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1.
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.
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2.
Uncertainties in the estimation of earthquake magnitudes in Greece   总被引：1，自引：0，他引：1  
Instrumental magnitudes in Greece have been reported as: a) Mmagnitudes based on the records of the Wiechert or Mainka seismographs,b) MLGR magnitudes based on the records of the Wood-Anderson(WA) seismographs (To = 0.8 sec, Veffective 1000) or othershort period seismographs calibrated against WA records and,c) MLSM magnitudes based on strong motion records(accelerograms). Comparison of such magnitudes with momentmagnitudes, Mw, for 329 earthquakes, with epicenters in thebroader Aegean area, performed in this study, showedthat M, MLGR+0.5 and MLSM are practically equalto Mw, with a small overall standard error ( = 0.23).Therefore, equivalent moment magnitudes, Mw *,estimated from these magnitudes and reported in the catalogues of theGeophysical Laboratory of the University of Thessaloniki are equal tomoment magnitudes for all practical purposes with reasonable uncertainties.It has been further shown that surface wave magnitudes, Ms,for Ms <6.0, can be also transferred into momentmagnitudes, Mw *, but the larger uncertaintiesencountered make its use rather problematic.  相似文献   

3.
Teleseismic observations of explosions tend to be richer in short-period energy than are earthquakes, thus the effectiveness of them b M s discriminant. At regional distances the same basic separation occurs for smaller events in terms ofM L M 0 (Woods et al., 1993) andm b M 0 (Patton andWalter, 1993). While these studies demonstrate the basic differences in excitation, they suffer in practical application because of the detailed information required in the retrieval ofM 0 . In this paper, we introduce a new method of discrimination, based on the energy strength (M E ) from broadband regional records that appears to be effective and efficient. In this method all events are processed as earthquakes, and explosions are distinguished by their stronger energy levels relative to their long-period amplitudes. Results from 29 events recorded by TERRAscope, sampling 15 explosions from NTS and 14 earthquakes from the southwestern United States, are represented, indicating complete separation (45 data points).M L =3.6 is the smallest event examined to date but the method can probably be extended to even smaller levels in calibrated regions.  相似文献   

4.
In the Solomon Islands and New Britain subduction zones, the largest earthquakes commonly occur as pairs with small separation in time, space and magnitude. This doublet behavior has been attributed to a pattern of fault plane heterogeneity consisting of closely spaced asperities such that the failure of one asperity triggers slip in adjacent asperities. We analyzed body waves of the January 31, 1974,M w =7.3, February 1, 1974,M w =7.4, July 20, 1975 (1437)M w =7.6 and July 20, 1975 (1945),M w =7.3 doublet events using an iterative, multiple station inversion technique to determine the spatio-temporal distribution of seismic moment release associated with these events. Although the 1974 doublet has smaller body wave moments than the 1975 events, their source histories are more complicated, lasting over 40 seconds and consisting of several subevents located near the epicentral regions. The second 1975 event is well modeled by a simple point source initiating at a depth of 15 km and rupturing an approximate 20 km region about the epicenter. The source history of the first 1975 event reveals a westerly propagating rupture, extending about 50 km from its hypocenter at a depth of 25 km. The asperities of the 1975 events are of comparable size and do not overlap one another, consistent with the asperity triggering hypothesis. The relatively large source areas and small seismic moments of the 1974 doublet events indicate failure of weaker portions of the fault plane in their epicentral regions. Variations in the roughness of the bathymetry of the subducting plate, accompanying subduction of the Woodlark Rise, may be responsible for changes in the mechanical properties of the plate interface.To understand how variations in fault plane coupling and strength affect the interplate seismicity pattern, we relocated 85 underthrusting earthquakes in the northern Solomon Islands Are since 1964. Relatively few smaller magnitude underthrusting events overlap the Solomon Islands doublet asperity regions, where fault coupling and strength are inferred to be the greatest. However, these asperity regions have been the sites of several previous earthquakes withM s 7.0. The source regions of the 1974 doublet events, which we infer to be mechanically weak, contain many smaller magnitude events but have not generated any otherM s 7.0 earthquakes in the historic past. The central portion of the northern Solomon Islands Arc between the two largest doublet events in 1971 (studied in detail bySchwartz et al., 1989a) and 1975 contains the greatest number of smaller magnitude underthrusting earthquakes. The location of this small region sandwiched between two strongly coupled portions of the plate interface suggest that it may be the site of the next large northern Solomon Islands earthquake. However, this region has experienced no known earthquakes withM s 7.0 and may represent a relatively aseismic portion of the subduction zone.  相似文献   

