bluuebunny/crossref_metadata_2025 huggingface dataset:
ANISOtime: Traveltime Computation Software for Laterally Homogeneous, Transversely Isotropic, Spherical Media
Authors: Konishi Kensuke , Borgeaud Anselme F. E. , Kawai Kenji , Geller Robert J.
Published: July 2021

Software packages for computing seismic traveltimes and raypaths in an isotropic, spherically symmetric, Earth model are well known and widely used. However, even though the theory for transversely isotropic (TI), spherically symmetric, models has been known since the late 1960s, readily available programs for traveltime calculations are restricted to isotropic models. We have developed a new software package, ANISOtime, for computing seismic traveltimes and raypaths in laterally homogeneous, TI, spherical media. This package calculates traveltime tables for both immediate and subsequent use. ANISOtime has both graphical user interface and command-line interface modes. The package is available for free public download. As it offers cross-platform compatibility through Java 8, it runs on Windows, macOS, Unix, and Linux.
Monotonic Neural Network for Ground-Motion Predictions to Avoid Overfitting to Recorded Sites
Authors: Okazaki Tomohisa , Morikawa Nobuyuki , Fujiwara Hiroyuki , Ueda Naonori
Published: July 2021

Data-driven machine-learning approaches are being increasingly applied to construct empirical ground-motion models (GMMs). It is a standard practice to divide observational records into learning and test datasets to correctly evaluate the predictive performance of a developed model. However, in this study, we show that division based on records or earthquakes is inappropriate for evaluating the generalization performance on recorded sites when GMMs include site-condition proxies as input variables. Complex models exhibit small residuals at sites used in the training process, but exhibit large residuals at new sites owing to overfitting to the trained sites. As a simple solution, we propose a neural network model that has monotonic dependence on some of the input variables. The model successfully obtains the generalization performance on recorded sites, although it lacks ability to represent oversaturation with input variables suggested in extreme ground-motion ranges. Therefore, alternative methods should be investigated to develop robust data-driven models under general conditions. Dividing the sites into learning and test data would play a fundamental role in developing such robust models.
On the possibility of the seismic detection of explosions from the signs of first onsets
Authors: Scheidegger A. E.
Published: July 2021

It is shown that, for an earthquake region, critical areas can be derived inside which one would expect seismic stations to observe a dilatation in the first onsets. Thus, if compressions are observed inside the critical area, one has grounds for the assumption that the “earthquake” was an explosion. As an example, critical areas are calculated for Turkmenia and the Tien Shan region.
Resonances in seismic noise under arctic sea-ice
Authors: Milne A. R. , Clark S. R.
Published: July 2021

abstract

Spectrograms of underwater seismic noise between 0.2 and 2.0 cps from the bottom of relatively shallow ice-covered seas show a line structure which indicates that vertical resonances occur within layers bounded at depth by the Mohorovičić discontinuity. The line structure appears with most clarity when recognizable transients are excluded. Under these quasi-stationary noise conditions, measurements of pressure spectra indicate that the seismic noise of the observed level can be generated by a vertical bobbing of the ice field as a whole.
First motions from seismic sources near a free surface
Authors: Burridge R. , Lapwood E. R. , Knopoff L.
Published: July 2021

abstract

The radiation patterns of first motions are calculated for the sudden occurrence of an arbitrarily oriented fault (dislocation) at the surface of a half space; the dislocation in the fault plane is also arbitrarily oriented and is assumed to occur over a very small area of the fault plane. Initially the source is considered at a finite depth and the solution is obtained by allowing the depth to tend to zero.

In general the results show a surprising directionality for the radiation of SV. In the focal plane projection the first motions of P and SH for a strike-slip fault show the familiar four-lobed radiation patterns. The first motions of SV show some reversals in polarity with angular distance from the source.

The first motions for all components of motion for a dip-slip fault have characteristics governed strongly by the presence of the free surface, and hence differ markedly from the usual radiation patterns for a deeply imbedded source.
The Hindu Kush earthquake of March 4, 1949 as recorded in Europe
Authors: Lehmann I.
Published: July 2021

abstract

The European records from distances 36°-50° of the deep Hindu Kush earthquake of March 4, 1949 were studied. The many clearly recorded deep-focus reflections lend to the records a characteristic appearance which is repeated in many other shocks from the same focal region. The ratios of the amplitudes of these phases vary somewhat from one shock to another. In the shock here considered sP and sPP are exceptionally large at most stations; in the Italian stations they are not so large, while pP is a clear phase. pP is not very well defined at most other stations.

Most of the 1949 records were from the old type long-period instruments having their highest magnification for periods from about 5 sec to 12 sec. Present day instruments of quite short or of very long proper period while admirable for many purposes do not record waves in this period range very well and therefore do not produce a satisfactory picture of the forerunners of earthquakes. The difference between the records obtained on different instruments is illustrated.

