bluuebunny/crossref_metadata_2025 huggingface dataset:
On Andean structure, Part II: Earthquake risk in Chile
Authors: Lomnitz C.
Published: July 2021

The problem of earthquake risk in Chile is discussed in terms of seismic history, geotectonic provinces and regional variations in earthquake incidence. Seismic zoning is impractical because all of Chile above Parallel 42° South has been within the epicentral area of some destructive earthquake.

The probability concept of earthquake risk is discussed in reference to the seismicity of Santiago. It is found that the earthquake risk at Santiago for an acceleration of 0.1 g is 62.5% in any 10-year period. For the same interval, the risk of a “destructive” earthquake (intensity 8 1/2) is of the order of 20%.
A new catalogue of earthquake fault plane solutions
Authors: Fara H. D.
Published: July 2021

The earthquake fault plane solutions available to spring, 1964 in the literature are listed in standard form.
The effect of boundaries on wave propagation in a liquidfilled porous solid: VIII. Reflection of planewaves at an irregular boundary
Authors: Deresiewicz H. , Wolf B.
Published: July 2021

A study is made of the reflected field arising from body waves incident on a rough boundary of a saturated porous medium. The perturbation method employed here imposes restrictions on the boundaries; these restrictions are discussed in detail and it is pointed out how the presence of dissipation in the medium places additional constraints on admissible boundaries. Further, it is shown that the reflected field consists of specularly reflected waves and scattered waves which may take on the character of surface motions, and that because of viscosity in the liquid phase of the medium all waves are dispersed and dissipated. In the presence of a periodic boundary, when the frequency of the field is such that these surface waves are true Rayleigh waves, resonances occur in the scattered waves. Finally, numerical and graphical results are given for the problem of reflections generated at a sinusoidal boundary by normally incident waves of low and high frequencies, respectively, in a medium whose material constands correspond to those for a certain sample of a kerosene-saturated sandstone.
Determination of the natural periods of buildings
Authors: Crawford R. , Ward H. S.
Published: July 2021

abstract

Random wind excitation has been used to find the first three modes of vibration of a nineteenstory building about the two major translational axes and the torsional axis of the building. Vibration records were obtained with Willmore electromagnetic seismometers feeding into a multichannel magnetic tape recorder. A harmonic analysis was then performed with the aid of an analogue computer to determine the first few vibration modes. Concurrently with the experimental program the building modes were computed two ways using different simplifying assumptions regarding the lateral stiffness of the structure.
Analysis of the convergence properties of Rubenstein's method for the determination of the lower modes of vibration of a multi-degree of freedom system
Authors: O'Kelly M. E. J.
Published: July 2021

abstract

This paper analyzes the convergence properties of a method, recently proposed by Rubenstein, for the determination of the lower modes of vibration of a multi-degree of freedom system from a reduced eigenvalue problem. It is shown that under certain conditions the method converges to the exact eigenvalues. It does not have global convergence and hence some care must be exercised when using it.
A Fourier transform method of apparent velocity measurement
Authors: Shima Etsuzo , McCamy Keith , Meyer Robert P.
Published: July 2021

abstract

By measuring the differences in phase angle of Fourier spectral densities across a fixed array, apparent velocities may be determined at a number of frequencies. Because the analysis is carried out in the frequency domain, determination may be objectively restricted to regions of the spectrum giving consistent phase relations between array elements.

Using an array of approximately 1.25 km length, the apparent velocities of the first arrivals from the 1962 North Carolina experiment were determined for shots from 45 to 155 km. No indication of an increase in velocity with range between 55 and 140 km was found.
Body force equivalents for seismic dislocations
Authors: Burridge R. , Knopoff L.
Published: July 2021

abstract

An explicit expression is derived for the body force to be applied in the absence of a dislocation, which produces radiation identical to that of the dislocation. This equivalent force depends only upon the source and the elastic properties of the medium in the immediate vicinity of the source and not upon the proximity of any reflecting surfaces. The theory is developed for dislocations in an anisotropic inhomogeneous medium; in the examples isotropy is assumed. For displacement dislocation faults, the double couple is an exact equivalent body force.
Travel-time curves for a low-velocity channel in the upper mantle
Authors: Dowling John , Nuttli Otto
Published: July 2021

abstract

Velocities within the earth can be determined from body wave time-distance (T-D) data by the Herglotz-Wiechert method provided the velocity does not decrease too rapidly with depth. Until the present time, the properties of T-D curves for rapid decreases of velocity with depth have been considered only qualitatively.