5.
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6.
The distribution of the focal mechanisms of the shallow and intermediate depth (h>40 km) earthquakes of the Aegean and the surrounding area is discussed. The data consist of all events of the period 1963–1986 for the shallow, and 1961–1985 for the intermediate depth earthquakes, withM s 5.5. For this purpose, all published fault plane solutions for each event have been collected, reproduced, carefully checked and if possible improved accordingly. The distribution of the focal mechanisms of the earthquakes in the Aegean declares the existence of thrust faulting following the coastline of southern Yugoslavia, Albania and western Greece extending up to the island of Cephalonia. This zone of compression is due to the collision between two continental lithospheres (Apulian-Eurasian). The subduction of the African lithosphere under the Aegean results in the occurrence of thrust faulting along the convex side of the Hellenic arc. These two zones of compression are connected via strike-slip faulting observed at the area of Cephalonia island. TheP axis along the convex side of the arc keeps approximately the same strike throughout the arc (210° NNE-SSW) and plunges with a mean angle of 24° to southwest. The broad mainland of Greece as well as western Turkey are dominated by normal faulting with theT axis striking almost NS (with a trend of 174° for Greece and 180° for western Turkey). The intermediate depth seismicity is distributed into two segments of the Benioff zone. In the shallower part of the Benioff zone, which is found directly beneath the inner slope of the sedimentary arc of the Hellenic arc, earthquakes with depths in the range 40–100 km are distributed. The dip angle of the Benioff zone in this area is found equal to 23°. This part of the Benioff zone is coupled with the seismic zone of shallow earthquakes along the arc and it is here that the greatest earthquakes have been observed (M s 8.0). The deeper part (inner) of the Benioff zone, where the earthquakes with depths in the range 100–180 km are distributed, dips with a mean angle of 38° below the volcanic arc of southern Aegean.  相似文献   

7.
From the events synthesized from the one-dimensional dynamical mass-spring model proposed byBurridge andKnopoff (1967), the relation between rupture length and earthquake momentM is studied for various model parameters. The earthquake moment is defined to be the total displacement of a connected set of mass elements which slide during an event. A parameter stiffness ratios is defined as the ratio of the spring constant between the two mass elements to that between one mass element and the moving plate. The velocity-dependent friction law (including weakening and hardening processes) is taken to control the sliding of a mass element. The distribution of the breaking strengths over the system is considered to be a fractal function. The cases for severals values and different velocity-dependent friction laws with different decreasing ratesr w of the frictional force with sliding velocity are studied numerically. The weakening process of the frictional force from the static one to the dynamic one obviously affects theM– relation. Meanwhile, a rapid weakening process rather than a slow weakening process can result in aM– relation, which is comparable to the observed one. Although an increase in thes value can yield an increase in the upper bound of the value and the number of events with largeM and values, the scaling of theM– relation is not affected by the change of thes value. For the cases in this study, the theoretical –M relations for small events withM<1 are almost in the form: M 1/2, while those for large events withM>1 have a scaling exponent less than but close to 1. In addition, the fractal dimension, the friction drop ratio and the roughness of the distribution of the breaking strengths over the fault surface are the minor parameters influencing the –M relation. A comparison between the theoreticalM– relation and the observed one for strike-slip earthquakes shows that for large events the theoreticalM– relation is quite consistent with the observed one, while for small events there is a one-order difference in the two relations. For the one-dimensional model, the decreasing rate of the dynamic frictional force with velocity is the main factor in affecting the characteristic value of the earthquake moment, at which the scaling of theM– relation changes.  相似文献   