It is shown in examples that the amplitude ratio PP:P may differ strongly at the same epicentral distance and also that pP may vary greatly with azimuth.

The deficiency of station readings is noted.

Travel times and their residuals are tabulated and travel times plotted versus epicentral distances.
Central U.S. crust—Upper mantle structure from Love and Rayleigh wave phase velocity inversion
Authors: McEvilly T. V.
Published: July 2021

abstract

Fundamental mode Rayleigh and Love wave phase velocities in the period range 5 to 80 seconds have been measured in the central U. S. Using the concept of constant partial derivatives of phase velocity with respect to layer parameters in a least-squares inversion scheme a structural model for the area was determined. Several parameters were constrained on the basis of other available data. It was not possible to fit satisfactorily both Rayleigh and Love data with a simple isotropic model. Conclusions require either anisotropy in the upper mantle or systematic errors in the phase velocity measurements.
Detection of small arrivals
Authors: De Bremaecker J. Cl.
Published: July 2021

Even, inverse noise filters have been used in an automatic scheme for the detection of small arrivals. Precursors have not been created by this method. The significance level is estimated by computing the number of points in a normal noise record which have the same amplitude. The method has been successfully tested on actual earthquakes. Starting with a digitized seismogram, it automatically finds the arrivals, prints their time, their direction, and their standard deviation, and plots the spectra and the seismograms. Examples are given.
Auditory discrimination of seismic signals from earthquakes and explosions
Authors: Frantti G. E. , Levereault L. A.
Published: July 2021

Magnetic tape recordings of short-period seismic signals from approximately 200 earthquakes and explosions were time-compressed by a factor of up to 512 to shift seismic frequencies to the audible range. These seismic data include the inhomogeneities introduced by substantial variations in the locations of sources and receivers (world-wide), propagation path length (32 to 7000 km), and source magnitude (M = 0.5 to M = 6.5). Subjects were trained with a representative set of the “seismic sounds.” Auditory experiments were conducted to determine the ability of the human auditory system to distinguish between seismic signals from earthquakes and explosions. The results of the experiments suggest that a trained listener can identify approximately two-thirds of the seismic sounds presented, where one half corresponds to chance performance.
Transmission of Rayleigh waves past a step change in elevation
Authors: Mal A. K. , Knopoff L.
Published: July 2021

Using a Green's function method of approximation, transmission and reflection coefficients are computed for the problem of Rayleigh waves incident upon a step change in the elevation of the surface of a homogeneous half-space. Several versions of these approximations are given, differing principally by the method in which the surface waves on the vertical face of the step are taken into account.
A seismic refraction and gravity study of the earth's crust in British Columbia
Authors: White W. R. H. , Savage J. C.
Published: July 2021

Explosion refraction studies have been carried out in areas adjacent to Vancouver Island, including the Strait of Georgia and Johnstone Strait and along the west coast of the Island. A refraction line has also been observed from a large explosion in Seymour Narrows, eastward through the mountains and for some distance across the plains of Alberta. Although the surficial strata complicate the interpretation, a series of short refraction profiles consistently reveal the presence of an intermediate layer with velocity of about 6.8 km/sec. in the coastal area. The depth to the upper boundary of this layer varies from about 11 km. along the west coast of the Island to less than 5 km. in the Strait of Georgia and along the east side of the Island. The longer range observations parallel to the coast indicate this layer to be more than 40 km. thick. On the profile eastward through the mountains a velocity of 7.8 km/sec. has been found for Pn, from an unreversed profile, with a crustal thickness of approximately 30 km.
Computer simulation of multi-storey structures subjected to ground motion
Authors: Ward H. S.
Published: July 2021

A hybrid computer system has been used to simulate some five-storey structures excited by ground motion. Five types of buildings were investigated to find the effect of some structural parameters on the dynamic response of the buildings. The factors that have been considered are the distribution of stiffness and mass in the structure, the type of foundation condition, and the action of viscous damping.

The results indicate that it may be reasonable to calculate the base shear forces, created by ground motion, as a function of the fundamental period of the building. In the case of lightly damped structures, however, the distribution of forces through the height of the structure is also shown to be dependent on the other dynamic characteristics of the building, as well as the frequency content of the ground motion.

Viscous damping considerably reduces the forces acting on a structure, and also tends to eliminate modes of vibration other than the fundamental. A pinned-end foundation condition is also shown to reduce the forces acting on the lower storeys of a building compared with the fixed-end condition.
Iberian Peninsula crustal structure from surface waves dispersion
Authors: Payo G.
Published: July 2021

abstract

Phase velocity of Rayleigh waves traveling from Toledo to Málaga have been determined from seven selected earthquakes. The direction of approach of the wave front in relation to the ground particle motion at the standard Spanish stations (Toledo and Málaga) and at Porto (Portugal) is discussed.

Also these three Observatories are considered as a triangular array to determine the phase velocity through the lberian Peninsula region; this result is compared with the dispersion data determined by the two stations at Toledo and Málaga.

