bluuebunny/crossref_metadata_2025 huggingface dataset:
Azimuth and slowness anomalies of seismic waves measured on the central California seismographic array. Part II. Interpretation
Authors: Otsuka Michio
Published: July 2021

abstract

An interpretation is given here of the azimuth and slowness anomalies for P waves measured on the seven northern stations in the telemetry seismographic array in central California which was described in Part I of this paper. The array provides a new method of investigating the structure of the upper mantle in an important oceanic-to-continental transition region. The structural feature which is primarily responsible for these anomalies is apparently located in the upper mantle within a depth of order 100 km of the surface. Examination shows that if these anomalies are attributed to a thickening crust alone, a dip of the Mohorovičić discontinuity as steep as 20° must be assumed. Further analysis suggests a solution having a moderately dipping Mohorovičić discontinuity below which a second, deeper interface has a velocity decrease downward and a steep dip toward the ocean.

An alternative hypothesis to that of Press and Biehler is formulated which explains traveltime anomalies of certain California stations in terms of a regional structural trend.
The field from an SH-point source in a continuously layered inhomogeneous medium: I. The field in a layer of finite depth
Authors: Vlaar N. J.
Published: July 2021

abstract

Expressions are derived for the field from an SH point source in a stratified heterogeneous layer of finite depth. It is found, that for a periodic disturbance, the contribution to the far field is mainly due to at most a finite number of unattenuated normal Love modes. The transient response of the medium is obtained by a Fourier synthesis. The final expressions are of a simple form, involving the eigenfunctions of a Sturm-Liouville boundary value problem. The excitation of a certain mode as a function of frequency and source depth is formulated in a concise form.
The earthquake sequence of November 1964 near Corralitos, California
Authors: McEvilly Thomas V.
Published: July 2021

abstract

A sequence of more than 100 aftershocks with magnitudes as low as −0.1 was recorded following a magnitude 5.0 earthquake on November 16, 1964, in the San Andreas fault zone of central California. The sequence was monitored in detail by three temporary seismographic stations at distances less than 15 km and the surrounding telemetry array. Nearly all of the 35 earthquakes which could be located clustered in a focal region about 4 km in diameter at a depth near 12 km and exhibited uniform first motion radiation patterns. First motion fault plane solutions are consistent with the right lateral transcurrent motion characteristic of the San Andreas fault. Exceptions to this uniform radiation pattern in the concentrated focal region occurred near the times of two large aftershocks apparently on another fault about 5 km away.
Coupled bending and torsional oscillations of a modern skyscraper
Authors: Medearis Kenneth
Published: July 2021

abstract

The use of electronic digital computers for solving complex engineering problems has instigated significant changes in solution methods. Matrix algebra and numerical analysis techniques are now being utilized extensively in dynamic and static structural analysis. Seismic structural problems which were previously solved using approximate methods, because of the impossibility of “exact” numerical calculations, are now being solved using these techniques in conjunction with computers.

Plresent day building codes advocate “static methods” of seismic structural anaysis where in the engineer is directed to use coefficients for determining design lateral loads. Empirical formulas are given for calculating T, the natural period of vibration of a structure. However, uncertainty exists regarding the empirical calculation of T for unsymmetrical structures subject to coupling between their bending and torsional vibratory motions. Dynamic coupling occurs when the center of mass and the center of rigidity of a structure are not coincident, resulting in motions, under seismic excitation, which may be of prime importance.

