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Showing papers in "Bulletin of the Seismological Society of America in 1966"


Journal ArticleDOI
TL;DR: The Knopoff-deHoop representation theorem has been used to calculate the form of the body waves radiated from an elliptical fault as discussed by the authors, where Rupture is assumed to initiate at one focus of the ellipse and then spread out radially on the fault plane.
Abstract: The Knopoff-deHoop representation theorem has been used to calculate the form of the body waves radiated from an elliptical fault. Rupture is assumed to initiate at one focus of the ellipse and then spread out radially on the fault plane. Two cases are considered: 1) constant slip everywhere on the fault surface and 2) a variable slip which approaches zero at the fault edge. The radiation is calculated for distances from the fault which are large compared to the fault dimensions. The body waves are described by the product of two factors, one of which is the familiar equivalent-force system radiation pattern. The other factor includes the time dependence of the signal; it does not depend upon the direction of slip. The body waves exhibit two stopping phases. The theory is used to estimate the fault dimensions associated with six deep-focus earthquakes studied by Kasahara. The estimated fault dimensions are about twice the dimensions of the focal sphere as found by Kasahara. Finally, the difference between the phase spectrums of shallow and deep-focus earthquake radiation observed by Kishimoto is shown to be related to a difference in shape of the two fault surfaces; shallow-focus earthquakes appear to be associated with elongated fault surfaces, whereas deep-focus earthquakes are associated with more circular fault surfaces.

196 citations


Book ChapterDOI
TL;DR: In this paper, the authors derived expressions for the total energy and energy spectral density of elastic waves radiated by a propagating fault by using a spacio-temporal autocorrelation of the acceleration of relative displacement over the fault plane.
Abstract: Previously derived expressions for the total energy and energy spectral density of elastic waves radiated by a propagating fault are rewritten in terms of a spacio-temporal autocorrelation of the acceleration of relative displacement over the fault plane. This is interpreted in a statistical sense as the average autocorrelation over an ensemble of earthquakes. An explicit form of autocorrelation function is assumed, depending upon two parameters, a correlation length, and a correlation time, and the total energy and energy spectral density are derived in terms of these parameters. By using scaling laws due to Bath and Duda for earthquake volume and radiation efficiency as functions of magnitude, the statistical parameters may also be related to magnitude.

177 citations


Journal ArticleDOI
TL;DR: In this article, the shear components of the earth strain were extracted from repeated triangulation surveys, where each triangle giving one measurement for γ1 and γ2, without requiring knowledge from adjoining areas.
Abstract: In deducing earth strains from repeated triangulation surveys, it is advantageous to extract the shear components of strain γ 1 = ( ∈ 11 − ∈ 22 ) and γ 2 = ( ∈ 12 + ∈ 21 ) separately. Unlike the dilation and rotation components of strain, 1 2 ( ∈ 11 + ∈ 22 ) and 1 2 ( ∈ 12 - ∈ 21 ) , these shear components can be determined locally, each triangle giving one measurement for γ1 and γ2, without requiring knowledge from adjoining areas. It follows that it should be possible to extract much more and better information about the shear strains from existing survey data than has been extracted hitherto.

151 citations


Journal ArticleDOI
TL;DR: The phase velocity method has been applied to the problem of the determination of the crust and upper mantle under the western Alpine crest and in the Alpine foreland to the north as discussed by the authors.
Abstract: The phase velocity method has been applied to the problem of the determination of the crust and upper mantle under the western Alpine crest and in the Alpine foreland to the north An extensive data processing package has been designed so that Fourier analysis is applied to the determination of phase velocities, rather than the more usual peak-and-trough method Effects of contamination by multipath interference, manifested in beats, can be minimized Advantage is made of apparent azimuthal variations in phase velocity to yield a further refinement in the method whereby the tripartite results are assigned to discrete lines in the network rather than to the area swept out by the wave front The results show that a well-developed low-velocity channel for S is found throughout the region with a velocity of S in the channel of 42 km/sec The top of the channel is at about 80 km depth A new analysis of P -wave data shows a likely horizon for reflections at 220 km; this is taken to be the depth of the lower boundary to the channel The mean P -wave velocity in the lower crust is at least as high as 67 km/sec The crustal and upper mantle structure vary significantly over relatively short distances The Mohorovicic discontinuity is deepest under the crest of the Alps and shoals to the north and west; a well developed root has been found