Two similar crustal-mantle structures, named IB1 and IB2, have been obtained for this Region, by modifying the models Dorman 8021 and CAN-SD respectively, and by means of the partial derivatives of the phase velocity with respect to the parameters of the layers. Both models IB1 and IB2 are almost identical and their corresponding dispersion curves fit the data with an error less than 0.1 km/sec. The crustal thickness given by these structures is about 33 km.

Group velocities of Love and Rayleigh waves from near earthquakes have been also studied. Some Algerian earthquakes yielded a dispersion curve for the arm of the Mediterranean Sea between Algeria and Spain. The curve appears almost typically oceanic.
Transmission of explosive sound through a fluid layer over an infinitely deep elastic bottom
Authors: Rosenbaum J. H.
Published: July 2021

abstract

The transmission of explosive sound through a thin fluid layer above an infinitely deep elastic bottom is described by means of an asymptotic theory. Special attention is paid to early oscillatory arrivals at large horizontal distances for the case of a high-velocity bottom, and it is found that these arrivals do not decay exponentially with horizontal distance. When the shear wave velocity in the bottom is low, the over-all response is very similar to the case of a fluid bottom, except for the Rayleigh mode, which becomes an important feature of the late signal. The response at long times may involve standing waves of finite horizontal wavelengths.
A portable strain meter with continuous interferometric calibration
Authors: Blayney J. L. , Gilman Ralph
Published: July 2021

A strain meter has been designed embodying techniques not used in earlier models. Minimum use of heavy and permanent auxiliary fixtures has made possible a portable instrument in which no components need be abandoned in order to change location. Provision for frequent or continuous interferometric calibration, and a photographic record therof, has provided uninterrupted strain measurements with long-term dependability. Also included is the usual continuous low-speed ink recorder for tidal and secular variations, and a high-gain band-pass recorder for the observation of long period waves and free oscillations of the earth. Details are given describing the method used for interpreting the interferometer image and its conversion to strain measurement.
The dynamic analysis of an apartment building
Authors: Shepherd R.
Published: July 2021

abstract

The normal-mode, response-spectra approach to the design of earthquake resistant structures forms the background to many design codes including the New Zealand one.

In order to apply these codes, and thus establish the seismic design loads, predictions of the elastic dynamic characteristics of a building must be made as part of the design process.

This paper describes the analysis undertaken using an electronic digital computer for the purpose of predicting the dynamic characteristics of one of New Zealand's tallest apartment buildings, the sixteen story Jerningham Apartments in Oriental Bay, Wellington.
Body wave radiation patterns from force applied within a half space
Authors: Gupta Indra N.
Published: July 2021

abstract

Expressions are derived for the horizontal and vertical displacements at an arbitrary depth within a homogeneous, isotropic, elastic half space when plane harmonic P, SV or SH waves are incident at any given angle. On the basis of the reciprocity theorem, these expressions represent also the far-field polar radiation patterns of P, SV and SH waves due to horizontal and vertical forces applied at a point within the half space. Numerical results for a few selected values of depth are shown for a solid half space.
The effect of initial stress or residual stress on elastic energy calculations
Authors: Burridge R. , Knopoff L.
Published: July 2021

abstract

The first aim of this paper is to define a stress-energy function which coincides with the conventional strain-energy function in a non-prestressed medium, but which has a more natural physical interpretation than strain energy for prestressed media.

The second aim is to show how consideration of prestress or residual stress may drastically change theoretical energy estimates for earthquakes.
Surface displacements due to an underground explosion
Authors: Bycroft G. N.
Published: July 2021

abstract

A simple solution to the problem of the surface displacements on an elastic half-space caused by an underground explosion is presented. The analysis is based on deriving the transient case for a point source from the steady state point source and then proceeding to the case of a finite spherical cavity by means of a retarded potential.

Theoretical values of maximum displacement compare favorably with measured values from the underground test shot Rainier, a nuclear explosion of Operation Plumbob.
A phase sensitive parametric seismometer
Authors: Rodgers P. W.
Published: July 2021

abstract

The steady-state response characteristics of a seismometer do not depend on the phase of the input. A phase-dependent response is obtained by parametric excitation of the seismometer, which is achieved with an electromagnetic spring. A theory is developed for this device and comparisons are made with experimental data. The construction of the seismometer and its application are discussed.
On the structure of the upper mantle
Authors: Lehmann I.
Published: July 2021

From apparent velocities of first arrivals across the Tonto Forest array for epicentral distances from 11° to 29° Niazi and Anderson derived a structure of the upper mantle deviating markedly from other structures as determined of late. It is found that on a different interpretation their data are in good accord with the Lehmann structure of the upper mantle.
Standing waves in a layered half space
Authors: Gupta I. N.
Published: July 2021

In a homogeneous elastic half space, plane harmonic waves give rise to standing waves only when on reflection there is no conversion from one wave-type to another. The existence of standing waves in a horizontally-layered half space is established for vertically propagating plane harmonic P, SV or SH waves. Expressions are derived for the particle displacements at the free surface and at any given depth. The layered system acts as a complicated filter suppressing certain periods while amplifying others. The results obtained may be helpful in an understanding of the ‘ground factor’, ambient seismic body-wave noise, and the vibration problem of a structure due to earthquake motion.
Dynamic response of a two-story steel frame structure
Authors: Bouwkamp J. G. , Blohm J. K.
Published: July 2021

A two-story steel rigid frame building with shear walls along two adjacent sides was subjected to dynamic tests. The tests were performed using a vibration generator to excite the building in its various modes.