The tallest structure in San Francisco, the forty-three story Wells Fargo Building, is subject to such coupling. The theoretical methods of analysis utilized in determining the dynamic characteristics of this uni-directionally symmetric structure are described in this paper. With three possible vibratory motions per story, two translational and one rotational, the resulting problem is seen to be a formidable one involving one hundred twenty-nine degrees of freedom. Using normal mode theory in conjunction with a matrix formulation of the governing coupled differential equations, frequency and vibratory mode solutions are obtained using a digital computer.
Surface waves across the Pacific
Authors: Saito Masanori , Takeuchi Hitoshi
Published: July 2021

Making use of Rayleigh and Love wave dispersion data, Santô divided the Pacific into seven regions. From his map and compiled group velocity dispersion curves, upper mantle structure in the Pacific in which the depths of the low velocity layer and the shear wave velocity are changing systematically from continent to ocean is obtained. In orogenic regions such as Japan and its surroundings, extremely low velocity layer in which the shear wave velocity is about 4.3 km/sec is just under the Moho. In the oceanic side of this region, the layer is overlain by the normal mantle material with shear wave velocity of about 4.6 km/sec and in the pure oceanic region this extremely low velocity layer disappears. The so-called ‘low velocity layer’ which is believed to begin at the depth of about 60 km under the ocean is present in the oceanic region but the shear wave velocity in the layer may be a little higher than that obtained by earlier works.
The field from an SH point source in a continuously layered inhomogeneous half-space
Authors: Vlaar N. J.
Published: July 2021

The propagation of waves due to the presence of an SH point source in the interior of a piecewise continuously stratified half-space is studied. The physical parameters governing the wave propagation, i.e. the rigidity and the density, are assumed to be arbitrary piecewise continuous functions of depth with constant finite limiting values as depth goes to infinity. The analysis is based on spectral theory of boundary value problems associated with ordinary linear second order differential equations. It is found for the time harmonic case that the final field representations are given in the form of a finite residue series, plus a branch line integral, the first representing the normal mode contribution to the field. The field expression appears to have a symmetrical form with respect to field point depth and source depth, involving solutions connected with free wave propagation. This enables one to draw immediately conclusions regarding the influence of the source depth and the frequency on the spectral excitation of the normal modes if numerical knowledge of free Love waves is assumed to be known.
Crustal refraction profile, Oregon coast range
Authors: Berg Joseph W. , Trembly Lynn , Emilia David A. , Hutt Jeremy R. , King John M. , Long Leland Timothy , McKnight William R. , Sarmah Survya K. , Souders Robert , Thiruvathukal John V. , Vossler Donald A.
Published: July 2021

Seven temporary and three permanent seismograph stations recorded seismic waves to a distance of about 270 km from a 110,000 pound quarry blast detonated near Depoe Bay, Oregon. The recording stations were in a north-south line along the northern coast range of Western Oregon and Washington. The travel-time data indicate an apparent shallow crustal thickness (about 16 km) for this region. The time versus distance data were not continuous beyond 130 km from the source which may have resulted from any combination of the following causes: (1) insufficient source energy; (2) lateral geological variations; and/or (3) a subcrustal negative velocity gradient.
On a moving stress discontinuity in a porous, liquid-saturated solid
Authors: Mukhopadhyay Arabinda
Published: July 2021

The stresses produced in a liquid-saturated porous solid by a moving discontinuity of shear stress inside the medium have been considered in this paper. The solutions for the stresses have been obtained in simple closed forms for certain types of moving stress discontinuity. It has been found that the nature of the stresses depends considerably on the velocity of the moving stress discontinuity. If this velocity exceeds a certain value, depending on the nature of the medium, the solutions for the stresses are found to contain delta functions, which are taken to represent discontinuities of stress along certain lines.
Short-period seismic noise
Authors: Douze E. J.
Published: July 2021

abstract

This report consists of a summary of the studies conducted on the subject of short-period (6.0-0.3 sec period) noise over a period of approximately three years. Information from deep-hole and surface arrays was used in an attempt to determine the types of waves of which the noise is composed. The theoretical behavior of higher-mode Rayleigh waves and of body waves as measured by surface and deep-hole arrays is described. Both surface and body waves are shown to exist in the noise. Surface waves generally predominate at the longer periods (of the period range discussed) while body waves appear at the shorter periods at quiet sites. Not all the data could be interpreted to define the wave types present.
Statistical properties of Rayleigh waves due to scattering by topography
Authors: Hudson J. A. , Knopoff L.
Published: July 2021