125 citations


Journal ArticleDOI
TL;DR: In this article, the authors derived cyclic functions of the direction of the source from the telemetry records of the central California seismographic array and compared these with the great circle azimuths and with the apparent velocities calculated from the Jeffreys-Bullen tables.
Abstract: Arrays of seismographs are usually considered to be detectors which give enhanced signals from distant earthquakes. They also provide, however, a new way of learning more about the structure of the crust and upper mantle. The deviation of the seismic-wave surface from its expected configuration may be regarded as a consequence of non-homogeneous and anisotropic conditions in the earth. The operations of the University of California network of telemetry stations in the Coast Ranges of California provides an opportunity to discover the practicality of this approach. The situation of this network near the continental margin gives the study particular interest. The differences in arrival-times between array elements of coherent peaks or troughs of P and pP phases from 28 teleseisms in the period of 1963-1964 were read from the telemetry records of the central California seismographic array. The direction of approach and velocities of the wave fronts were then determined and compared with the great circle azimuths and with the apparent velocities calculated from the Jeffreys-Bullen tables. The observed anomalies in direction of approach and apparent velocites are found to be cyclic functions of the direction of the source. The amplitudes of these functions are almost 10 degrees in azimuth anomaly and 1.0 sec/deg in slowness anomaly. Error analyses show that the anomaly functions cannot be attributed to the measurement errors. The derived anomaly functions provide a powerful means of examining crustal and upper mantle structure under the array and perhaps at the source. Variations between subsets of the array indicate significant differences in structure between portions of the Coast Ranges to the north and to the south of Hollister.

95 citations


Journal ArticleDOI
TL;DR: In this article, the first-arrival data of the Lake Superior Experiment of 1963 have been interpreted by the time-term method, and the results appear to be consistent and meaningful.
Abstract: The first-arrival data of the Lake Superior Experiment of 1963 have been interpreted by the time-term method. The analysis has shown the method to be well suited to this type of survey, and the results appear to be consistent and meaningful. Approximately 500 first-arrivals from head waves generated at the Mohorovicic discontinuity, have been reduced to estimates of crustal time-terms at over 100 locations. A much shallower refracting surface (here called the Upper Refractor) furnished nearly 1,000 observations to yield upper crustal time-terms at the same locations. The analysis reveals the material beneath the UR and beneath the M to have velocities of 6.63 and 8.10 km/sec respectively. The surface of the Upper Refractor, on the basis of a simple interpretation of the time-terms, is revealed as undulating, coming close to the surface at the edges of the lake and reaching maximum depths of approxmately 15 km to the east and west of the Keweenaw Peninsula. On a similar basis the Mohorovicic discontinuity is revealed as an easterly dipping surface, having a depth of approximately 35 km at the west end of the lake and reaching a maximum depth of about 60 km in the region just west of the Keweenaw Peninsula. Eastwards, the time-term values fluctuate but do not increase or decrease systematically. The velocity of the material lying above the Upper Refractor is not well determined, but appears to be roughly 5.5 km/sec. A perusal of geological literature suggests that this low velocity material is mostly sedimentary, filling a well-known synclincal basin whose axis bends around the Keweenaw Peninsula. This mainly sedimentary section is known to be underlain by a great thickness of igneous rocks, which in all probability corresponds to the Upper Refractor mapped by the seismic studies.