8.
We analyzed the broadband body waves of the 1992 Nicaragua earthquake to determine the nature of rupture. The rupture propagation was represented by the distribution of point sources with moment-rate functions at 9 grid points with uniform spacing of 20 km along the fault strike. The moment-rate functions were then parameterized, and the parameters were determined with the least squares method with some constraints. The centroid times of the individual moment-rate functions indicate slow and smooth rupture propagation at a velocity of 1.5 km/s toward NW and 1.0 km/s toward SE. Including a small initial break which precedes the main rupture by about 10 s, we obtained a total source duration of 110 s. The total seismic moment isM o =3.4×1020 Nm, which is consistent with the value determined from long-period surface waves,M o =3.7×1020 Nm. The average rise time of dislocation is determined to be 10 s. The major moment release occurred along a fault length of 160 km. With the assumption of a fault widthW=50 km, we obtained the dislocationD=1.3 m. From andD the dislocation velocity isD=D/0.1 m/s, significantly smaller than the typical value for ordinary earthquakes. The stress drop =1.1 MPa is also less than the typical value for subduction zone earthquakes by a factor of 2–3. On the other hand, the apparent stress defined by 2E s /M o , where andE s are respectively the rigidity and the seismic wave energy, is 0.037 MPa, more than an order of magnitude smaller than . The Nicaragua tsunami earthquake is characterized by the following three properties: 1) slow rupture propagation; 2) smooth rupture; 3) slow dislocation motion.  相似文献   

9.
S-wave spectral analysis is applied to 174 strong motion accelerationrecords to obtain the source parameters of 27 aftershocks(3.1 ML 4.3) of the May 13, 1995, Mw 6.6,Kozani-Grevena (NW Greece) earthquake. The data are derived from atemporary network, of three-component digital accelerographs, deployedwithin the strongly affected area some days after the mainshock occurrence.Site effects were evident in the strong motion records at 3 out of the 4stations used, and a correction was applied to account for theoverestimation of seismic moment due to amplification of thelow-frequency part of the spectrum. The data from this analysis arecomplimented with previously obtained source parameters for earthquakesin Greece, in order to study the applicability of the empirical scalingrelations used so far, towards smaller magnitudes. In general, a goodcorrelation was observed in most cases, validating the use of empiricalrelations that are applicable to the Aegean area. Empirical relations aredetermined between seismic moment and seismic slip, as well as, betweenseismic moment and stress drop, applicable to small magnitude earthquakes(ML < 4.3). Stress drop values were found to be relatively small,ranging from 2 to 41 bars, indicative of inter-plate environments. Thevalues of fc and of fmax were found in good agreement withrelations based on observations from larger worldwide earthquakes.  相似文献   

10.
The most complete and reliable data of strong (M s6.5), shallow (h<70 km) earthquakes which occurred in the inner Aegean seismic zone have been utilized to describe its seismicity time variation during 1800–1986 by two independent statistical models. The first is a sequentially stationary model of seismicity rates which shows that intervals of low seismicity rate, lasting for some 37 years, alternate with high rate intervals of 8–12 years duration. The second model is a statistical model according which seismic energy released within 5-year time windows approximates a harmonic curve within a period of about 50 years. This model is in agreement with the notion that the time series of strong earthquake occurrences in the inner Aegean seismic zone consists of a random (shocks withM s=6.5–6.8) and a nonrandom component (M s6.9). Maxima and minima of the harmonic curve coincide with the high and low rate intervals, respectively. A model of regional stationary accumulation of thermal stresses along certain seismic belts and their cyclic relaxation may explain this periodicity.  相似文献   

11.
We use 576 earthquakes of magnitude, M w, 3.3 to 6.8 that occurred within the region 33° N–42.5° N, 19° E–30° E in the time period 1969 to 2007 to investigate the stability of the relation between moment magnitude, M w, and local magnitude, M L, for earthquakes in Greece and the surrounding regions. We compare M w to M L as reported in the monthly bulletins of the National Observatory of Athens (NOA) and to M L as reported in the bulletins of the Seismological Station of the Aristotle University of Thessaloniki. All earthquakes have been analyzed through regional or teleseismic waveform inversion, to obtain M w, and have measured maximum trace amplitudes on the Wood–Anderson seismograph in Athens, which has been in operation since 1964. We show that the Athens Wood–Anderson seismograph performance has changed through time, affecting the computed by NOA M L by at least 0.1 magnitude units. Specifically, since the beginning of 1996, its east–west component has been recording systematically much larger amplitudes compared to the north–south component. From the comparison between M w and M L reported by Thessaloniki, we also show that the performance of the sensors has changed several times through time, affecting the calculated M L’s. We propose scaling relations to convert the M L values reported from the two centers to M w. The procedures followed here can be applied to other regions as well to examine the stability of magnitude calculations through time.  相似文献   