Two rotational modes were investigated in considerable detail. Mode shapes and frequencies were determined for each mode of vibration, and values of damping were computed. A computer analysis of the building was made to study the comparison between experimental and analytical results.

The anticipated response of the building under an actual earthquake (El Centro) was determined also.
Model and theoretical seismicity
Authors: Burridge R. , Knopoff L.
Published: July 2021

abstract

A laboratory and a numerical model have been constructed to explore the role of friction along a fault as a factor in the earthquake mechanism. The laboratory model demonstrates that small shocks are necessary to the loading of potential energy into the focal structure; a large part, but not all, of the stored potential energy is later released in a major shock, at the end of a period of loading energy into the system. By the introduction of viscosity into the numerical model, aftershocks take place following a major shock. Both models have features which describe the statistics of shocks in the main sequence, the statistics of aftershocks and the energy-magnitude scale, among others.
Scale-model study of refraction along an irregular interface
Authors: Howell B. F. , Baybrook T. G.
Published: July 2021

abstract

An irregular boundary scatters waves refracted along it, acting as a low-pass filter. This effect is similar to that produced by refraction along an intermediate velocity layer or by a linear-gradient transition zone. Such scatter could explain why reflections from the Mohorovicic discontinuity are hard to record. If this is the case, they should be easier to record if frequencies above 5 cps are filtered out of the seismogram.
Scattering of steady elastic waves by surfaces of arbitrary shape
Authors: Sharma Darbari L.
Published: July 2021

abstract

The three-dimensional problem of scattering of steady elastic waves from an arbitrarily shaped body is formulated in terms of simultaneous singular integral equations. These basic equations defining the displacement potentials at the surface are two-dimensional Fredholm integral equations of the second kind over the surface of the body. In contrast to other formulations, this procedure is not restricted to a particular shape of the scattering object or to the nature of the incoming wave. However, by assuming the scatterer to be a body of revolution, the integral equations are reduced to one-dimensional Fredholm equations along the axis of the scatterer. The formulation is presented for a fixed rigid scatterer.

The integral equations are solved by approximating the potential functions by a set of polynomials and by satisfying the integral equations in the sense of least squares. The solution yields the surface potentials from which the field potentials or the field displacements are obtained by surface integrals. The numerical results are in good agreement with those obtained by the eigenfunction method for an incoming wave which is a plane shear or longitudinal wave.
Investigation of a method for determining stress accumulation at depth
Authors: Eisler Joseph D.
Published: July 2021

abstract

Measurement of stress accumulation at depth could serve as the basis for predicting earthquakes. Based on laboratory measurements of rock samples, stress accumulation in the earth can be estimated from changes in the compressional velocity of seismic waves traversing the stressed region, provided the travel times can be determined with precision. An initial field experiment performed in the Gabilan Range near Salinas, California, using 100-kilogram charges, demonstrated that seismic energy could be reliably detected up to 42 kilometers away, that the character of the recorded signal was closely repeatable for successive shots, and that the precision of timing was better than ±1 millisecond.
Local seismic monitoring—Fairview Peak Area, Nevada
Authors: Westphal W. H. , Lange A. L.
Published: July 2021

Four portable seismographs were used in January-February and April to August 1965 to record local earthquakes in the Fairview Peak Area of west-central Nevada. During these recording periods, which totalled 129 days, over 1300 local shocks were recorded of which 224 were large enough for determination of focal points. These hypocenters defined the active faults of the region and provided a means for mapping the probable extent of these faults at depth.
Preliminary determination of crustal structure in the Katmai National Monument, Alaska
Authors: Berg Eduard , Kubota Susumu , Kienle Jurgen
Published: July 2021

Seismic and gravity observations were carried out in the active volcanic area of Katmai in the summer of 1965. A determination of hypocenters has been aftempted using S and P arrivals at a station located at Kodiak and two stations located in the Monument. However, in most cases, deviations of travel times from the Jeffreys-Bullen tables were rather large. Therefore hypocenters are not well located. A method based on P- and S-wave arrivals yields a Poisson's ratio of 0.3 for the upper part of the mantle under Katmai. This higher value is probably due to the magma formation. The average depth to the Moho from seismic data in the same area is 38 km and 32 km under Kodiak. Using Woollard's relation between Bouguer anomaly and depth to the Moho, a small mountain root under the volcanoes with a depth of 34 km was found dipping gently up to 31 km on the NW side. The active volcanic cones are located along an uplift block. This block is associated with a 35 mgal Bouguer anomaly. The Bouguer anomaly contour map for the Alaska Peninsula is given and an interpretation attempted.
Seismoscope results from three earthquakes in the Hollister, California, Area
Authors: Cloud William K.
Published: July 2021