abstract

The two-dimensional problems of the scattering of harmonic body waves and Rayleigh waves by topographic irregularities in the surface of a simplified model of the earth are considered with especial reference to the processes of P-R, SV-R and R-R scattering. The topography is assumed to have certain statistical properties; the scattered surface waves also have describable statistical properties. The results obtained show that the maximum scattered seismic noise is in the range of wavelengths of the order of the lateral dimensions of the topography. The process SV-R is maximized over a broader band of wavelengths than the process P-R and thus the former may be more difficult to remove by selective filtering. An investigation of the process R-R shows that backscattering is much more important than forward scattering and hence topography beyond the array must be taken into account.
Body waves as normal and leaking modes. Part I: Introduction
Authors: De Bremaecker J. Cl.
Published: July 2021

abstract

Realistic artificial seismograms may be computed by considering body waves as sums of normal or leaking modes of surface waves: the S wave and those arriving after S may be considered as sums of higher normal modes of Rayleigh waves (RiN) and Love waves (LiN); in this case the apparent velocity c < βn. Earlier arrivals are generally due to the first kind of leaking modes of Rayleigh waves (RiL1) for which βn < c < αn. Deep reflections in seismic prospecting are RiL2 for which c > αn. Synthetic seismograms can be computed by double Fourier transformation in those two last cases. Alternately the method of residues followed by a single Fourier (or Laplace) transformation may be used in all cases. Earth-stretching approximations should give excellent results for SH waves and may give satisfactory results for P and SV waves.
Reflection and transmission from a plane layered core-mantle boundary
Authors: Teng Ta-Liang
Published: July 2021

abstract

A class of transfer functions in terms of layer matrices is derived, giving the transmission and reflection of plane harmonic P or S waves incident from either side of a plane layered core-mantle boundary. A computer program coded in complex arithmetic for the IBM 7094 is used to evaluate these functions. Numerical values obtained from five suggested models of the core-mantle boundary are compared and discussed. The aim is to formulate the method, and to establish some general guide, for the studies of the structure of the core-mantle boundary and the affenuation of seismic waves inside the core.
Master curves for the response of layered systems to compressional seismic waves
Authors: Fernandez Luis M.
Published: July 2021

abstract

The layers of the earth's crust act as a filter with respect to seimic energy arriving at a given station. Consequently the motion recorded at the surface depends not only on the frequency content of the source and on the response characteristics of the recording instrument, but also on the elastic parameters and thicknesses of the transmitting layers. This latter dependence is the basis for a method of investigating the structure of the crust and upper mantle.

To facilitate this investigation a set of master curves for the transfer functions of the vertical and horizontal component of longitudinal waves and their ratios is presented. The calculation of these curves is in terms of a dimensionless parameter. This calculation allows one to group the curves corresponding to different crustal models into families of curves. The characteristics of these curves are discussed from the point of view of their “periodicity” in the frequency domain and of their amplitude in order to investigate the influence of the layer parameters. Considerations, either of constructive interference or of Fourier analysis of a pulse multiply reflected within the layer system, reveal that the amplitudes of the transfer curves depend on the velocity contrasts at the interfaces of the system. The “periodicity” or spacing of the peaks depends on the time lags between the first arrivals and the arrivals of the different reverberations. Closely spaced fluctuations correspond to large-time lags, and widely spaced fluctuations to short-time lags.
Seismic wave arrivals from LONGSHOT, 0° to 27°
Authors: Carder D. S. , Tocher Don , Buee Charles , Stewart S. W. , Eisler Joseph , Berg Eduard
Published: July 2021

abstract

Seismic wave arrivals from the LONGSHOT underground nuclear explosion were recorded at 27 locations in Alaska and two in Yukon Territory at distances between 2.6° and 26.6° from ground zero on Amchitka Island. Five of these stations were on the island chain, three on the Alaska Peninsula, two on other islands, and the remainder on the mainland from Anchorage to Point Barrow. The upper mantle travel times to distances of about 11° may be represented by t = Δ(km)/7.85 + 4.9 ± 0.5. The estimated depth to the M discontinity beneath the island chain is from 22 to 26 km depending on the choice of data.