12.
Some comparisons between mining-induced and laboratory earthquakes   总被引：3，自引：0，他引：3  
Although laboratory stick-slip friction experiments have long been regarded as analogs to natural crustal earthquakes, the potential use of laboratory results for understanding the earthquake source mechanism has not been fully exploited because of essential difficulties in relating seismographic data to measurements made in the controlled laboratory environment. Mining-induced earthquakes, however, provide a means of calibrating the seismic data in terms of laboratory results because, in contrast to natural earthquakes, the causative forces as well as the hypocentral conditions are known. A comparison of stick-slip friction events in a large granite sample with mining-induced earthquakes in South Africa and Canada indicates both similarities and differences between the two phenomena. The physics of unstable fault slip appears to be largely the same for both types of events. For example, both laboratory and mining-induced earthquakes have very low seismic efficiencies where a is the apparent stress and is the average stress acting on the fault plane to cause slip; nearly all of the energy released by faulting is consumed in overcoming friction. In more detail, the mining-induced earthquakes differ from the laboratory events in the behavior of as a function of seismic momentM 0. Whereas for the laboratory events 0.06 independent ofM 0, depends quite strongly onM 0 for each set of induced earthquakes, with 0.06 serving, apparently, as an upper bound. It seems most likely that this observed scaling difference is due to variations in slip distribution over the fault plane. In the laboratory, a stick-slip event entails homogeneous slip over a fault of fixed area. For each set of induced earthquakes, the fault area appears to be approximately fixed but the slip is inhomogeneous due presumably to barriers (zones of no slip) distributed over the fault plane; at constant , larger events correspond to larger a as a consequence of fewer barriers to slip. If the inequality a / 0.06 has general validity, then measurements of a =µE a /M 0, where is the modulus of rigidity andE a is the seismically-radiated energy, can be used to infer the absolute level of deviatoric stress at the hypocenter.  相似文献   

13.
A re-assessment of the historic seismicity of the central sector of the Colombian Eastern Cordillera (EC) is made by revision of bibliographic sources, by calibration with modern instrumental earthquakes, and by interpretations in terms of current knowledge of the tectonics and seismicity of the region. Throughout the process we have derived an equation to estimate Mw for shallow crustal earthquakes in Colombia using the length of isoseismal VIII, LVIII:
We also derived an equation to evaluate Mw for Colombian crustal earthquakes using the rupture length, L, estimated generally from the aftershock distribution of strong earthquakes:
We calculated average attenuation parameters for intermediate depth and shallow earthquakes that may be used, combined with other observations, to estimate the focal depth of historical events. Our final picture shows three distinct regions of the Colombian Eastern Cordillera (EC) where historical earthquakes are distributed. (a) The southern sector, from the Páramo de Sumapaz down to the Colombian Massif where the largest crustal earthquakes have occurred (1827, M 7 3/4; 1967, Mw = 7.0). (b) The central sector, between the Páramo de Sumapaz and Tunja with moderate to large earthquakes associated to the reverse faults on the piedmonts (the 1805 earthquake, M 6 3/4, on the western flank, and the 1743, 1923 and 1995 with M 6 1/2, 6 3/4, and 6.5, respectively, on the eastern flank). (c) The northern sector, to the north of Tunja, which is characterized by recurrent earthquakes probably associated with major reverse faults in the axial zone (e.g., 1646, I0 = VIII; 1724, M 6 3/4; 1755, I0 VIII; and 1928, M 5 3/4). Two events appear to be related to the axial faults to the south of Bogotá: those in 1644 (M 6) and 1917 (M = 7.1). The 1785 earthquake might have been an intraplate event in the subducting plate under the EC. Events in 1616 and 1826, which caused damage along the axial zone of the Cordillera near Bogotá, have no historical records precise enough to allow the estimation of their location and size, but their epicentres are probably not farther than some tens of kilometers from Bogotá.  相似文献   