Seismoscope records from a magnitude 5.0 earthquake on January 19, 1960, and from magnitudes 5.6 and 5.5 earthquakes on April 8, 1961, in the Hollister, California, area, like those from the Parkfield, California, main shock of June 27, 1966, indicate that the maximum amplitude of transient motion in close proximity to the San Andreas Fault was oriented at a large angle to the strike of the fault.
Vibration and soil-structure interaction tests of a nine-story reinforced concrete building
Authors: Jennings Paul C. , Kuroiwa Julio H.
Published: July 2021

abstract

The Millikan Library Building, a nine-story reinforced concrete shear-wall structure at the California Institute of Technology, was tested dynamically by means of two eccentric mass vibration generators located on the ninth floor. The response levels ranged up to a maximum acceleration of 0:02 g. The natural periods of vibration, the mode shapes and the energy dissipation were measured for the first and second E-W translational modes, the N-S fundamental mode and the first torsional mode. Soil-structure interaction was investigated by measuring foundation motion and nearby soil surface movements during resonent vibrations in the N-S and E-W fundamental modes. Other tests included “man-excited” vibrations at low stress levels and a measurement of resonance of an air handling unit on the roof, which was found to magnify the roof response by a factor of 8.5.

The measured fundamental periods were short compared to typical framed structures of this height, 0.50 sec in the N-S direction, 0.66 in the E-W direction and 0.46 in torsion. These values increased roughly 3 per cent over the range of testing. The energy dissipation as measured by a viscous damping factor, varied between 0.70 and 2.00 per cent of critical. This large variation over the testing range indicates that tests at higher stresses are needed to determine the energy dissipation expected during the response to strong earthquake motions.

The soil-structure interaction measurements showed that the building responded very nearly as if fixed at the foundation; rocking contributed less than 1 per cent to the total roof motions of the structure and foundation translation about 2 per cent. Although negligible as far as the building motion is concerned, the results demonstrate the possibility of performing full-scale soil-structure interaction experiments.
A source of damping produced by the interaction of close-standing buildings
Authors: Rea Dixon , Bouwkamp Jack G.
Published: July 2021

abstract

A multi-story steel-framed building was subjected to dynamic tests at various stages of construction. In the interval between two test series, a dramatic change occurred in the building's dynamic properties. In the first series, the damping capacities of the two fundamental modes (East-West and North-South) were about 2 per cent of critical; of the other modes, 0.4 per cent to 0.9 per cent of critical. In the second test series, the damping capacities of all modes ranged from 5 per cent to 10 per cent of critical.

This increase in damping has been attributed to the interaction of the building with its service tower. The second test series was conducted after the service tower was structurally complete. The two buildings were found to be dynamically coupled, with connections at only the basement and foundation levels. Non-structural connections, in the form of steel plates providing access between the buildings, existed at every floor level and dissipated energy in relative motion between the buildings. From experimental data it was possible to estimate the amount of energy this source of Coulomb damping could dissipate and to show that it was responsible for the large change noted in damping capacity.
Head waves from a transition layer
Authors: Nakamura Yosio
Published: July 2021

abstract

Ultrasonic, seismic-model experiments have been performed to re-examine the nature of head waves from a transition layer. Two-dimensional, layered structures, some with a transition zone and some with a sharp discontinuity, constructed by lead-aluminum laminations have served as the models. Amplitude and phase responses have been measured in a frequency range of 25 kHz to 200 kHz. At low frequencies, where the wavelength is much longer than the thickness of the transition zone, little difference is observed between head waves from a transition zone and those from a sharp discontinuity. At a certain frequency range where the wavelength is close to the thickness of the transition zone, the measured head-wave amplitude from a transition zone becomes a few times larger than that from a sharp discontinuity. This is characteristic of head waves from a poorly defined boundary and may be used to estimate the thickness of a general transition layer. A sharp, high-frequency cutoff is again observed, but the cutoff frequency is not consistent with previous studies.
New techniques for the determination of surface wave phase velocities
Authors: Bloch S. , Hales A. L.
Published: July 2021

