中强地震前地震波参数异常的研究

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。     

地方震尾波震级的初步研究

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。     

Regional characteristics of coda attenuation after the lancang-Gengma earthquake

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/β_m=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.     

Improved seismic discrimination using pattern recognition

The problem of discriminating between earthquakes and underground nuclear explosions is formulated as a problem in pattern recognition. As such it may be separated into two stages, feature extraction and classification. The short-period (SP) features consist of m_b and autoregressive parameters characterising the preceding noise, signal and coda. The long-period (LP) features consist of LP power spectral estimates taken within various group velocity windows. Contrary to common usage we have extracted features from horizontal Rayleigh waves and Love waves as well as vertical Rayleigh waves. The classification is performed by approximating the statistical distribution of earthquake and explosion feature vectors by multivariate normal distributions.The method has been tested on a data base containing 52 explosions and 73 earthquakes from Eurasia recorded at NORSAR between 1971 and 1975. Several of these events are difficult on the m_b : M_s diagram [m_b(PDE) and M_s (NORSAR) have been used]. The data set was divided into a learning and an independent data set. All of the events both from the learning data set and the independent data set were correctly classified using the new procedures. Furthermore, the increase in separation as compared to the m_b : M_s discriminant is significant.     

Rediscovering Signal Complexity as a Teleseismic Discriminant

We re-examine the utility of teleseismic seismic complexity discriminants in a multivariate setting using United Kingdom array data. We measure a complexity discriminant taken on array beams by simply taking the logarithm of the ratio of the P-wave coda signal to that of the first arriving direct P wave (β_CF). The single station complexity discriminant shows marginal performance with shallow earthquakes having more complex signatures than those from explosions or deep earthquakes. Inclusion of secondary phases in the coda window can also degrade performance. However, performance improves markedly when two-station complexity discriminants are formed showing false alarm rates similar to those observed for network m_b − M_s. This suggests that multistation complexity discriminants may ameliorate some of the problems associated with m_b – M_s discrimination at lower magnitudes. Additionally, when complexity discriminants are combined with m_b – M_s there is a tendency for explosions, shallow earthquakes and deep earthquakes to form three distinct populations. Thus, complexity discriminants may follow a logic that is similar to m_b – M_s in terms of the separation of shallow earthquakes from nuclear explosions, although the underlying physics of the two discriminants is significantly different.     

Magnitude estimation of Koyna-Warna earthquakes, India

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 (M_dnew) scale, for 1 mmlevel, is:M_dnew = –0.594 + 2.04 log₁₀ – 0.0435 (log₁₀)²The estimated M_dnew are compatible with the reported M_S values of the NGRI observatory and the m_b values of the United States Geological Survey (USGS). These magnitudes can be obtained within the standard deviation of ± 0.26 units of M_S (NGRI). A new relatively homogeneous catalog for Koyna earthquakes of M_dnew 4.0 is obtained. The momentmagnitudes for 34 Koyna-Warna events of M_dnew 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 M_W} 4.4 to 5.4, we abbreviate the magnitude using M_A. The second is based on the S-wave spectrum of short period seismograms of local earthquakes (M_W < 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 (M_W > 3.8). We recommend the use of both techniques whenever possible. The estimated moment magnitudes and M_dnew show an almost linear relationship with a standard deviation of ± 0.05.     

Magnitudes of large shallow earthquakes from 1904 to 1980

In order to obtain a uniform magnitude catalogue, surface-wave magnitudes M_s and broad-band body-wave magnitudes m_B have been determined for large shallow earthquakes from 1904 to 1980. In making the catalogue homogeneous, the author consistently adheres to the original definitions of M_s and m_B given by Gutenberg (1945) and Gutenberg and Richter (1956). The determinations of M_s and m_B are all based on the amplitude and period data listed in Gutenberg and Richter's unpublished notes, bulletins from stations worldwide, and other basic information. m_B is measured on broad-band instruments in periods of ～8 s. Consistency of the magnitude determinations from these different sources is carefully checked in detail. More than 900 shallow shocks of magnitude 7 and over are catalogued. The meaning of the magnitude scales in various catalogues is examined in terms of M_s and m_B. Most of the magnitudes listed by Gutenberg and Richter (1954) in their “Seismicity of the Earth” are basically M_s for large shocks shallower than 40 km, but are basically m_B for large shocks at depths of 40–60 km. The surface-wave magnitudes given by “Earthquake Data Reports” are higher than M_s by 0.2 unit whether the combined horizontal amplitude or the vertical amplitude is used. m_B and the currently used 1 s body-wave magnitude are measured at different periods and should not be directly compared.     