This paper presents a technique for calculating a T-D curve for any velocity distribution, including continuous and discontinuous increases and decreases of velocity with depth. Some properties of T-D curves are quantitatively studied by systematically varying the characteristics of a single model and noting the corresponding variations in the calculated T-D curves. From this it is concluded that a significant low-velocity channel may not be evidenced by a shadow zone but rather by an overlapping of two distinct branches of the T-D curve. It is further concluded that the presence of a shadow zone implies a very gentle velocity gradient below the low-velocity channel.

By fitting a calculated T-D curve to observed data one can determine velocity as a function of depth even when the velocity decreases rapidly with depth, when a low-velocity channel exists. Observed T-D data for two underground nuclear explosions (gnome and bilby) measured in four different azimuths were fitted with T-D curves calculated for assumed velocity distributions. It is concluded that these data can be satisfied by a low-velocity channel for P waves in the upper mantle. The character of this channel (depth, thickness and velocity) was determined in each azimuth. The depth to its top was shallow (70 ± km) in the western U.S. and deep (125 ± km) in the eastern U.S. The velocity gradient below the channel is sharp enough to produce no prominent shadow zones. There are significant lateral changes in upper mantle velocities in the western U. S.
Ground motion accelerogram analysis including dynamical instrumental correction
Authors: Jenschke V. A. , Penzien J.
Published: July 2021

abstract

Due to inertial and damping characteristics of strong motion seismographs, recorded ground motion accelerograms may in some cases be sufficiently in error to significantly affect the results obtained when generating standard response or Fourier spectra. Therefore, the objectives of this paper are to present an analytical method of generating standard spectra which will eliminate the above instrumental error and to show the significance of this error by presenting some sample results obtained from accelerograms representing both earthquakes and underground nuclear explosions.
High frequency microearthquakes recorded at Quetta, Pakistan
Authors: DeNoyer John
Published: July 2021

Numerous high frequency microearthquakes were recorded on a special portable magnetic tape recorder at Quetta, Pakistan, during the summer of 1962. These microearthquakes have maximum particle velocities between 30 and 50 cps. Many of the foci appear to be almost directly under the recording station. The focal distance for the nearest of these earthquakes appears to be less than one km. The source for these earthquakes may result from “bedding plane slippage” as strain accumulates in sediments adjacent to a high angle fault.
The S wave project for focal mechanism studies earthquakes of 1962
Authors: Stauder William , Bollinger G. A.
Published: July 2021

The Department of Geophysics of Saint Louis University has instituted a routine program for the determination of the focal mechanism of the larger earthquakes of each year using methods developed for the use of S waves in focal mechanism studies. Suites of records from selected stations are assembled from the WWSS microfilm file for each earthquake of interest. A combination of P-wave first motion and S-wave polarization data is then used to determine graphically the mechanism of the earthquakes.

Thirty-six earthquakes of 1962 were selected for study. The focal mechanism solutions are presented for twenty-three of these shocks. There is evidence of patterns characteristic of the focal mechanism of earthquakes occurring in Kamchatka, the Aleutian Islands and South America.

A complete presentation of all the data and of all the solutions is available in a more lengthy report.
Wave propagation phenomena at an irregular infinite interface: Part I: Theory
Authors: Adams W. M. , Chang Chung-Po
Published: July 2021

Much wave propagation theory is not generally useful because plane parallel boundaries are assumed. In the present paper, the theory for wave phenomena at an irregular infinite interface is developed. As most interfaces in the earth are irregular, such a theory should prove valuable. The Weber integral solution to the wave equation is interpreted using a dual limiting procedure to define a principal value. Numerical formulation and applications are given in following articles.
Computer program for automatic processing of Basin and Range seismic data
Authors: Ryall Alan , Jones Austin E.
Published: July 2021