abstract

A number of new techniques have been developed for the determination of phase velocities from the digitized seismograms from pairs of stations. One of these techniques is to Fourier analyze the sum (or difference) of the two seismograms after time shifting in steps to correspond to steps in phase velocity. The amplitude of the summed seismogram is a maximum for any particular period when both seismograms are in phase at that period. Another method is to pass both seismograms through a narrow bandpass digital filter centered at various periods and form the cross product of the filtered seismograms, after time shifting. The average of the resultant time series is a maximum when the two signals are in phase. The computer output is a matrix consisting of amplitudes or averages as a function of phase velocity and period. The phase velocity dispersion is determined from the contoured matrix. Using these techniques, interstation phase velocities of Rayleigh waves have been determined for the “World Wide Network Standard Stations” at Pretoria, Bulawayo and Windhoek. The method using cross-products is the most efficient.
Diffraction of elastic waves by a rigid 90° wedge Part I
Authors: Kraut Edgar A.
Published: July 2021

abstract

The problem of calculating the reflected and diffracted elastic waves generated when a plane compressional wave strikes a rigid quarter plane is formulated as a Wiener-Hopf problem in two complex variables. It is shown how a closed form solution of this two variable Wiener-Hopf problem can be obtained provided that the Laplace transform of the elastic wave Green's function can be factorized into factors having suitable analytic properties.
Seismological measurements and measurement error
Authors: Freedman Helen W.
Published: July 2021

A seismological measurement, such as arrival time or, less directly origin time, is an example of a measurement variable which can be considered as the sum of a parameter—the quantity being measured—and an error variable. Optimal methods for the estimation of this parameter vary with the probability distribution of the error variable. In particular, estimation in the presence of bias or of gross errors is discussed, together with the related problem of precision versus accuracy of the estimate.

Errors in estimates of arrival times, origin times and hypocentral location contribute to variation in travel-time estimates; these are analyzed separately. Each of these, with the exception of focal depth, has a distribution which can be fitted to a mixture of a normal distribution and some contamination. The degree of contamination varies; methods for truncation are suggested. The presence of possible, often undetectable, bias in locations and travel times may make confidence statements about these parameters unreliable.
Estimation ofPKPtravel times
Authors: Bolt Bruce A.
Published: July 2021

A set of travel times for the core-waves PKP is derived from observations and tabulated. All values are tied to the 1968 P travel times for a surface-focus for 75° < Δ < 95° due to Herrin et al. Times for both the branches DF (110° < Δ < 180°) and GH (125° < Δ < 156°) are listed in more detail than previously, together with an extension for PKiKP waves, from 110° back to 105°. Checks on the DF branch for 120° < ° < 150° using observations from the 1965 LONGSHOT explosion provide no evidence for further revision of the first section of this branch.

Special attention is given to the times of the DF branch (PKIKP) for ° > 150°. Delays at 153° and 162° of order 3 sec reported by K. Ergin in 1967 are not confirmed. There is a group of small arrivals about 1 sec earlier than predicted by the present PKIKP tables for 156° < Δ < 160°, approximately. These may indicate an error in the curvature of the present curve and, perhaps, a slight discontinuity in the inner core near a radius of 850 km. No evidence is found for a layer of any consequence with reduced P velocity in the inner core.

Revised times for the branch AB (denoted by PKP2) are based mainly on readings from the Indian Ocean earthquake of February 17, 1966 and the Jeffreys-Bullen times. For Δ > 155°, the new empirical times confirm the curvature of the Jeffreys-Bullen curve for PKP2; however, near Δ = 152°, the observed curvature appears to be greater than that of the latter curve. The new times are of the order of one second earlier; this result is highly sensitive to the precision of the epicenter estimated for the 1966 shock. PKP2 is traceable at least out to 185°.

Depth allowances for PKP have been computed, by T. Qamar, to provide a set consistent with the model Earth used by Herrin et al in the 1968 P tables.
Epicentral uncertainties and detection probabilities from the Yellowknife seismic array data
Authors: Manchee E. B. , Weichert D. H.
Published: July 2021

Analog recording tapes from the Yellowknife seismic array have been processed diqitally in Canada for over a year, with concentration on the automatic detection and epicenter location of events between 26° and 90° distance, using short period P-wave arrivals.

For the purpose of detection, signals from the individual seismometers in the two arms of the cross array are analog band-pass filtered, digitized at 20 samples/s, multiplexed into a digital computer, velocity and azimuth filtered and correlated, using an exponentially weighted integration over time with an equivalent width of 1.6 s. The correlograms for up to 168 phased beams are scanned for values exceeding a preset trigger level and an event is recorded when the level is passed consistently a number of times. In late 1966, during a seismically quiet period and with the array fully operational, the 50 per cent automatic detection level achieved by this method for events in the Third Zone to Yellowknife was m4.0 ± 0.1, slightly better than the level of an analog trigger operated at the station which uses the correlogram method for a single unphased beam only. The 50 per cent detection level of the Yellowknife standard station is about m4.4 and thus the array-computer automatic detection method gives about Δm0.4 improvement, which is expected from the processing method used if the noise is largely uncorrelated. No significant variations in the detection level with azimuth have yet been observed.

Approximate epicenter locations are determined from the best apparent arrival vector. The best vector is assumed to be given by the maxima of parabolas at constant azimuth and wave number interpolated between the highest values of the correlograms.