Research on the regionalQ values of coda waves for the aftershocks in Luquan,Yunnan of 1985

Based on the scattering coda model by which local and regional earthquakes are interpreted (K. Aki, 1969), and using observational coda data of 68 aftershocks of the 1985 Luquan, Yunnan earthquake registered by the VGK seismographs installed at 12 stations in the Yunnan regional short-period network, theQ-values of coda waves are calculated respectively for 6 time intervals. It is observed that within the frequency range of 0.40–1.65 Hz of the observed data, theQ-values are closely related with the frequencies and the calculated codaQ ranges between 80–240 with the coefficient of frequency dependence η=0.45. The calculated source factorsB(f> _p) of the coda waves which indicate the scattering strength are mostly within the order 10^?23–10^?24. Areas with lowQ-values present high scattering. It should be noted that by comparing data obtained before and after the Luquan earthquake, clear changes can be detected in theQ-values measured at stations close to the epicentral region, and that theQ-values of the aftershock coda are less than about one half of the pre-shock values. It may be mentioned that the time-dependent regional variations of theQ-values might possibly bring about practical significance in earthquake prediction. Moreover, aftershock focal parameters are determined. Through discussions on the quantitative relations between the focal parameters, we get: 1gE=1.59M _L+ 11.335;E=(2.10 × 10^?5)M ₀; length of focal rupturea=0.40?0.80 km for 3.0≤M _L<5.0 events; stress drop Δσ=(6.0–130) ×10⁵ Pa. Through interpretation of the data, we have also learned the important characteristics that there is no linear relation between the stress drops and the earthquake magnitudes.     

A slow earthquake

Anomalous earthquakes such as creep events, tsunami earthquakes and silent earthquakes have been reported in the recent literature. In this paper we discuss an anomalous “slow earthquake” that occurred on June 6, 1960 in southern Chile. Although the surface-wave magnitude of this event is only 6.9, it excited anomalously large long-period multiple surface waves with a seismic moment of 5.6 · 10²⁷ dyn cm. The Benioff long-period seismogram of this earthquake recorded at Pasadena shows an extremely long, about 1.5–2 h coda of Rayleigh waves, with a period of 10–25 s. The coda length for other events with a comparable magnitude which occurred in the same region is about 10 min. This observation suggests that the long coda length is due to a long source rupture process which lasted at least 1 h. Although at least 15 distinct events can be identified in the coda, no short-period body waves were recorded corresponding to these, except for the first one. These results suggest that a relatively small (M_s ? 6.9) earthquake triggered a series of slow events; the duration of the whole sequence being longer than 1 h. This event probably occurred on a transform fault on the extension of the Chile Rise and provides important information regarding the nature of the transform fault.     

Quantification relations between the sourc eparameters

The various useful source-parameter relations between seismic moment and common use magnitude lg(M ₀) andM _s,M _L,m _b; between magnitudesMs andM _L,M _s andm _b,M _L andm _b; and between magnitudeM _s and lg(L) (fault length), lg (W) (fault width), lg(S) (fault area), lg(D) (average dislocation);M _L and lg(f _c) (corner frequency) have been derived from the scaling law which is based on an “average” two-dimensional faulting model of a rectangular fault. A set of source-parameters can be estimated from only one magnitude by using these relations. The average rupture velocity of the faultV _r=2.65 km/s, the total time of ruptureT(s)=0.35L (km) and the average dislocation slip rateD=11.4 m/s are also obtained. There are four strong points to measure earthquake size with the seismic moment magnitudeM _w.

1. The seismic moment magnitude shows the strain and rupture size. It is the best scale for the measurement of earthquake size.
2. It is a quantity of absolute mechanics, and has clear physical meaning. Any size of earthquake can be measured. There is no saturation. It can be used to quantify both shallow and deep earthquakes on the basis of the waves radiated.
3. It can link up the previous magnitude scales.
4. It is a uniform scale of measurement of earthquake size. It is suitable for statistics covering a broad range of magnitudes. So the seismic moment magnitude is a promising magnitude and worth popularization.

     

Theoretical and Observed Depth Correction for M s

—?Modal summation technique is used to generate 5000, three-component theoretical seismograms of Love and Rayleigh waves, assuming modified PREM (PREM-C) and AK135F global earth models. The focal depth h and the geometrical fault parameters are randomly chosen so as to uniformly cover possible source mechanisms and obtain uniform distribution of log h in the interval 1?h?h?M _s of the form:¶ΔM _s (h)=0 forh< 20km, ΔM _s (h)=0.314log(h)-0.409 for 20≠h< 60km, ΔM _s (h)=1.351log(h)-2.253 for 60≠h< 100km, ΔM _s (h)=0.400log(h)-0.350 for 100≠h< 600km .¶After applying the above correction, the relationship between the surface wave magnitude and the scalar seismic moment for the observational data set significantly improves, and becomes independent of the source depth. In relation to CTBT, no depth correction is needed for M _S when the m _b ???M _S discriminant is computed, because the proposed correction is zero for earthquakes with foci above 20?km.     