In the distance range from 6° to 16°, recognition of the initial P wave may be a function of the signal-to-noise ratio. From 13° to 27°, travel times average from 1.5 to 4.0 seconds earlier than the JB surface-focus travel times with a steep gradient from “early” in central and northern Alaska to an average of 2 seconds “more early” in eastern Alaska including Circle Hot Springs, Anchorage, and Kodiak. PcP arrivals are recognized at 15.5° and beyond, and their travel times are parallel to the JB PcP times but are from 2 to 4 seconds earlier. PcP was not anomalously early at Kodiak as was P. P wave amplitude maxima to the Bering Sea stations of St. Paul Island and Cape Romanzoff were an order less than at like distances on the Peninsula.
Gravitationally perturbed elastic waves
Authors: Gilbert Freeman
Published: July 2021

abstract

In Lamb's problem there are two interface pulses. When the half-space is incompressible the locked Rayleigh (S) pulse travels with a speed γ = 0.9553β where β is the shear pulse speed, and the leaking Rayleigh (P) pulse travels with a speed δ = 1.966β. In the absence of gravity the S pulse is the more dominant pulse. The effect of gravity is to enhance the importance of the P pulse. In the presence of gravity, as β decreases, the S pulse becomes insignificant, and the P pulse becomes dispersive and approaches the behavior of the classical gravity wave. Several theoretical seismograms and hodographs, for the buried line source problem, are presented to illustrate the effect of gravity.

The effect of gravity on dispersion curves, for a layer with a rigid bottom, is to distort them in such a way that one should observe wave groups similar to the classical gravity waves in a fluid layer when the shear speed is small. In addition one should observe very slow wave groups with wavelengths shortened by gravity. Several dispersion curves are presented to illustrate these features.

The propagator coefficient matrix is given for a transversely isotropic material including gravity.
Analysis of current methods of interpreting strong-motion accelerograms
Authors: Schiff Anshel , Bogdanoff John L.
Published: July 2021

abstract

The influence on the calculated velocity and displacement and shock spectra of accelerogram vagaries is examined. The effect of centerline adjustment, small random reading errors, and chart paper distortion are considered in detail. An expression for the variance of the final displacement error, which is caused by accelerogram reading errors, is presented. This expression can be used to determine the reading accuracy required to evaluate the final ground displacement to desired accuracy. The utility of determining ground velocity and displacement is discussed. The effects of accelerogram reading errors on shock spectra is considered. The adequacy of current chart paper speeds and transducer natural frequencies is discussed.

It is concluded: (a) that the use of accelerograms to accurately determine net ground displacement is impractical; (b) that reading errors in accelerogram charts, as they are currently being recorded, can cause significant errors in response spectra; (c) that limitations of current seismographs may attenuate the high frequency content in strong-motion seismic shocks or distort low-frequency spectra due to aliasing.
A model study of elastic waves in a layered sphere
Authors: Gregson Victor
Published: July 2021

abstract

Elastic waves produced by an impact were recorded at the surface of a solid 12.0 inch diameter steel sphere coated with a 0.3 inch copper layer. Conventional modeling techniques employing both compressional and shear piezoelectric transducers were used to record elastic waves for one millisecond at various points around the great circle of the sphere. Body, PL, and surface waves were observed. Density, layer thickness, compressional and shear-wave velocities were measured so that accurate surface-wave dispersion curves could be computed. Surface-wave dispersion was measured as well as computed.

Measured PL mode dispersion compared favorably with theoretical computations. In addition, dispersion curves for Rayleigh, Stoneley, and Love modes were computed. Measured surface-wave dispersion showed Rayleigh and Love modes were observed but not Stoneley modes. Measured dispersion compared favorably with theoretical computations. The curvature correction applied to dispersion calculations in a flat space has been estimated to correct dispersion values at long-wave lengths to about one per cent of correct dispersion in a spherical model. Measured dispersion compared with such flat space dispersion corrected for curvature proved accurate within one per cent at long wave lengths.