USCGS P.D.E. information is used in conjunction with the J-B tables to calculate an expected apparent arrival vector. The difference between the expected and best interpolated arrival vectors has an average deviation of about 6 ms/km. Their distribution does not suggest a simple crustal or upper mantle cause under the array station.
Theoretical changes in seismic travel time with changes in stress at depth
Authors: Sawyers Kenneth N.
Published: July 2021

Use is made of second-order elasticity theory to derive an estimate of the change in travel time with stress for compressional waves which propagate along the principal stress directions in a homogeneous layer of material. A particular stress system is chosen which can be expected to produce a strike-slip fault. Laboratory data for Barre, Vt. granite, interpolated for a depth of approximately 1 km, are used to obtain a numerical result. The change in travel time is found to be 0.075 milliseconds per km per bar. Since timing precision of better than ±1 millisecond can be achieved for refraction paths up to some 40 km in length, it is concluded that stress changes of several bars are within the range of detectability in carefully executed field experiments.
Displacement on the San Andreas fault subsequent to the 1966 Parkfield earthquake
Authors: Smith Stewart W. , Wyss Max
Published: July 2021

Immediately following the 1966 Parkfield earthquake a continuing program of fault displacement measurements was undertaken, and several types of instruments were installed in the fault zone to monitor ground motion. In the year subsequent to the earthquake a maximum of at least 20 cm of displacement occurred on a 30 km section of the San Andreas fault, which far exceeded the surficial displacement at the time of the earthquake. The rate of displacement decreased logarithmically during this period in a manner similar to that of the decrease in aftershock activity. After the initial high rate of activity it could be seen that most of the displacement was occurring in 4–6 day epochs of rapid creep following local aftershocks. The variation of fault displacement along the surface trace was measured and shown to be consistent with a vertidal fault surface 44 km long and 14 km deep, along which a shear stress of 2.4 bars was relieved.
Shear waves from a nuclear explosion in a salt cavity
Authors: Perret William R.
Published: July 2021

Shear waves have been resolved in records of acceleration and particle velocity resulting from the Sterling event which was a nuclear explosion (380 tons) within the stable cavity left by the Salmon detonation in the Tatum salt dome. These shear waves were strongest in records of vertical motion at stations in the shot-level plane, but were detectable in records from both vertical and horizontal radial gages above and below that elevation. No appreciable shear waves could be identified in records from shot-level gages oriented horizontally. All instruments involved in this study were situated within the salt dome at ranges between 166 and 660 meters from the explosive source. Shear waves carried over 95 per cent of the energy in vertical shot-level records, but they represented less than 11 per cent of the energy in horizontal radial shot-level records. Cause of the vertically polarized shear waves is considered to be vertical asymmetry at the source.
Liquefaction of soils induced by earthquakes
Authors: Ambraseys N. , Sarma S.
Published: July 2021

abstract

The conditions under which liquefaction effects are known to have occurred are reviewed and their generic cause is discussed. Special attention is given to the delay with which liquefaction effects appear on surface deposits originally stable. It is suggested that little damage to structures on poor soil can occur as the direct result of severe earthquake shaking and that damage observed is almost entirely due to foundation failures, ground settlements and tilting. Field evidence suggests that earthquakes are incapable of densifying originally loose deposits to a stable mass, and that such deposits neither experience nor transmit severe shaking.
Rays and travel times in a spherical anisotropic earth
Authors: Vlaar N. J.
Published: July 2021

abstract

A study is made of the propagation of stress discontinuities (wave fronts) in a radially inhomogeneous, tangentially isotropic model of an anisotropic spherical medium.

The results are given in the form of integrals for the ray paths and the travel times for the several body-wave phases.

A special case of anisotropy dealt with in Section 2.2 (3) gives rise to considerable simplifications. This case in particular appears to be useful for the construction of travel-time tables for an Earth in which anisotropy occurs.
A study of the Koyna earthquake of December 10, 1967
Authors: Gupta Harsh , Narain Hari , Rastogi B. K. , Mohan Indra
Published: July 2021

abstract

Data now available on the Koyna earthquake have been studied in detail. Different origin times and epicenters given by India Meteorological Department and Central Water and Power Research Station, field evidences and nature of the seismograms for this earthquake suggest a multiple event. A fault plane solution has been obtained by using the sense of first motions. Seismicity in Koyna region has been found to increase with the increase of water level in the reservoir and vice-versa with a certain time lag. The two major earthquakes of this region have similar foreshock-aftershock pattern, corresponding to type 2 of Mogi's (1963) models. Aftershocks of this earthquake are related by a function Log N = a + bM, value of b being −0.8. Possibility of predicting maximum expected magnitude at a certain seismic activity level of Koyna region has been also pointed out.
A note on the statistical interpretation of Båth's law
Authors: Vere-Jones D.
Published: July 2021