A program has been written for the University of Nevada's IBM 1620 computer which locates epicenters of seismic events that occur within the Nevada region. The program utilizes readings of P and S waves at up to eight stations in the Basin and Range province, rejects readings for events which are outside the region of interest or events recorded at less than three Nevada stations, establishes trial epicenters for locatable events by means of a table-lookup routine which utilized P-time differences between pairs of stations, and adjusts the trial epicenter by a least-squares procedure based on a sloping-layer crustal model derived from recent U. S. Geological Survey seismic refraction profiles. Epicenters calculated by the program are found to be affected by near-surface structural irregularities in the vicinity of receiving stations, but are not affected either by near-surface features in the source region or direction of the propagation paths with respect to regional structural trends. Traveltime delays calculated for Nevada seismographic stations appear to correlate with Bouguer gravity anomalies at the station sites.
The stopping phase on seismograms
Authors: Savage J. C.
Published: July 2021

Model studies have demonstrated that the stopping phase, an event which originates at the termination of rupture, may be identified on long-period strain recordings. A tentative identification of the stopping phase has been made on ultra-long-period seismograms of three major earthquakes (Montana, 1959; Chile, 1960; and Alaska, 1964). The events chosen lead to reasonable estimates of the length of rupture associated with the earthquake. A second type of stopping event, the breakout phase, must occur when a rupture intersects a free surface. A two-dimensional model study indicates that the breakout phase should be a prominent seismic event, particularly if the first motion is emergent. A review of studies of seismograms of earthquakes which produced surface faulting indicates that a prominent second event is often observed. However, there does not appear to be an adequate criterion to distinguish the breakout phase from the pP phase. Thus no certain identification can be made.
Surface wave dispersion in the Tonga-Fiji region
Authors: Hunkins Kenneth , Kuo John T.
Published: July 2021

Rayleigh, Love, and PL wave dispersion was investigated for 33 shocks in the Tonga-Kermadec arc and for one shock in the New Hebrides arc. The records were taken at Suva, Fiji with Lamont-I.G.Y. long-period seismographs. All epicenters were relocated using a least squares method programmed for a high-speed digital computer. The shocks were classified into five groups according to region. Each group has similar dispersion characteristics. Lower velocities were found beneath the Lau and South Fiji basins than beneath the Tonga and Fiji ridges. PL waves reflect the same velocity changes found for Rayleigh and Love waves. All the dispersion data are intermediate between typical continental and typical oceanic values. A theoretical calculation for a 21.5 km thick crust with a 7.0 km/sec lower layer was found to fit a path which includes the Lau basin, Fiji ridge, and Fiji basin.
Note on the use of reciprocity theorem for obtaining radiation patterns
Authors: Gupta Indra N.
Published: July 2021

Expressions for the horizontal and vertical displacements at the surface of an elastic half space when plane harmonic P or SV waves are incident at any given angle are already known. On the basis of the reciprocity theorem, these expressions are used to obtain “far-field” radiation patterns of P and SV waves due to horizontal and vertical forces applied at the free surface.
Radiation pattern of mantle Rayleigh waves and the source mechanism of the Hindu Kush earthquake of July 6, 1962
Authors: Chander Ramesh , Brune James N.
Published: July 2021

abstract

The source mechanism of the Hindu Kush earthquake of July 6, 1962 (magnitude 634-7, focal depth 218 km) was studied by comparing the observed amplitude and phase radiation patterns of mantle Rayleigh waves of 150 sec and 200 sec period with theoretical radiation patterns of Rayleigh waves from single- and double-couple point sources, and by considering evidence from Love waves and the shape of P and S pulses. The solution for the source mechanism, which is consistent with all the body wave and surface wave data available for this earthquake, is a double couple acting as a step function in time, with nodal planes oriented as determined from P wave data. Since for waves with periods greater than about 5 sec, the source appears to be an ideal point source, the radius of the equivalent source volume is estimated to be less than 10 km. For Rayleigh waves of 150 sec period, the agreement between observed and theoretical phases (for the above source model) is greatly improved by assuming regional phase velocities instead of a uniform phase velocity for all areas. It is concluded that with the accuracy currently attainable, a study of Rayleigh waves alone cannot determine the source mechanism of an earthquake uniquely.
Propagation of a P-pulse in a solid sphere
Authors: Alterman Z. , Abramovici F.
Published: July 2021

abstract

An exact solution is obtained for the displacement of the surface of a uniform elastic solid sphere of radius a due to an impulsive compressional pulse from a point-source situated at a distance b from the center. The duration of the source is δa/c where c denotes the shear-wave velocity, and its time-variation is such that the surface-displacement stays finite when the time tends to infinity.