14.
Vrancea is one of the few singular seismic regions of the world where intermediate-depth earthquakes are permanently generated (around 10 events/month with M L > 3) within an extremely confined focal volume. This particularity and the relatively large number of short-period waveforms recorded by the Romanian local network provides us the opportunity to test the performance of the empirical Green's function technique in retrieving the source time function and source directivity of the Vrancea earthquakes. Three earthquakes that occurred on March 11, 1983 (M L = 5.4), April 12, 1983 (M L = 5.1) and August 7, 1984 (M L = 5.1) in the lower part of the subducting lithosphere (h 150 km) were analyzed. A set of 28 adjacent events (3.0 < M L < 4.4) which occurred between 1981 and 1997 were selected as corresponding empirical Green's functions. To test the confidence of the retrieved source time function, we compare the deconvolved pulses using Green's functions of different sizes and recorded simultaneously by short-period and broad-band instruments. Our tests show that the durations of the source time function is well-constrained and is not affected by the limited frequency range of the short-period instruments, or by the relative difference in the focal mechanism between the main event and Green's event. The apparent duration of the source time function outlines source directivity effects, and when these effects are sufficiently strong, they can identify the real fault plane. Relatively short source duration and correspondingly high stress drop values are in agreement with other previous results emphasizing a specific seismic regime in the lower part of the Vrancea subducting lithosphere.  相似文献   

15.
The rates and configuration of seismic deformation in the North Aegean trough-North Anatolian fault are determined from the moment tensor mechanisms of the earthquakes that occurred within this region. The analysis is based onKostrov's (1974) formulation. The fault plane solutions of the earthquakes of the period 1913–1983 withM s 6.0 are used. The focal mechanism of some of the past events (before 1960) is assumed, based on the present knowledge of the seismotectonics as well as on the macroseismic records of the area studied. The analysis showed that the deformation of the northern Aegean is dominated by EW contraction (at a rate of about 15 mm/yr) which is relieved by NS extension (at a rate of about 9 mm/yr). It was also shown that the northern part of North Anatolia (north of 39.7°N parallel) undergoes contraction in the EW direction (at a rate of about 9 mm/yr) and NS extension as the dominant mode of deformation (at a rate of about 5 mm/yr). It may be stated therefore, that the pattern of deformation of the northern Aegean and the northern part of North Anatolian fault is controlled by the NS extension the Aegean is undergoing as a whole, and the dextral strike-slip motion of the North Anatolian fault. The southern part of North Anatolia is undergoing crustal thinning at a rate of 2.3 mm/yr, NS extension (at a rate of 5 mm/yr) as well as EW extension (at a rate of 4 mm/yr), which are consistent with the occurrence of major normal faulting and justify the separation of North Anatolia into two separate subareas.  相似文献   

16.
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   

17.
We review earthquake distributions associated with hydrocarbon fields in the context of pore pressure diffusion models, poroelastic stress transfer and isostasy theory. These three mechanisms trigger or induce seismic instabilities at both local scale (D5 km) and at regional scale (D20 km). The modeled changes in stress are small (1 MPa), whatever the tectonic setting. Each mechanism corresponds to different production processes. (1) Local hydraulic fracturing due to fluid injection induces seismic-slip on cracks (M L3) within the injected reservoir through decreasing the effective stress. (2) Pure fluid withdrawal causes pore pressure to decrease within the reservoir. It triggers adjustments of the geological structure to perturbations related to the reservoir response to depletion. Poroelastic mechanisms transfer this stress change from the reservoir to the surrounding levels whereM L5 seismic instabilities occur either above or below the reservoir. (3) Massive hydrocarbon recovery induces crustal readjustments due to the removal of load from the upper crust. It can induce larger earthquakes (M L6) at greater distance from the hydrocarbon fields than the two other mechanisms.Due to the mechanical properties of the shallow rock matrices involved, seismic slip triggered either by mechanism (1) or (2), is a second-order process of the main elastoplastic deformation. for a minimum of 80% of commercially productive basins, most of the local deformation is reported as aseismic, i.e., there is no evidence forM L3 earthquakes. Nevertheless, the induced stresses vary as a function of time in a manner that depends on the hydraulic diffusivity (i.e., permeability) of the reservoir and surrounding rocks. Because small earthquakes (M L3) indicate changes in stress and pore pressure, monitoring of seismicity is a means of assessingin situ reservoir behavior.The less constrained seismic response to hydrocarbon recovery is the possible connection between local fluid manipulations, triggered earthquakes and major regional earthquakes. Positive feedback mechanisms suggest that the region of seismic hazard changes is much larger than the area where hydrocarbons are extracted. These observations and models testify that fluid movement and pore pressure changes (increase or decrease) play important roles in the mechanics of earthquakes and in the triggering of natural earthquakes.  相似文献   