Two sets of surface waves were observed. One set was associated with body waves radiating outward from impact. The other set was associated with body waves reflecting at the pole opposite impact. For each set of surface waves, measured dispersion compared favorably with computed dispersion.
Surface motion from pulse transmission through anisotropic layered structures
Authors: Robinson Edwin S. , Costain John K.
Published: July 2021

abstract

Theoretical surface motion for pulse transmission through transversely isotropic elastic structures is computed by Fourier inversion of the product of the Fourier transform of an input pulse, complex transfer coefficients for anisotropic structures, and a seismograph system response. Introduction of anisotropy causes subtle changes in the angles of refraction and shifts in arrival times of different phases as intuitively expected from Snell's law. Computations suggest no obvious criteria for recognizing anisotropy from surface motion.
Standing waves in layered fluid half-space
Authors: Gupta I. N.
Published: July 2021

abstract

The existence of standing waves in a horizontally-layered fluid half-space is established for plane harmonic compressional waves propagating from below. The angle of incidence and the velocity-distribution within the layers are assumed to be arbitrary. Expressions are derived for the particle velocity at the free surface and at any given depth. The results show features similar to those for normally incident waves in a multilayered solid half space.
Travel-time curves and upper-mantle structure from long period S waves
Authors: Ibrahim Abou-Bakr K. , Nuttli Otto W.
Published: July 2021

abstract

Long-period S-wave travel-time data, including second and later arrivals, are presented for distances of 3° to 65° for focal depths of 33 and 120 km. Onset times were determined on the basis of particle motion diagrams and the product of the horizontal radial and vertical components of motion. Because the recording stations principally were LRSM units, the travel times represent data for an “average” United States earth model.

An S-wave velocity distribution calculated for the upper mantle provides a satisfactory fit to the empirical travel-time data for focal depths of both 33 and 120 km. The proposed model contains a pronounced low-velocity layer at a depth of about 150-200 km, and secondary low-velocity layers at depths of 340-370 km and of 670-710 km. In addition, there are regions of rapidly increasing velocity beginning at depths of about 400 and 750 km, and constant velocity zones at 220-350 km and 400-670 km.
Intensity map and structural damage, Parkfield, California earthquake of June 27, 1966
Authors: Cloud William K.
Published: July 2021

The isoseismal map indicates that the main shock of the Parkfield, California earthquake series on June 27, 1966 was felt over an area of approximately 20,000 square miles, and that the maximum Modified Mercalli scale intensity of VII was limited to a small area near the San Andreas fault zone in the vicinity of the Parkfield and Cholame. In this area an acceleration pulse of 0.5g was recorded. However, damage to homes, pipe lines, bridges and other structures was not severe.
Love wave spectra and the mechanism of the 1966 Parkfield sequence
Authors: Filson John , McEvilly Thomas V.
Published: July 2021

The spectra of the Love waves from five earthquakes of the 1966 Parkfield sequence, recorded at Berkeley, have been studied for evidence of source propagation. Independent field evidence suggests that the source of the main shock of the sequence may have propagated on a line away from Berkeley. The modulation effect of propagation on the amplitude spectrum is discussed. This effect is dependent upon the total time delay due to rupture down the fault and seismic wave propagation back to the point of initial rupture, and is thus ambiguous with respect to fault rupture velocity and fault length. Consideration of a frequency dependent phase velocity can resolve this ambiguity. The amplitude spectrum of the largest (M = 5.5) Parkfield earthquake shows sharp minima at periods where the spectra of the other four shocks are relatively smooth. Considered individually, these minima are consistent with a fault rupture length of about 30 km and a rupture velocity of 2.2 km/sec. The pattern of the minima varies from that predicted by the assumed phase velocities, thus the rupture velocity/fault length ambiguity is not resolved. Similarity in the shape of the spectra of the four smaller (M = 3.8-5.1) events suggests similar focal dimensions for these earthquakes. The Love wave amplitude spectra of the four smaller earthquakes scale well, at intermediate periods, to the magnitudes determined on standard short-period intruments; the main shock scales to a much higher magnitude. The spectra are considered in the light of proposed seismic scaling laws.
Note on strong motion records from the June 1966 Parkfield, California, earthquake sequence
Authors: Murray G. F.
Published: July 2021