The solution is applied to a source at a distance of one-eighth of the radius below the surface, approximating a deep-focus earthquake. Theoretical seismograms, radial and angular component, are given at distances 0 < ϑ < π for a source of duration 0.03a/c. Rayleigh waves are clearly seen at ϑ ≧ 45 °. Groups of reflected waves, especially predominant in the angular component, have the velocity of the lowest Airy phase in the group-velocity dispersion-curves. Diffracted waves, discussed in a previous paper, are found here again and in certain cases have an amplitude seven times larger than the amplitude of the direct pulse and also larger than any of the reflected pulses at the same distance. The transformed phases PSn, P2Sn have in general larger amplitude than the reflected Pn. Arrival times, initial amplitudes, reflection and convergence coefficients of pulses are obtained by steepest descents analysis and compared with the complete results.
Physical and topographic factors as related to short-period wind noise
Authors: Robertson Herbert
Published: July 2021

abstract

Median values of seismic noise in the period range of 0.3 to 1.3 sec were obtained from recordings at vaults of the Pole Mountain and WMSO arrays. Interquartile ranges were used to measure dispersions about the medians. Medians of the noise at Pole Mountain ranged from 0.91 mµ to 2.20 mµ in November 1962. The former value was obtained for a vault that was located in dense granite at the base of a massive granite outcrop; the latter value was obtained for a vault in a slab of dense granite located on a grassy plain. This indicated that topographic shielding from wind rather than density of bedrock affected noise. As a test of this idea, wind protection numbers were assigned to vaults Z1 through Z9 of the WMSO array based on comparative topographic shielding with respect to a known wind direction. Noise values increased as wind numbers decreased. Topographic protection and vault construction limited wind noise at WMSO.
A study of the Northwestern Pacific upper mantle
Authors: Walker Daniel A.
Published: July 2021

abstract

A fundamental problem of earthquake seismology is the occurrence of the upper mantle low-velocity channel. This study is intended to examine its existence in the upper mantle below the Northwestern Pacific on the basis of body-wave arrivals at a bottom-mounted hydrophone near Wake Island. A comparison of the observed travel times and the Jeffreys-Bullen travel times shows an extreme anomaly in the 21- to 33-degree range for both P and S waves. Assumed linear paths suggest a P-wave-channel upper boundary between 165 km and 185 km, and a lower boundary between 290 km and 542 km. Travel times for P and S waves indicate that the velocities in the channel remain constant at 8.1 km/sec and 4.65 km/sec respectively.
Determination of inhomogeneous media for which the vector wave equation of elasticity is separable
Authors: Hook Joseph F.
Published: July 2021

A generalization of the Helmholtz potential representation has recently been employed to effect separation of the vector wave equation of elasticity for certain types of inhomogeneous media whose properties vary with a single cartesian coordinate, z. For separability, the three constitutive parameters of the medium must satisfy a pair of simultaneous nonlinear differential equations. In this paper, integrals of these equations are derived which are explicit expressions for two of the parameters in terms of the third, whose functional form may be chosen arbitrarily. In the most general case, the arbitrary parameter must be the rigidity, μ For a special subcase, any one of the three parameters may take this role.
Deduction of earth strains from survey data
Authors: Frank F. C.
Published: July 2021

abstract

In deducing earth strains from repeated triangulation surveys, it is advantageous to extract the shear components of strain γ1=(∈11−∈22)andγ2=(∈12+∈21) separately. Unlike the dilation and rotation components of strain, 12(∈11+∈22)and12(∈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.
A comparison of earthquake magnitude with T-phase strength
Authors: Johnson R. H. , Northrop J.
Published: July 2021