89 citations


Journal ArticleDOI
TL;DR: In this paper, a general formulation for the computation of the apparent azimuth and the angle of incidence at the free surface is obtained, for a number of combinations of various dip angles and velocity contrasts, for the first and second derivatives of the travel time data are to be analyzed.
Abstract: When a layer is overlying a half-space with a tilted interface, the refraction of seismic waves emerging from the half-space generally results in their reorientation away from the vertical plane of incidence, with the following consequences: The angle of emergence at the free surface and, thus, dT/dΔ will become azimuth dependent. The apparent direction of approach of the wave front along the free surface will, in general, be different from the true azimuth. The azimuthal deviation also varies with azimuth. A general formulation for the computation of the apparent azimuth and the angle of incidence at the free surface is obtained. The numerical corrections to the observed dT/dΔ and apparent azimuth, for a number of combinations of various dip angles and velocity contrasts, are tabulated. The tables may be helpful when the first and second derivatives of the travel time data are to be analyzed. Their comparison with observation can also be useful for making inferences about the underlying crustal structure. The latter approach has been applied to the P arrivals across the Tonto Forest Seismological Observatory (TFSO) array, Arizona. On the basis of these observations, the Mohorovicic discontinuity under the observatory appears to be dipping locally as much as eight degrees in the N70° ± 5°E direction.

88 citations


Journal ArticleDOI
TL;DR: In this article, a relatively small perceptible earthquake occurred near a tripartite net of high sensitivity in central Japan, a substantial difference was found between its 25 foreshocks and 173 aftershocks in the relation of frequency of occurrence and magnitude.
Abstract: When a relatively small perceptible earthquake occurred near a tripartite net of high sensitivity in central Japan, a substantial difference was found between its 25 foreshocks and 173 aftershocks in the relation of frequency of occurrence and magnitude. For that study the coefficient “ b ” in the magnitude versus frequency equation is 0.35 for the former and 0.76 for the latter. A similar investigation has been carried out on the great Chilean earthquake of 1960, also accompanied by many foreshocks and aftershocks. Using four sensitive and suitably located U.S.C.G.S. stations, Eureka, Tucson, South Pole, and Byrd, foreshocks and aftershocks were located in addition to those reported by U.S.C.G.S. or B.C.I.S. Forty-five foreshocks and 250 aftershocks were found in a period of 33 hours before and 33 hours after the main shock. The same characteristic found for the Japanese earthquake was also found for the Chilean earthquake; i.e. the foreshocks showed a different picture from the aftershocks for the frequency of occurrence, and an appreciably smaller value seems to be valid for “ b ” of the foreshocks.

87 citations


Journal ArticleDOI
TL;DR: The most active continuous seismic zone in this region during historic time extended over 750 km, from a point off the California coast near Ventura to Winnemucca in north-central Nevada as mentioned in this paper.
Abstract: Maps of tectonic flux are presented for the conterminous United States west of longitude 109°W, for periods of time before and after 1932, and for the entire historic period through 1961. The most active continuous seismic zone in this region during historic time extended over 750 km, from a point off the California coast near Ventura to Winnemucca in north-central Nevada. Although this zone is characterized by a discontinuous line of historic surface faulting, it is neither sharply defined by, nor closely related to structures along its path that are generally considered to be the major tectonic elements of the region. The broad areal extent of this, and five other active zones, suggests that the tectonic processes causing earthquakes and surface faulting in the western United States are distributed over broad regions, and are not confined to geologic or physiographic provinces. Seismicity maps for different periods indicate that seismic activity in some areas has shifted with time. Within major seismic zones, gaps in the seismicity pattern are filled in by successive large earthquakes. Recurrence curves support a high level of activity for the Ventura-Winnemucca zone, and they indicate a lower rate of activity for the San Andreas fault zone than for other areas in the region studied. Recurrence curves for the central California area indicate that the rate of activity in a given region may remain practically constant over periods at least as long as a century, whether or not large earthquakes occur in the region during those periods. There appears to be a general correlation between observed shear or slippage, and the seismicity of any given region. Based on these results, it is proposed that, where historic faulting has occurred in areas with little or no historic seismic activity, such faulting is due to the propagation of fractures into the inactive areas from adjoining seismic zones.