Data is presented for all the June 1966 Parkfield, California, earthquakes that were recorded by five strong motion accelerographs in the Shandon-Cholame area south of Parkfield. The instrument array is generally transverse to the fault, intersecting it just north of the southern limit of the June surface fractures. Unique seismograms were obtained of small shocks during wave motion from the principal earthquake; in the three minutes following the onset at 0426, June 28 GMT, two of the strong motion instruments recorded completely ten different earthquakes. Approximate epicenters are located for thirteen of the earthquakes.
T-phase radiators in the western Aleutians
Authors: Johnson Rockne H. , Norris Roger A.
Published: July 2021

abstract

The aftershocks of the Rat Islands earthquake of 4 February 1965 provided a large volume of data for processing with a T-phase source location program. Although the earthquake epicenters were somewhat uniformly distributed through the Rat and Near Islands region, the computed T-phase sources were grouped in six clusters along the Aleutian arc. The clusters are considered to represent radiation from distinct sites along the Aleutian Ridge. These sites are probably submarine promontories which, due to their exposure, radiate energy over broader arcs of the Pacific than do intervening regions. The relationship of T-phase strength to earthquake magnitude varied little among these radiators; however, T phases from sources south of the Aleutian trench were significantly weaker. Identification of the arrivals from separate radiators in the signal from the main shock allowed an estimation of the length and velocity of the faulting. The estimate was 500 km and 3.5 km/sec.
Earthquake of August 19, 1966, Varto Area, eastern Turkey
Authors: Wallace Robert E.
Published: July 2021

abstract

The earthquake of August 19, 1966, centered near Varto in eastern Turkey. It occurred at 12:22:09.6 G. M. T. and had a Richter magnitude of about 7. Intensities reached IX. Reportedly, 2,529 people were killed, 1,500 injured, and 19,013 buildings were demolished or heavily damaged.

Surface fractures in three parallel fracture zones trending N65-70°W display both right-lateral strike slip and extension normal to the fractures. The trend and sense of displacement relate the fracturing to the North Anatolia fault system.

Examples of geologic factors influencing damage include: (1) degree of saturation of the alluvium, (2) proximity to landslides, and (3) relation to surface faulting.
Effect of anisotropy on Love wave propagation
Authors: Negi Janardan G. , Upadhyay S. K.
Published: July 2021

abstract

A study on Love wave propagation in a transversely isotropic layer with stress free upper surface and underlying rigid base, is presented. The characteristic frequency equation is obtained and frequency dependence of the velocity parameters for different modes is analysed in detail. Several distinctive propagation phenomena which differ considerably from those in isotropic case are listed below:
A photoelastic analysis of propagation of Rayleigh waves past a step change in elevation
Authors: Dally J. W. , Lewis David
Published: July 2021

A series of photoelastic tests were conducted to determine transmission and reflection coefficients associated with the stress and displacement of a Rayleigh wave propagating past a step change in elevation on the boundary of an elastic half plane. The determinations were made on six different models with ratios of step height to wave length (h/λ) ranging from 0 to 1.1. The stress transmission coefficient varied from 1 to about 1.5 as h/λ increased from 0 to 1.1. The stress reflection coefficient increased from 0 to 0.7 with increasing h/λ. The displacement transmission coefficient increased initially to a value of 1.4 for h/λ = 0.2 then decreased monotonically to about 1 as h/λ increased to 1.1. Finally, the displacement reflection coefficient increased monotonically from 0 to about 1.5 as h/λ increased over the range considered.