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.
An interpretation of the first-arrival data of the Lake Superior Experiment by the time-term method
Authors: Berry M. J. , West G. F.
Published: July 2021

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 Mohorovičić 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 Mohorovičić 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.
Hayward fault slippage in the Irvington-Niles districts of Fremont, California
Authors: Cluff Lloyd S. , Steinbrugge Karl V.
Published: July 2021

abstract

Right lateral slippage on the Hayward fault has faken place in the Irvington and Niles districts of Fremont since the well known 1868 Hayward earthquake which produced surface ruptures from San Leandro to Warm Springs. This post-1868 movement has been occurring without being identified with strong earthquakes. Structures and railroads crossing the Hayward fault in the area under study date back to 1866. Fault slippage can be observed at ten separate locations along the strike of the Hayward fault in the Irvington-Niles districts of Fremont, California. One location predates the 1868 earthquake. There is no evidence for parallel lines of fault slippage within the approximately 200-foot wide fault zone. The slippage appears to have occurred within a 10-foot wide band parallel to the strike of the fault.

Structures and railroads built at different times during the past 100 years give a time-history of the slippage. The slippage, if any, between 1868 and 1909 is unknown. From 1909 until as late as 1949 or early 1950, there was no observed fault slippage. Approximately one-half foot of slippage occurred between about 1949 or early 1950 and about 1957, and no measurable slippage since 1957.
Displacements in the Claremont Water Tunnel at the intersection with the Hayward fault
Authors: Blanchard F. B. , Laverty G. L.
Published: July 2021

abstract

Three cracks have been observed in the concrete lining of the Claremont Water Tunnel at its point of intersection with the Hayward fault zone. The cracks, which show a right-lateral sense of displacement, were not present fourteen years ago in 1950.
Damage to culvert under memorial stadium, University of California, Berkeley, caused by slippage in the Hayward fault zone
Authors: Radbruch Dorothy H. , Lennert Ben J.
Published: July 2021

abstract

Slippage on a fault plane or sheared band in the Hayward fault zone has seriously damaged a cast-in-place concrete box culvert which carries the waters of Strawberry Creek under the University of California Memorial Stadium at Berkeley, California. Right-lateral movement of the fault or sheared band has offset the southwest-flowing creek so that the southwest portion has moved northwest with respect to the northeast portion. The two parts are connected by a northwest-trending section about 1,200 feet long, which flows in the culvert under the stadium. The fault crosses the culvert at a low angle, trending about 20° more to the north than the culvert, so that both tension and shear have been exerted on the culvert by right-lateral movement of the fault since the culvert was built in 1923. Two transverse cracks 1 inch wide and about 5 feet apart were noted when the culvert was inspected in 1948. In 1965 the maximum width of the cracks was 312 inches and the section of the culvert between the cracks appeared to have rotated clockwise. Total average separation of the cracks due to tension is about 4.1 inches; lateral offset of the culvert is 1.25 inches. Movement along the fault is estimated at 0.1 inch per year over the 42 years since the culvert was built.
Corrections to apparent azimuths and travel-time gradients for a dipping Mohorovičić discontinuity
Authors: Niazi Mansour
Published: July 2021

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:

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 Mohorovičić discontinuity under the observatory appears to be dipping locally as much as eight degrees in the N70° ± 5°E direction.
Radiation from a realistic model of faulting
Authors: Savage J. C.
Published: July 2021

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.
The “little variable factor” a statistical discussion of the reading of seismograms
Authors: Freedman Helen W.
Published: July 2021

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.
Spectral analysis of tsunami records from stations in the Hawaiian Islands
Authors: Loomis Harold G.
Published: July 2021

abstract

Power spectra for the tsunamis of 13 October 1963, 20 October 1963, and 28 March 1964, as recorded at several stations in the Hawaiian Islands and at Midway, were obtained by digital methods. It was found that a given tsunami produced spectra at nearby stations which were somewhat similar for low frequencies (i.e., out to 1.6 c/ks), but that even this similarity disappeared between spectra from stations as far apart as Hilo and Honolulu (210 miles). Likewise, a given station gives its spectral imprint to tsunamis incident upon it, but only qualitative similarities are found. The time changes in spectra of a tsunami at a given station do not support any simple picture of a local response function. The energy decay curves show the arrival of reflected energy from (sometimes) identifiable sources. The time changes in the mean frequency of incident energy are not immediately explainable.
Analysis of surface-foci travel times
Authors: Carder D. S. , Gordon D. W. , Jordan J. N.
Published: July 2021