71 citations


Journal ArticleDOI
TL;DR: In this paper, the authors defined the transition zone as the range from 12° to 18° where the first apparent arrival depends on signal-to-noise conditions and showed that the P t (P -teleseismic) phase dominates the longitudinal portion of recordings from the transversal phase and the observer often interprets P t as the first arrival.
Abstract: Regional and teleseismic P wave travel times from surface foci entail relatively fast crustal and upper mantle velocities beneath continental shields, and, in contrast, slow velocities beneath cordillera. On the average, teleseismic readings from continental and oceanic sources give arrival times 2 sec earlier than the Jeffreys-Bullen zero depth curve. In addition to this overall offset an empirical surface focus curve shows prominent departures from the J-B predictions in the neighborhood of 30° and 60°, where the observations are approximately 2 1/2 to 3 sec earlier than the J-B curve. As defined in this paper the term “transition zone” refers to the range from 12° to 18° (circa) where the first apparent arrival depends on signal-to-noise conditions. Given sufficient signal strength P r , an emergent phase which fits a slope of 8.6 to 8.8 km/sec on travel time plots, arrives first. Amplitudes associated with the P t ( P -teleseismic) phase dominate the longitudinal portion of recordings from the transition zone and, if noise levels are high, the observer often interprets P t as the first arrival.

70 citations


Journal ArticleDOI
TL;DR: In this article, the dispersive characteristics of a single elastic layer overlying an elastic half-space are examined in detail for the fundamental and the first and second higher modes of Rayleigh waves.
Abstract: The dispersive characteristics of a single elastic layer overlying an elastic half-space are examined in detail for the fundamental and the first and second higher modes of Rayleigh waves. Phase velocity, group velocity, and the ratio of horizontal to vertical surface displacement are computed as functions of dimensionless quantities proportional to period and wave number. The significant range for the independent variable, B 1 T/H , proves to be largely independent of the parameters of the structure. The range is 1 to 20 for the fundamental, 0.3 to cutoff for the first higher mode, and 0.2 to cutoff for the second higher mode. The most important parameter of the structure for Rayleigh wave dispersion is the shear velocity ratio. Variations in the Poisson9s ratio in the surface layer and the density contrast may produce substantial effects. Poisson9s ratio in the half-space is of least significance. The dependence on model parameters of the long-period cutoff for the higher modes is determined. Specific results are given for the following geophysical examples: continental crust, continental ice cap, sedimentary basin, alluvial overburden, and laboratory seismic models.

Journal ArticleDOI
TL;DR: A survey of available literature indicates that at least 27 tsunamis and seismic seiches in the Indian Ocean have been reported as mentioned in this paper, most of these were reported from the coastal regions of the seismically active Indonesian Arc, whereas progressively fewer such waves were reported in coastal regions adjacent to the Bay of Bengal, Arabian Sea and the southeastern coast of Africa and the western coast of Australia.
Abstract: References have been made in the past to the absence of tsunamis and seismic seiches in the Indian Ocean. However, a survey of available literature indicates that at least 27 such waves have been reported. Most of these were reported from the coastal regions of the seismically active Indonesian Arc, whereas progressively fewer such waves were reported from the coastal regions adjacent to the Bay of Bengal, Arabian Sea, and the southeastern coast of Africa and the western coast of Australia.