The photoelastic method provides information over the entire region of interest which is very useful in the understanding and analysis of the complex wave interaction problems associated with Rayleigh wave propagation past boundary irregularities.
Minimum recording times for determining short-term seismicity from microearthquake activity
Authors: Sanford Allan R. , Singh Surendra
Published: July 2021

The minimum microearthquake recording time needed to determine short-term seismicity (10 years) has been established for regions having the same degree of activity as Socorro (400 shocks with M ≧ 0 each year). An analysis of more than five years of microearthquake data indicates that about six months of recording are required for a reliable estimate of the earthquake-frequency relation from which seismicity is calculated. If the slope b of the earthquake-frequency law is assumed, the short-term seismicity can be determined from four weeks of recording. The danger in the latter method is that a small error in b leads to relatively large errors in seismicity.

Long-term seismicity (100 years) predicted from the present level of micro-earthquake activity in Socorro is much lower than the known seismicity of the region. The number of microearthquakes would have to increase by a factor of 20, or the value of b decrease approximately 20 per cent, before prediction would agree with the long-term seismic activity of the region.
The Peru earthquake of October 17, 1966
Authors: Lomnitz Cinna , Cabré Ramón
Published: July 2021

Seismological results on the destructive Peru earthquake (M = 7.5) include: determination of epicenter, isoseismal map, tsunami observations, and some correlations between intensity and geologic factors. The epicenter was located off the coast of central Peru, about 190 kilometers northwest of Lima. Isoseismals were elongated in the direction of the coast line. The intensity exceeded VIII in the region of Huacho, and reached VII in Lima. There are indications, however, that accelerations and frequencies were both higher than would be expected in the regions for which the Modified Mercalli Scale was developed. Local high intensities were frequently correlated with the occurrence of recent alluvium. Analysis of tsunami travel times shows that the Peru Trench acts as a high-velocity channel which refracts wave energy to the coast in advance of the expected arrival over the direct path.
The East Coast Onshore-Offshore Experiment, II. Seismic refraction measurements on the continental shelf between Cape Hatteras and Cape Fear
Authors: Dowling John J.
Published: July 2021

abstract

During the East Coast OnShore-OffShore Experiment (ECOOE) two lines of instrumented buoys were anchored on the continental shelf between Cape Hatteras and Cape Fear. One line extended from near shore to the 100 fathom depth contour. The other line was parallel to and near the 100 fathom depth contour. Shots varying in size from 20 lbs. to 10 tons were fired along both lines. From the refraction data obtained and well-control available, crystalline basement is correlated with a layer of velocity 5.96 km/sec, the top of the Lower Cretaceous with a layer of velocity 3.54 km/sec and the top of the Upper Cretaceous with a layer of 2.44 km/sec. A contour map of the depth to crystalline basement under the shelf and structure sections are presented. The statistical uncertainties in the depths and velocities were determined.
Preliminary report on some aftershocks of the December 28, 1966 earthquake in Northern Chile
Authors: Pitt Andrew M. , Ellis James O.
Published: July 2021

abstract

Epicenters of aftershocks of the December 28, 1966 earthquake in northern Chile lie in a 75-km north-trending zone 20 to 30 km off the coastline. The epicenter for the main shock, as determined by the U.S. Coast and Geodetic Survey, is about 10 km north of the southern end of the aftershock zone. The aftershocks are about 30 km deep in the model used for locations; this places them in the lower crust or upper mantle. The aftershocks have no apparent relation to any surface faults.
Effect of soil conditions on damage in the Peru earthquake of October 17, 1966
Authors: Lee Kenneth L. , Monge E. Joaquin
Published: July 2021

abstract

This earthquake was centered offshore, 140 miles north of Lima. A strong motion record at Lima indicated a peak acceleration of 0.4g, and several pulses of 0.3g. The earthquake caused severe damage to many adobe and quincha houses and to some modern reinforced concrete structures in Lima, where the observed damage indicated MM intensities from VI to VIII. At towns closer to the instrumental epicenter the MM intensities ranged up to a maximum of about IX. There was some indication that the higher intensities were associated with the poorer soil foundation conditions. This was especially true where the soil and foundation conditions were particularly bad, but the trend was less definite where the soils provided good to fair foundations. Damage to well built tall buildings and other modern structures was mostly limited to broken windows and cracking in wall panels. In the few cases where modern structures did sustain more serious damage, it appeared to be due to inadequate provision for seismic forces in the designs, or to poor construction practices. Some slumping and landslides developed in dry sands and partially cemented granular soils.
A local magnitude scale for South Austalian earthquakes
Authors: White R. E.
Published: July 2021

