctuations of apparent crustal attenuation in the band 1.5 to 24 Hz. Apparent attenuation was estimated using a single back scattering model of coda waves. The coda wave data were restricted to 30 s following the origin time; this insures that crustal effects dominate the results as the backscattered shear waves thought to form the coda would not have had time to penetrate much deeper. Results indicate a strong variation in apparent crustal attenuation at high frequencies between the Franciscan and Salinian regions of central California and the Long Valley area of the Sierra Nevada. Although the codaQ measurements coincide at 1.5 Hz (Q c =100), at 24 Hz there is a factor of four difference between the measurements made in Franciscan (Q c =525) and Long Valley (Q c =2100) with the Salinian midway between (Q c =900). These are extremely large variations compared to measures of seismic velocities of comparable resolution, demonstrating the exceptional sensitivity of the high frequency codaQ measurement to regional geology. In addition, the frequency trend of the results is opposite to that seen in a compilation of codaQ measurements made worldwide by other authors which tend to converge at high and diverge at low frequencies, however, the worldwide results generally were obtained without limiting the coda lengths and probably reflect upper mantle rather than crustal properties. Our results match those expected due to scattering in random media represented by Von Karman autocorrelation functions of orders 1/2 to 1/3. The Von Karman medium of order 1/3 corresponding to the Franciscan codaQ measurement contains greater amounts of high wavenumber fluctuations. This indicates relatively large medium fluctuations with wavelengths on the order of 100 m in the highly deformed crust associated with the Franciscan, however, the influence of scattering on the codaQ measurement is currently a matter of controversy.  相似文献   

16.
3He/4He ratios in lavas erupted during the last 360 years at Mt. Vesuvius are between 2.2 and 2.7 RA (RA = atmospheric ratio of 1.39 × 10−6), and are among the lowest values measured in young volcanic rocks. They are also identical to values measured in summit crater fumaroles sampled during 1987–1991. This agreement indicates that the 3He/4He ratio in the crater fumaroles faithfully tracks the magmatic value. The relatively low and uniform 3He/4He ratio in the lavas reflects either a mantle source enriched in (U + Th)/3He, or a mixture of magmatic and crustal components.  相似文献   

17.
Crustal attenuation for Jamaica, West Indies   总被引：1，自引：0，他引：1  
The S and coda wave spectra of small earthquakes on the island of Jamaica were used to determine the near surface and coda Q attenuation, and Q c,respectively. Q c determined by the single-station method was found in the range of 1 to 10 Hertz to be given by the relationship, Q c= 60 ± 5f 0.87±0.05. This suggests that the Jamaican crust is highly attenuating which is further supported by the observation of rapid intensity fall-off with distance for earthquakes that have affected the island in the past. , determined from S-wave spectra with short travel times was found to be 0.058 ± 0.012 on the central crustal block, which makes up nearly two-thirds of the island, and 0.080 ± 0.014 in surrounding belt sub-regions. The pattern of values seems to fit with the surface geology in that the central block has areas of exposed outcrops of older and harder rock than the belts, which are characterized by thicker sedimentary sequences as well as intense fracturing and faulting.Atkinson and Boore (1998) and Atkinson(2001) presented an alternative method to stochastic modelling for ground motion in Eastern North America, whereby California attenuation relationships were modified to account for crustal differences invelocity-depth profile, Q and between both regions. Following their example, the California spectral attenuation relation of Boore, Joyner and Fumal (1997) was modified to account for differences between the California and Jamaica crust, resulting in an attenuation relation that is deemed to be more appropriate for Jamaica. Spectral accelerations for Jamaica when compared to California, are especially reduced beyond 20 km from the source and at high frequencies, f 1 hertz.The study concludes that the Jamaican crust, although having an oceanic composition is highly attenuating, which may be a result of intensive tectonic processes, whereas is consistent with near-rock conditions on the central block and soft rock conditions elsewhere on the island.  相似文献   

18.
中强地震前地震波参数异常的研究   总被引：2，自引：0，他引：2  
本文研究了1988年5月26日库尔勒M_35.5地震和1991年2月25日柯坪M_s6.5地震前后的地震波初动符号、振幅比、尾波持续时间比和尾波衰减系数的变化特征,发现两次地震前上述参数均具有不同程度的异常。地震前(?)波初动一致性越强,持续时间越长,未来地震震级越大;振幅比突跳是即将发生地震的明显信号;首先出现尾波衰减系数异常的台站距未来地震震中较近。根据上述参数的异常特征,成功地预报了1991年6月6日和静M_s5.2地震。  相似文献   

19.
For short-period near-earthquake records in eastern China, from the empirical attenuation formula of coda ground motion amplitudeA with timeτ: lgA=G?2. 235 lgτ, using the single scattering theory modified with epicentral distance, we obtain the curve family of corrected coda amplitudeA c(r,t), andω/2Q c values for each time interval of coda. From this,Q c(f,h) values, which correspond to each observational average frequency and sampling depth, are calculated. The results substantially agree with those observationalQ c values in Yunnan, Beijing and central Asia.  相似文献   

20.
Estimation of seismic wave attenuation in the shallow crust in terms of coda wave Q structure previously investigated in the vicinity of Cairo Metropolitan Area was improved using seismograms of local earthquakes recorded by the Egyptian National Seismic Network. The seismic wave attenuation was measured from the time decay of coda wave amplitudes on narrow bandpass filtered seismograms based on the single scattering theory. The frequency bands of interest are from 1.5 to 18 Hz. In general, the values obtained for various events recorded at El-Fayoum and Wadi Hagul stations are very similar for all frequency bands. A regional attenuation law Q c = 85.66 f 0.79 was obtained.  相似文献   

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