Journal ArticleDOI
TL;DR: In this paper, the effect of a corrugated interface on wave propagation is considered by using the method that was first applied to acoustical gratings by Rayleigh, where quantities of the order of the square of corrugation amplitude are taken into account.
Abstract: The effect of a corrugated interface on wave propagation is considered by using the method that was first applied to acoustical gratings by Rayleigh. The problem is what happens when a plane P wave is incident on a corrugated interface that separates two semi-infinite media. As is well known, there are irregular (scattered) waves as well as regular waves. By assuming both the amplitude and the slope of a corrugated interface to be small, quantities of the order of the square of corrugation amplitude are taken into account. In the case of normal incidence for three models considered, the effect of corrugation on reflection is larger than the effect of corrugation on refraction; the amplitude of the regularly reflected waves decreases, and that of the regularly refracted waves and of the irregular waves increases, as the corrugation amplitude becomes larger. Generally, the larger the velocity contrast, the larger the variation of wave amplitude with the wavelength and the amplitude of corrugation. The S wave component generally becomes larger as the wavelength of corrugation becomes smaller. Boundary waves exist, depending upon the ratio of wavelength of corrugation to that of the incident wave. For a specified interface, it is possible that there is a significant difference in wave amplitude as a function of the elastic constants. In the case of oblique incidence, computation was carried out for angles of incidence smaller than 15° for one model. For these small angles of incidence, almost all results for the case of normal incidence still hold. Furthermore, it can be concluded that the effect of the angle of incidence on reflected S waves is larger than for the other waves and that large differences in the amplitudes of waves at different angles of incidence may be expected for the irregular waves.

Journal ArticleDOI
TL;DR: In this paper, the transmission and reflection coefficients for Rayleigh waves normally incident upon the corner of a homogeneous elastic wedge formed by two stress-free planes are computed using a Green9s function method of approximation.
Abstract: Using a Green9s function method of approximation, transmission and reflection coefficients are computed for the problem of Rayleigh waves normally incident upon the corner of a homogeneous elastic wedge formed by two stress-free planes. The Rayleigh waves are incident from infinity and travel along one surface of the wedge. The transmitted waves on the second surface and the reflected waves on the first surface are calculated by the application of Huygens9 principle. A pair of coupled integral equations for the displacements are obtained by means of a representation theorem. Neglecting the diffracted body waves near the corner, the coupled integral equations are reduced to a pair of algebraic equations. A new feature of the calculation involves consideration of diffracted surface waves travelling toward the vertex. Numberical values of the phase shifts and attenuation factors in the transmitted and reflected waves are computed as functions of the wedge angle. Comparison with experimental results show considerably better agreement than has been obtained previously.

Journal ArticleDOI
TL;DR: In this paper, the second derivative with respect to pressure of the bulk modulus is assumed to be negligible at all pressures considered, and this assumption is checked by computing the compression (V/V 0 ) in the pressure range by equations of state using the assumption, and comparing the resulting values with measured compression.
Abstract: By using the accuracy inherent in ultrasonic velocity measurements taken at pressures less than 10 kb, the seismic parameter φ = v p 2 − ( 4 3 ) v S 2 can be computed at very high pressures. The equation used requires the assumption that the second derivative with respect to pressure of the bulk modulus be negligible at all pressures considered. This assumption is checked by computing the compression ( V/V 0 ) in the pressure range by equations of state using the assumption, and comparing the resulting values with measured compression. Illustrations are given for MgO and Al 2 O 3 .

Journal ArticleDOI
TL;DR: In this paper, a separation of P - and S -wave potentials is achieved for an inhomogeneous medium in which density is constant and Lame9s parameters, λ and μ, are assumed to vary as λ/λ 1 = μ/μ 1 = (1 + bz ) 2 where λ 1, μ 1 and b are constants.
Abstract: A separation of P - and S -wave potentials is achieved for an inhomogeneous medium in which density is constant and Lame9s parameters, λ and μ , are assumed to vary as λ/λ 1 = μ / μ 1 = (1 + bz ) 2 where λ 1 , μ 1 and b are constants. The resulting equations are solved for an arbitrary angle of incidence. Plane wave reflection coefficients are obtained for the situation when the material mentioned above forms a transition layer between two homogeneous, elastic half-spaces. First and/or second-order discontinuities in material properties are permitted at the boundaries of the transition layer. Some numerical results are given.