Identification Criteria for Sources of T Waves Recorded in French Polynesia

—?From a data set of 150 digital records of T phases from 71 sources obtained on seismometers of the Polynesian Seismic Network, we define a discriminant separating earthquake and explosion sources, which uses the maximum amplitude of recorded ground velocity, measured on its envelope, e _Max (in μm/s), and the duration of the phase measured at 1/3 of maximum amplitude, τ_1/3 (in seconds). Earthquake sources and man-made explosions are effectively separated in a log-log space by the straight line ¶¶log₁₀ e _Max = 4.9 log₁₀τ_1/3 - 4.1 .¶¶Other criteria in both the time and frequency domains fail to reliably separate the populations of the various kinds of events. The application of this technique to analog records of large-scale man-made explosions carried out in the 1960s confirms that it provides an adequate discriminant over 3.5 orders of magnitude of ground velocity.     

Coda Q Estimates in the Koyna Region, India

—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.     

Attenuation characteristics of coda waves and estimation ofQc values in eastern China

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.     

Regional Magnitude Scaling, Transportability, and Ms:mb Discrimination at Small Magnitudes

—?Data sets of m _b (Pn) and m _b (Lg) measurements are presented for three continental regions in order to investigate scaling relationships with moment magnitude M _w and event discrimination at small magnitudes. Compilations of published measurements are provided for eastern North American and central Asian earthquakes, and new measurements are reported for earthquakes located in western United States. Statistical tests on M _w :m _b relationships show that the m _b (Lg) scale of Nuttli (1973) is transportable between tectonic regions, and a single, unified M _w :m _b (Lg) relationship satisfies observations for M _w ～4.2–6.5 in all regions. A unified relationship is also developed for nuclear explosions detonated at the Nevada Test Site and test sites of the former Soviet Union. Regional m _b for explosions scale at higher rates than for earthquakes, and of significance is the finding that m _b (Pn) for explosions scales at a higher rate than m _b (Lg). A model is proposed where differences in scaling rates are related to effects of spectral overshoot and near-field Rg scattering on the generation of Pn and Lg waves by explosions. For earthquakes, m _b (Pn) and m _b (Lg) scale similarly, showing rates near 1.0 or 2/3?·?log₁₀ M _o (seismic moment).¶M _w :m _b (Lg) scaling results are converted to unified M _s :m _b (Lg) relationships using scaling laws between log M _o and M _s . For earthquakes with M _s greater than 3.0, the scaling rate is 0.69?·?M _s , which is the same as it is for nuclear explosions if M _s is proportional to 1.12?·?log M _o, as determined by NTS observations. Thus, earthquake and explosion populations are parallel and separated by 0.68 m _b units for large events. For small events (M _s ?M _s :m _b (Lg) plots for stable and tectonic regions, respectively. While the scaling rate for explosions is ～0.69, this value is uncertain due to paucity of M _o observations at small yields. Measurements of [m _b (P)???m _b (Lg)] for earthquakes in the western United States have an average value of ?0.33?±?.03 m _b units, in good agreement with Nuttli's estimate of m _b bias for NTS. This result suggests that Nuttli's method for estimating test site bias can be extended to earthquakes to make estimates of bias on regional scales. In addition, a new approach for quick assessments of regional bias is proposed where M _s :m _b (P) observations are compared with M _s :m _b (Lg) relationships. Catalog M _s :m _b (P) data suggest that m _b bias is significant for tectonic regions of southern Asia, averaging about ?0.4 m _b units.     

Application of Network-averaged Teleseismic P-wave Spectra to Seismic Yield Estimation of Underground Nuclear Explosions

—?A set of procedures is described for estimating network-averaged teleseismic P-wave spectra for underground nuclear explosions and for analytically inverting these spectra to obtain estimates of m _b /yield relations and individual yields for explosions at previously uncalibrated test sites. These procedures are then applied to the analyses of explosions at the former Soviet test sites at Shagan River, Degelen Mountain, Novaya Zemlya and Azgir, as well as at the French Sahara, U.S. Amchitka and Chinese Lop Nor test sites. It is demonstrated that the resulting seismic estimates of explosion yield and m _b /yield relations are remarkably consistent with a variety of other available information for a number of these test sites. These results lead us to conclude that the network-averaged teleseismic P-wave spectra provide considerably more diagnostic information regarding the explosion seismic source than do the corresponding narrowband magnitude measures such as m _b , M _s and m _b (L _g ), and, therefore, that they are to be preferred for applications to seismic yield estimation for explosions at previously uncalibrated test sites.     