18.
The surface-wave magnitudes of a selection of New Zealand earthquakes have been determined on a consistent basis using the ‘Prague formula’ and station corrections. The earthquakes range in magnitude from about 5 to 7.8, covering the instrumental period 1901–1988. Magnitudes for many of the earlier events had not been properly determined previously; and some significant discrepancies from the traditional magnitudes were found. The use of European station data (160° < D < 175°) is important to New Zealand because of its geographical isolation. These distant data were found to give consistently slightly higher Ms than closer stations, but could be used without bias through the station correction procedure. The relationship between Ms and ML was found for 31 ‘shallow’ New Zealand events and much of the scatter was explained as a function of depth. Significant differences in Ms/ML expressions from Europe and California were also found. The limited New Zealand data for Mw and M0 related well to Californian and global relationships with Ms.  相似文献   

19.
In this paper we evaluate the present state of the seismic regime in Southern California using the concentration parameter of seismogenic faults (K sf ,Sobolev andZavyalov, 1981). The purpose of this work is to identify potential sites for large earthquakes during the next five or ten years. The data for this study derived from the California Institute of Technology's catalog of southern California earthquakes, and spanned the period between 1932 to June 1982. We examined events as small asM L 1.8 but used a magnitude cutoff atM L =3.3 for a detailed analysis. The size of the target earthquakes (M M ) was chosen as 5.3 and 5.8.The algorithm for calculatingK sf used here was improved over the algorithm described bySobolev andZavyalov (1981) in that it considered the seismic history of each elementary seismoactive volume. The dimensions of the elementary seismoactive volumes were 50 km×50 km and 20 km deep. We found that the mean value ofK sf within 6 months prior to the target events was 6.1±2.0 for target events withM L 5.3 and 5.41.8 for targets withM L 5.8. Seventy-three percent of the targets withM L 5.8 occurred in areas whereK sf was less than 6.1. The variance of the time between the appearance of areas with lowK sf values and the following main shocks was quite large (from a few months to ten years) so this parameter cannot be used here for accurate predictions of occurrence time.Regions where the value ofK sf was below 6.1 at the end of our data set (June, 1982) are proposed as the sites of target earthquakes during the next five to ten years. The most dangerous area is the area east of San Bernardino whereK sf values are presently between 2.9 and 3.7 and where there has been no earthquake withM L 5.3 since 1948.  相似文献   

20.
The Timiskaming earthquake, which occurred near the Quebec-Ontario border at the northwest end of the Western Quebec seismic zone in 1935, is one of the five largest instrumentally recorded southeastern Canadian earthquakes. Previous studies of this earthquake concentrated on modeling teismograms recorded at regional distances, a better constrained focal mechanism is obtained. The waveforms indicate thrust faulting on a moderately dipping northwest striking plane at a depth of 10 km. TheM w of 6.1 determined in this study is in good agreement with previous magnitude estimates (m b 6.1,M s 6.0, andm bLg 6.2–6.3). The focal mechanism is similar to those of many recent small to moderate earthquakes in the region, and the inferred (from theP axis) acting stress of northeast compression is consistent with the overall eastern North American stress field. The Lake Timiskaming Rift Valley in which the earthquake occurred, comprises several northwest striking faults consistent with the strike of the 1935 event. Thus, the 1935 earthquake appears to be a result of faulting on the reactivated Timiskaming graben.  相似文献   

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