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 Pr, 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 Pt (P-teleseismic) phase dominate the longitudinal portion of recordings from the transition zone and, if noise levels are high, the observer often interprets Pt as the first arrival.
A mobile seismograph array
Authors: Lehner Francis E. , Press Frank
Published: July 2021

abstract

A movable array composed of Ranger type seismometers, 60-day film recorders and 7-day magnetic tape recorders housed in compact trailers, has been developed. The array is useful for research requiring frequent instrument relocation such as P-delay, micro-seismicity, aftershock and signal-to-noise ratio studies.

The array unit combines the functions found in conventional fixed stations with a high degree of mobility. Conveniences such as solid state amplifiers, radio and clock circuitry, internal calibration, and minimum installation time are special features.

With the battery supply provided, a one week period of unattended film and tape recording is possible. With commercial power, the instruments can operate unattended for up to sixty days.

Useful magnification up to several million is available, depending on the frequency band selected.
Microearthquake activity recorded by portable seismographs of high sensitivity
Authors: Oliver Jack , Ryall Alan , Brune James N. , Slemmons David B.
Published: July 2021

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.
The application of rational approximations to the solution of problems in theoretical seismology
Authors: Longman I. M.
Published: July 2021

The method of approximate Laplace transform inversion, by analytically inverting suitable rational approximations to a function f¯(p) of the operator p is applied to the problem of propagation from a quadratic pulse point source in a uniform liquid sphere.

Apart from this main problem, the relation between the efficiency of the method and the sharpness of the pulse is illustrated in a general way by considering three simple examples (in descending order of sharpness), namely the approximate inversion of the Laplace transforms of a delayed Heaviside unit function, a simple function having a gradient discontinuity, and a quadratic pulse.

In order to suggest how the technique can be extended to more difficult theoretical seismic problems, the method is applied to a simple function f¯(p) having branch points on the imaginary axis in the p-plane.
A survey of complaints of seismic-related damage to surface structures following the Salmon underground nuclear detonation
Authors: Power Dean V.
Published: July 2021

Following Salmon, the undergound nuclear experiment in Mississippi, numerous unexpected complaints of damage were received from residents living 15 to 40 km away. The claimed damage is mostly cracks in plaster, stucco, and masonry. Ground motion at these ranges was within the predicted safe limits, with peak surface-particle velocities roughly an order of magnitude below the 5 to 10 cm/sec previously accepted as the threshold for damage. It is shown that a 5 cm/sec damage threshold is not descriptive of this area, and revised damage criteria need to be formulated that take into consideration geology, frequency spectrum, and other factors in addition to surface particle velocity. The claimed damage is similar to that normally found in Mississippi homes where it is attributed to poor foundation soils. The triggering by Salmon of naturally occurring settlement cracks is suspected, but the physics of the process is not yet fully understood. Further complications in the survey are introduced by local sociology and psychology and public relations factors.
Estimating the accuracy of source “parameters”
Authors: Freedman Helen W.
Published: July 2021

abstract

This paper suggests a method for estimating the errors accompanying estimates of seismic source parameters. While the method is a general one, the data here are from 1962 and 1963 and special attention is given to a few regions, such as the Kuriles. The overall standard error of about one-half a degree in each coordinate and four seconds in origin time are broken down and analyzed in terms of magnitude, depth, and location in space and time. The most accurately located earthquakes appear to be those of intermediate depth and magnitude in highly seismic areas. There is some indication that these errors are decreasing with time.
Correction of atmospheric errors in electronic measurements of earth crust movements
Authors: Thompson M. C. , Janes H. B.
Published: July 2021

abstract

Three methods are described for correcting electronic measurements of earth movement for atmospheric refraction effects. The effectiveness of two of these is discussed in terms of experimental studies using microwave systems. Precision of the order of 1 part in 109 (1 micron in 1 km) appears to be attainable.
Body wave radiation patterns from elementary sources within a half space
Authors: Gupta I. N.
Published: July 2021