Earthquake parameters and spatial distribution of coseismic effects in Southern Siberia and Mongolia

In this work we review earthquakes that happened in Southern Siberia and Mongolia within the coordinates of 42°–62° N and 80°–124° E and first propose relationships between earthquake parameters (a surface-wave earthquake magnitude M _s and an epicentral intensity(I ₀) based on the MSK-64 scale) and maximal distances from an earthquake epicenter (R _{e max}), hypocenter (R _{h max}), and a seismogenic fault (R _{f max}) to the localities of secondary coseismic effects. Special attention was paid to the study of these relationships for the effects of soil liquefaction. Hence, it was shown that secondary deformations from an earthquake were distributed in space away from an earthquake epicenter, than from an associating seismogenic fault. The effects of soil liquefaction are manifested by several times closer to a seismogenic fault, than all other effects, regardless of the type of tectonic movement in a seismic focus. Within the 40 km zone from an earthquake epicenter 44% of the known manifestations of liquefaction process occurred; within the 40 km zone from a seismogenic fault—90%. We propose the next relationship for effects of soil liquefaction: M _s = 0.007 × R _{e max} + 5.168 that increases the limits of the maximum epicentral distance at an earthquake magnitude of 5.2 ≤ M _s ≤ 8.1 as compared to the corresponding relationships for different regions of the world.     

Physical size of tsunamigenic earthquakes of the northwestern Pacific

The new scale M_t of tsunami magnitude is a reliable measure of the seismic moment of a tsunamigenic earthquake as well as the overall strength of a tsunami source. This M_t scale was originally defined by Abe (1979) in terms of maximum tsunami amplitudes at large distances from the source. A method is developed whereby it is possible to determine M_t at small distances on the basis of the regional tsunami data obtained at 30 tide stations in Japan. The relation between log H, maximum amplitude (m) and log Δ, a distance of not less than 100 km away from the source (km) is found to be linear, with a slope close to 1.0. Using three tsunamigenic earthquakes with known moment magnitudes M_w, for calibration, the relation, M_t = log H + log Δ + D, is obtained, where D is 5.80 for single-amplitude (crest or trough) data and 5.55 for double-amplitude (crest-to-trough) data. Using a number of tsunami amplitude data, M_t is assigned to 80 tsunamigenic earthquakes that occurred in the northwestern Pacific, mostly in Japan, during the period from 1894 to 1981. The M_t values are found to be essentially equivalent to M_w for 25 events with known M_w. The 1952 Kamchatka earthquake has the largest M_t, 9.0. Of all the 80 events listed, at least seven unusual earthquakes which generated disproportionately-large tsunamis for their surface-wave magnitude M_s are identified from the relation. From the viewpoint of tsunami hazard reduction, the present results provide a quantitative basis for predicting maximum tsunami amplitudes at a particular site.     

The empirical relationships between M s, m b and M L for China and vicinity

Data from 753 earthquakes are used to determine a relationship between surface-wave magnitude (M _s) and bodywave magnitude (m _b), and from 541 earthquakes to determine a relationship between surface-wave magnitude (M _s) and local magnitude (M _L) for China and vicinity: M _s=0.9883 m _b-0.0420, M _s=0.9919 M _L-0.1773. The relationship of M _s versus m _b is obtained for 292 events occurred in the Chinese mainland in the time period from 1964 to 1996, 291 events occurred in Taiwan in the time period from 1964 to 1995 and 170 events occurred in the surrounding area. Standard deviation of the fitting is 0.445. Relationship of M _s versus M _L is obtained for 36 events occurred in the Chinese mainland, 293 events occurred in Taiwan, China and 212 events occurred in the surrounding area. The total amount is 541 events. Standard deviation of the fitting is 0.4673. The uncertainties of the converted M _s in different magnitude intervals can be estimated using complementary cumulative distribution function (CCDF). In the relationship of M _s versus m _b, taking ±0.25 as a range of uncertainties, in magnitude interval m _b 4.0–4.9, the probabilities for the converted M _s taken value less than (M _s-0.25) and more than (M _s+0.25) are 17% and 27% respectively. Similarly, we have probabilities for m _b 5.0–5.9 are 34% and 20% and that for m _b 6.0–6.9 are 11% and 47%. In the relationship of M _s versus M _L, if the range of uncertainties is still taken as ±0.25, the corresponding probabilities for magnitude interval M _L 4.0–4.9 are 22% and 38%, for M _L 5.0–5.9 are 20% and 15% and for magnitude interval M _L 6.0–6.9, are 15% and 29%, respectively. The relationships developed in this paper can be used for the conversion of one magnitude scale into another magnitude scales conveniently. The estimation of uncertainties described in this paper is more accurate and more objective than the usual estimation expressed by deviation. The estimations described in this paper indicate various dispersions in different magnitude intervals of original data. The estimations of uncertainties described by probabilities can be well connected with the total estimations of uncertainties in seismic hazard assessment.