Journal ArticleDOI
TL;DR: In this article, the authors used portable seismographs with ultra-high sensitivity to record a sufficient number of nearby micro-earthquakes in a short interval of time, say one day, so that some measure of the seismic activity of a very local area might be obtained very quickly.
Abstract: The increase in frequency of occurrence of earthquakes with decreasing magnitude is well known. In a few cases observations have shown that this relation holds for extremely small events, including those with magnitudes well below zero, and that the energy of the smaller shocks is confined largely to the higher seismic frequencies. These facts suggest that portable seismographs with ultra-high sensitivity might record a sufficient number of nearby microearthquakes in a short interval of time, say one day, so that some measure of the seismic activity of a very local area might be obtained very quickly. This idea was tested in west central Nevada where ten sites were occupied for short intervals of time. Microearthquakes were recorded at rates ranging from several per day to over two hundred per day. Generally, consistently high microseismicity was observed in areas of recent faulting. A lower level of activity, well above that of aseismic areas however, was observed at other sites in Nevada. Some indication of variation of microearthquake activity with time was obtained. Activity averaged over an interval of about ten hours usually differed by less than a factor of two from the overall mean for that site. In one case, however, swarms of events persisting for a total interval of a little more than one day were observed. The method appears very promising as a technique for monitoring current tectonic activity.

Journal ArticleDOI
TL;DR: In this paper, a single pass program, COAST, was developed for use on the IBM 7030 (STRETCH) computer, which discriminates compatible time groups from an uncorrelated chronological data file, computes a first approximation to the hypocenter using only five stations, and determines the refined hypocenter and earthquake magnitude using all relevant data without any intervention by a seismologist.
Abstract: At the U. S. Coast and Geodetic Survey, where preliminary earthquake locations must be computed from as many as 30,000 initially unrelated P arrivals and associated phases per month, considerable effort has been devoted toward making this process more fully automatic. A single pass program, COAST, has now been developed for use on the IBM 7030 (STRETCH) computer. This program discriminates compatible time groups from an uncorrelated chronological data file, computes a first approximation to the hypocenter using only five stations, and determines the refined hypocenter and earthquake magnitude using all relevant data without any intervention by a seismologist.

Journal ArticleDOI
TL;DR: In this paper, Santo divided the Pacific into seven regions and compiled group velocity dispersion curves, upper mantle structure in the Pacific in which the depths of the low velocity layer and the shear wave velocity are changing systematically from continent to ocean.
Abstract: Making use of Rayleigh and Love wave dispersion data, Santo divided the Pacific into seven regions. From his map and compiled group velocity dispersion curves, upper mantle structure in the Pacific in which the depths of the low velocity layer and the shear wave velocity are changing systematically from continent to ocean is obtained. In orogenic regions such as Japan and its surroundings, extremely low velocity layer in which the shear wave velocity is about 4.3 km/sec is just under the Moho. In the oceanic side of this region, the layer is overlain by the normal mantle material with shear wave velocity of about 4.6 km/sec and in the pure oceanic region this extremely low velocity layer disappears. The so-called ‘low velocity layer’ which is believed to begin at the depth of about 60 km under the ocean is present in the oceanic region but the shear wave velocity in the layer may be a little higher than that obtained by earlier works.

Journal ArticleDOI
TL;DR: In this paper, a simple theoretical model was used to calculate the frequencies of vibration of the buildings; the model was based on the assumption that there was no joint rotation in the building frames.
Abstract: This paper outlines the methods that have been used to determine the frequencies and modes of vibration of multistory buildings from their wind-induced vibrations. Three buildings of ten, thirty-eight and forty-seven stories were investigated. A simple theoretical model was used to calculate the frequencies of vibration of the buildings; the model was based on the assumption that there was no joint rotation in the building frames. A comparison of the theoretical and measured values of the frequencies showed that this simple model was a realistic representation of only the smaller building. It is concluded that a model that includes joint rotation would be more realistic for the taller buildings. Auto-correlation and power spectrum analysis of the vibration records were used to obtain an estimate of the damping characteristics of the buildings. The values obtained were 1 to 3 per cent of the critical amount of damping.