abstract

The known expressions for the polar radiation patterns due to a horizontal or a vertical force, applied at a point within a uniform half space, are used to obtain the body wave radiation patterns from several other elementary seismic sources. Polar radiation patterns from seven elementary line sources, i.e., horizontal and vertical double forces without moment, horizontal and vertical single couples, center of dilatation, center of ratation, and double couple without moment, are first derived. Similar point sources in the three-dimensional space are also considered and the corresponding polar as well as azimuthal radiation patterns are obtained for P, SV, and SH waves. The results obtained include the effect of finite depth of the source below the free surface. Some of the results of Burridge et al for double-couple type seismic sources near a free surface are reproduced in a simple manner.

For the elementary point sources considered here, the azimuthal radiation patterns for a uniform half-space are found to be identical with those for an infinite homogeneous medium. However the polar radiation patterns appear to be profoundly affected by the proximity of the free surface.
Reservoir-dam interaction during earthquakes
Authors: Chopra Anil K.
Published: July 2021

abstract

Most work, to date, on analysis of effects of earthquakes on dams treats dam and reservoir as two uncoupled systems. The importance of interaction between the two systems in determining the water pressure on dams during earthquakes is investigated herein. The analysis presented is approximate in nature and is aimed at exploring the problem. The earthquake acceleration is first modeled as a stationary gaussian white noise process. For this excitation, certain qualitative conclusions concerning importance of interaction effects are derived. Hydrodynamic responses of typical systems subjected to El Centro (1940) earthquake are then determined to confirm these conclusions. On basis of these results, it appears that the interaction between reservoir and dam has significant influence on the hydrodynamic responses during earthquake excitations.
Estimating earthquake magnitude
Authors: Freedman Helen W.
Published: July 2021

abstract

The disturbingly large variation in magnitude estimates observable in any earthquake catalogue has led to this analysis of one particular catalogue and definition of magnitude. The method employed by the United States Coast and Geodetic Survey is analyzed in the light of theoretical considerations concerning the probability distribution of the variables involved. In particular, the truncation procedure seems to lead to estimates which consistently underestimate the magnitude for large earthquakes, and perhaps overestimate the magnitude for small ones. An alternative method is suggested which leads to estimates which are in good agreement with those provided by Berkeley, Pasadena and Lamont.
The earthquakes of 1965-66 in the Peloponnesus, Greece; a field report
Authors: Ambraseys N. N.
Published: July 2021

abstract

Between March 1965 and September 1966, two strong earthquakes occurred in central Peloponnesus (Greece). The first shock was connected with the west wall of the Megalopolis depression and it was followed by another shock connected with the opposite wall. The cumulative effects of these two earthquakes, and of two other deeper shocks that occurred further north, were considerable and difficult to separate. Landslides and slumping of the ground accounted for the majority of the damage which increased progressively with the shocks that followed. Submarine and coastal slumping caused additional damage and a small sea-wave was set off in the Gulf of Corinth. At the epicentral area the ground movements were severe, with a double amplitude on the record-plate of a standard Wilmot seismoscope reaching 60 millimeters.
A comparison of free oscillations of oceanic and continental earth models
Authors: Derr John S.
Published: July 2021

It is well-known that Rayleigh and Love waves over continental and oceanic structures have different periods (T) for a given order (n). In the present study, this difference is explored over the spectrum for periods greater than 30 sec, and particularly the graver fundamental spheroidal and torsional oscillations. The oceanic model is the same as the continental model below 400 km depth, is adjusted between 400 km and 10 km to preserve the Earth's overall mass and moment of inertia, and has 5 km of crust and 5 km of ocean. For torsional oscillations, the difference in period for a given n is about 3 sec throughout the range of 2 ≦ n ≦ 120, or 0.15 per cent to 3.3 per cent. However, the two curves for the group velocity are not parallel in this range: continental group velocity increases monotonically with T, while oceanic group velocity has a broad minimum for 160 < T < 200 sec. For spheroidal oscillations, the difference in period for a given n is about 2 sec for 2 ≦ n ≦ 120, or 0.1 per cent to 2 per cent. For n = 0, the difference is only 0.1 sec or 0.008 per cent. The two group velocity curves are almost parallel with minima in the same range, 205 to 210 sec. Comparison with the precision of available measurements of the free periods shows that the presence of an oceanic crust and upper mantle is important for fitting models of the earth to any set of free oscillation observations.
A note on the focal mechanism of the Parkfield, California earthquake of June 28, 1966
Authors: Algermissen S. T. , Harding Samuel T.
Published: July 2021