A theoretical version of Båth's law is derived on the assumption that aftershock magnitudes are independently and exponentially distributed. The results are not fully in agreement with the commonly accepted picture of Båth's law, but it is shown by reference to Utsu's table of Japanese aftershock sequences that the discrepancies may be due to bias introduced by the method of selecting the data for study.
Beam-steering of large randomly-spaced long-period arrays
Authors: Evernden Jack F.
Published: July 2021

Beam-steering of a U.S.-wide randomly-spaced array of long-period LRSM instruments is shown to yield a √n gain in signal to noise ratio for P signals and somewhat less for S signals. Bandpass filtering of these records yields an additional factor of 2 gain in signal to noise ratio.
Upper mantle structure in the Basin and Range province, western North America, from the apparent velocities of S waves
Authors: Kovach Robert L. , Robinson Russell
Published: July 2021

The variation of shear velocity with depth in the upper mantle for the Basin and Range province of western North America has been studied with direct measurements of dT/dΔ for S waves in the distance range 14° < Δ < 40°. Three orthogonal components of digital data were used and onset times were determined using the product of the horizontal radial and vertical components of motion and particle motion diagrams. A linear LRSM array in Arizona was used for the measurement of dT/dΔ.

An S-wave velocity distribution is derived, compatible with P-wave velocity models for the same region. The derived model consists of a thin lid zone of shear velocity 4.5 km/sec overlying a low-velocity zone and a change in velocity gradient at a depth of 160 km. Two regions of high-velocity gradient are located at depths beginning at 360 km and 620 km.
Universal dispersion tables
Authors: Anderson Don L. , Kovach Robert L.
Published: July 2021

The effect of a small change in any parameter of a realistic Earth model on the periods of free oscillation is computed for both spheroidal and torsional modes. The normalized partial derivatives, or variational parameters, are given as a function of order number and depth in the Earth. For a given mode it can immediately be seen which parameters and which regions of the Earth are controlling the period of free oscillation. Except for oSo and its overtones the low-order free oscillations are relatively insensitive to properties of the core. The shear velocity of the mantle is the dominant parameter controlling the periods of free oscillation and density can be determined from free oscillation data only if the shear velocity is known very accurately. Once the velocity structure is well known free oscillation data can be used to modify the average density of the upper mantle. The mass and moment of inertia are then the main constraints on how the mass must be redistributed in the lower mantle and core.
Dispersion of Rayleigh waves for purely oceanic paths in the Pacific
Authors: Piermattei Rodolfo , Nowroozi Ali A.
Published: July 2021

abstract

Thirty-five shallow, distant earthquakes located in the Pacific and recorded at the Ocean Bottom Geophysical station OBS III by a long-period vertical seismograph and a long-period crystal hydrophone were selected for analysis of the dispersion of Rayleigh waves. The sensors are part of the Lamont Geological Observatory's instrument package implanted in May 1966 approximately 200 km west of San Francisco at a water depth of 3.9 km. The location of the station and that of the epicenters, all in the ocean, give us for the first time the oppurtunity of studying purely oceanic paths. The group velocity dispersion curves in the period range 12-40 sec show minor regional differences in the oceanic crustal structure. For the comparison, dispersion curves were obtained for 24 of these earthquakes from the seismographs recorded at the Berkeley seismograph station, BKS. Most of the pairs of dispersion curves show no significant differences due to crossing the continental margin. However, the group velocities of Rayleigh waves from southern Alaska and Easter Island are higher at OBS III than at Berkeley by as much as 0.1 km/sec. Realizing that theoretical models based exclusively on surface-wave data are not unique, and taking Dorman's oceanic model 8099 as our starting point, we were able to fit our experimental dispersion curves using models characterized by a low-velocity zone starting at a depth of about 60 km. According to this type of solution the crust is thicker along the paths from south Alaska and Easter Island, parallel to the coast, than along the other paths examined. The pressure-to-displacement ratio (P/D) is not very sensitive to changes in the models for periods greater than 12 sec. It is, however, useful in determining the local sedimentary structure from short-period waves.
Analysis of seismic peak amplitudes from underground nuclear explosions
Authors: Murphy J. R. , Lahoud J. A.
Published: July 2021

abstract

Ground-motion data recorded from 99 underground nuclear event ranging in yield from about 1 to 1200 kilotons are analyzed. The data were measured at more than 500 different stations with the source-to-station distance ranging from less than 1 to approximately 600 kilometers. More than 2,900 acceleration, displacement and velocity peak resultant vector data points are included in the analysis. A comparison of the results of the statistical analysis with expected results using cube-root of yield scaling concepts, shows that the peak amplitude of displacement cube-root scales whereas peak acceleration and velocity do not cube-root scale. Relative differences in the yield and distance exponents, n and m, obtained from the statistical analysis, are explained in terms of differences in the frequency content of recorded ground motions.

The observed peak amplitude measurements from the events Boxcar and Benham are compared with predictions based on the statistical analyses.