Journal ArticleDOI
TL;DR: A sequence of more than 100 aftershocks with magnitudes as low as −0.1 was recorded following a magnitude 5.0 earthquake on November 16, 1964, in the San Andreas fault zone of central California as discussed by the authors.
Abstract: A sequence of more than 100 aftershocks with magnitudes as low as −0.1 was recorded following a magnitude 5.0 earthquake on November 16, 1964, in the San Andreas fault zone of central California. The sequence was monitored in detail by three temporary seismographic stations at distances less than 15 km and the surrounding telemetry array. Nearly all of the 35 earthquakes which could be located clustered in a focal region about 4 km in diameter at a depth near 12 km and exhibited uniform first motion radiation patterns. First motion fault plane solutions are consistent with the right lateral transcurrent motion characteristic of the San Andreas fault. Exceptions to this uniform radiation pattern in the concentrated focal region occurred near the times of two large aftershocks apparently on another fault about 5 km away.

Journal ArticleDOI
TL;DR: In this paper, seven temporary and three permanent seismograph stations recorded seismic waves to a distance of about 270 km from a 110,000 pound quarry blast detonated near Depoe Bay, Oregon.
Abstract: Seven temporary and three permanent seismograph stations recorded seismic waves to a distance of about 270 km from a 110,000 pound quarry blast detonated near Depoe Bay, Oregon. The recording stations were in a north-south line along the northern coast range of Western Oregon and Washington. The travel-time data indicate an apparent shallow crustal thickness (about 16 km) for this region. The time versus distance data were not continuous beyond 130 km from the source which may have resulted from any combination of the following causes: (1) insufficient source energy; (2) lateral geological variations; and/or (3) a subcrustal negative velocity gradient.

Journal ArticleDOI
TL;DR: Second Love and Rayleigh modes are found to propagate in Eurasia across many different structures; the continent is divided into regions where these higher modes have the same dispersion characteristics, suggesting that the modes have some form of elastic linkage as mentioned in this paper.
Abstract: Second Love and second Rayleigh modes are found to propagate in Eurasia across many different structures; the continent is divided into regions where these higher modes have the same dispersion characteristics. The condition for constructive interference breaks down only when the second modes cross areas of rapid change at oceanic-continental boundaries and under some mountain ranges. There are many paths in Eurasia where the second Love and Rayleigh modes have the same dispersion, and along these paths the two wave trains have a constant phase relationship, suggesting that the modes have some form of elastic linkage.

Journal ArticleDOI
TL;DR: In this article, phase velocities of Rayleigh waves in Finland have been measured by the Fourier analysis method, using records of three stations which are on almost the same great circle as several Greek earthquakes.
Abstract: Phase velocities of Rayleigh waves in Finland have been measured by the Fourier analysis method, using records of three stations which are on almost the same great circle as several Greek earthquakes. Phase velocities of Love waves of two of these earthquakes have also been determined. The wave groups were divided by numerical filtering into two frequency channels, which were analyzed separately. The resulting phase velocity curves were then smoothed in order to remove rapid oscillations, and mean values were calculated. The phase velocities were found to be slightly lower than those found for the Canadian Shield (Brune and Dorman, 1963), the maximum deviation for Rayleigh waves being about 0.07 km/sec at a period of 28 seconds and for Love waves about 0.15 km/sec at a period of 32 seconds. At periods shorter than 40 seconds the phase velocities of Rayleigh waves were found to be equal in Northern and Southern Finland within the accuracy of measurement.