S-wave polarization studies of the Parkfield, California earthquake of June 28, 1966, indicate that a type II or double couple focal mechanism best fits the available data. The direction of greatest stress derived from the S-wave solution is in general agreement with results derived from geological evidence.
An analysis of seismoscope data from the Parkfield earthquake of June 27, 1966
Authors: Hudson D. E. , Cloud W. K.
Published: July 2021

An analysis of 16 seismoscope records from the Parkfield earthquake indicates the general usefulness of seismoscope data as a supplement to information produced by recording accelerographs. The pattern of attenuation of response spectrum values with distance from the fault was obtained, and certain anomalies in response spectrum values which may be related to local geologic structure were noted. The initial strong ground motions in the immediate vicinity of the fault were found to be predominately transverse to the fault. A direct comparison of seismoscope results with response spectrum curves calculated from recorded groud acceleration-time curves indicates an overall seismoscope accuracy under typical field conditions of 10-15 per cent.
The Parkfield, California, earthquakes of 1966
Authors: McEvilly T. V. , Bakun W. H. , Casaday K. B.
Published: July 2021

The characteristics of the Parkfield, California earthquake sequence of 1966 are presented. Historically, the epicentral region is one of the three most seismic areas along the San Andreas fault in central California. It is characterized, however, by a relatively high incidence of large earthquakes in proportion to smaller shocks, compared to other active zones. The 1966 sequence occurred in an area where measured deformation across the fault for 1959-1965 shows a decrease from about 2 cm/year to the north to zero to the south of the area. Neither micro-earthquake nor normal seismic activity prior to the sequence gave indication of its coming. Seismicity before the sequence was confined to the north of the active zone, with some indication of convergence of foci toward the location of the initial shocks. The early aftershock distribution extended 20 km south of the main shock; cracking occurred to 33 km south of the main shock; and intense aftershock activity for the entire sequence extended 27 km south of the main shock. At least 95 per cent of the earthquakes, including the three largest, have P-wave radiation patterns consistent with right lateral transcurrent motion on the San Andreas fault. Earthquakes of the sequence fall very closely along the fault trace. About 75 per cent of the total strain release for the sequence can be accounted for by earthquakes in the main shock region, the principal shock (M = 5.5) contributing only 25 per cent of the total. The sequence is characterized by a high incidence of large aftershocks, an extensive area of aftershock activity, and average focal depths near 5 km-three properties apparently related, and distinguishing two types of sequence traits in central California.
Deconvolution techniques for linear systems
Authors: Ko Hon-Yim , Scott Ronald F.
Published: July 2021

Superposition for linear systems is the process in which the input is convolved with the transfer function of the system, whereby the output is obtained. Deconvolution is, therefore, the reverse process in which an unknown input (or the transfer function) is calculated from the measured output and a known transfer function (or input). Several methods of deconvolution are developed and the advantages and disadvantages of each are discussed. Examples are given to demonstrate the applicability of the methods.
The Varto Üstükran (Anatolia) earthquake of 19 August 1966 summary of a field report
Authors: Ambraseys N. N. , Zátopek A.
Published: July 2021

abstract

The Varto-Üstükran earthquake of August 19, 1966, occurred near the east end of the Anatolian fault system in Turkey, very near where an earlier, less intense earthquake had caused damage in 1946. The earthquake had a magnitude 6.8 and it was felt over an area of over 300,000 square kilometers. The epicentral intensity did not exceed IX (MM) and there were signs of faulting over a length of about 30 kilometers. The earthquake killed some 2,500 and wounded 1,300 people. Over 19,000 houses of poor construction were destroyed and 100,000 people were made homeless.