Journal ArticleDOI
TL;DR: A number of independent readings of “typical” seismographic records are made, primarily in order to estimate the errors involved in determining the onset of a P wave.
Abstract: A number of independent readings of “typical” seismographic records are made, primarily in order to estimate the errors involved in determining the onset of a P wave. The degree of consistency with which a disturbance is called “the first arrival from an earthquake” is discussed. The distribution of reading errors is estimated and a method suggested for improving the accuracy of routine reading of seismograms. This model provides estimates for reading errors of several types of onsets. Finally the classification of onsets into “ i ” and “e” is shown to be inadequately defined.

Journal ArticleDOI
TL;DR: In this article, P-wave amplitude data for underground nuclear explosions are correlated with detonation energy, and the relationship of teleseismic magnitude to explosive yield for various types of low coupling is established.
Abstract: P -wave amplitude data for underground nuclear explosions are correlated with detonation energy. Amplitudes are reproducible to 25 per cent when source media and propagation path differences are minimized. These data verify theoretical scaling relationships and establish relative coupling for various shot media. An empirical correlation of these amplitude data with dry porosity of the detonation medium indicates that a medium with 60 per cent dry porosity may couple explosive energy one-fourth or one-fifth as efficiently as does alluvium. The relationship of teleseismic magnitude to explosive yield for various types of low coupling shows that dry porous media give a significant reduction of seismic signals generated by underground nuclear explosions.

Journal ArticleDOI
TL;DR: The San Salvador earthquake of May 3, 1965 was preceded by a local seismic swarm of three months duration as discussed by the authors, and the main shock was destructive in densely populated area of not more than 15 km in radius; the same general area was damaged in the earthquakes of 1576, 1659, 1798, 1839, 1854, 1873, 1880, 1917, and 1919.
Abstract: The San Salvador earthquake of May 3, 1965 was preceded by a local seismic swarm of three months duration. The main shock was destructive in a densely populated area of not more than 15 km in radius; the same general area was damaged in the earthquakes of 1576, 1659, 1798, 1839, 1854, 1873, 1880, 1917, and 1919. Over 120 casualties were reported. The epicenter has been located on the south rim of the Median Trough, a post-Pliocene structure which accounts for the high seismic and volcanic activity in the region. The observed intensity is attributable to shallow focal depth and to the presence of thick inhomogeneous beds of fluviatile pumice. The tectonic setting and shallow subsurface factors should be recognized in future building codes and zoning regulations.

Journal ArticleDOI
TL;DR: In this article, travel-time curves for the most prominent phases of the longitudinal and shear waves for epicentral distances between zero and 140 km and between 200 km and 1000 km have been constructed for Southeastern Europe.
Abstract: Travel-time curves for the most prominent phases of the longitudinal and shear waves for epicentral distances between zero and 140 km and between 200 km and 1000 km have been constructed for Southeastern Europe. Both refraction and reflection methods indicate a three-layered crust. The average thickness is approximately 16 km for the top layer, 15 for the middle layer and 11 for the lower layer. The crustal thickness varies from about 47 to 32 km from place to place. The extremely low value 0.21 for Poisson9s ratio was found in the top as well as in the middle layer.

Journal ArticleDOI
TL;DR: In this article, records from SOFAR-depth hydrophones at widely spaced locations in the North Pacific were scanned for earthquake T phases and a source solution was obtained by computer for T-phase arrivals correlated at four or more hydrophones.
Abstract: Records from SOFAR-depth hydrophones at widely spaced locations in the North Pacific were scanned for earthquake T phases. A source solution was obtained by computer for T -phase arrivals correlated at four or more hydrophones. Using this method, the number of events located on the North Pacific rim is about ten times greater than the number located by conventional body-wave determinations. It is concluded that the capability for location of earthquakes by T -phase arrivals extends down to about magnitude 3.6 for the North Pacific rim.