abstract

A magnitude scale has been developed for South Australian earthquakes which is simple and consistent. It is based on the maximum trace amplitude among the S waves recorded on vertical component Benioff seismographs, and applies to distances up to 3.6 degrees. The extension of the Richter local magnitude scale to Benioff and other highly sensitive instruments used to study small magnitude earthquakes and with no consideration of differences in crustal structure has drawbacks and, in the present case, could lead to significantly erroneous estimates of magnitudes. The South Australian scale has resulted from an application of the “time-term” method to the study of amplitudes. This has produced satisfactory agreement with a theoretical amplitude-distance relation. Tentative relations are given between the South Australian scale and other scales and an energy-magnitude relation is derived. The maximum S ground velocity between 1° and 3° is due to the reflection from the Mohorovičić discontinuity.
Computational methods
Authors: Engdahl Eric R. , Taggart James , Lobdell John L. , Arnold E. P. , Clawson Gerald E.
Published: July 2021

The surface-focus P travel times estimated by Herrin et al (1968) entail a P velocity distribution from which smoothed travel-time tables of P, pP, PP and PcP are constructed. Theoretical considerations and computational procedures used are described. It is shown that computational errors in the new travel-time tables are less than 0.01 second.
The response of the horizontal pendulum seismometer to Rayleigh and Love waves, tilt, and free oscillations of the earth
Authors: Rodgers P. W.
Published: July 2021

The horizontal pendulum seismometer is sensitive not only to acceleration along its sensitive axis but also to tilt, variations in the angle of inclination, and along-the-boom acceleration. The complete steady-state response of this type of seismometer to Rayleigh and Love waves, tilt, and free oscillations of the Earth is treated. An equation of motion is developed which includes the effects of tilt, variation in the angle of inclination, and along-the-boom acceleration. An approximate solution to this equation is obtained which separates out the response due to each effect. The response, including these effects, is developed for Rayleigh and Love waves and the conditions under which along-the-boom acceleration and variations in the angle of inclination are important are stated. The question “How much of the seismogram is due to tilt?” is answered in detail for long period Rayleigh waves and free oscillations. It is shown that the seismograms resulting from such waves can require sizable corrections depending on the wave parameters. A correction factor for Rayleigh waves is developed which is universal in the sense that it is independent of the parameters of the particular seismometer and thus applies to all pendulous horizontal seismographs. For Rayleigh waves it is a function only of ellipticity, phase velocity, and period. Correction factor curves for long-period retrograde Rayleigh waves are presented. For circular particle motions a ten per cent correction is required for a three hundred second Rayleigh wave. The problem of obtaining the horizontal ground motion is treated. The response of the horizontal seismometer as a tilt meter is examined; a conversion factor between displacement and tilt magnification is developed. The complete response to simultaneous spheroidal and torsional free oscillations of the Earth is developed. It is shown that the principal response to the low-order spheroidal modes is as a tilt meter. The relationship between the horizontal and vertical seismogram is developed.
A note on the New England earthquake of November 18, 1755
Authors: Rothman Robert L.
Published: July 2021

Reports of a seismic sea wave associated with the New England earthquake of November 18, 1755 were investigated. Evidence indicates that such reports were erroneous and that the tsunami in question occurred on November 1, 1755 and was associated with the Lisbon earthquake of that date.
Source parameters of earthquakes, and discrimination between earthquakes and nuclear explosions
Authors: Davies John B. , Smith Stewart W.
Published: July 2021

The first part of this study describes a technique by which the source parameters of an earthquake can be obtained from the spectrum of compressional waves. The source parameters defined are fault length, fracture velocity, and fault plane attitude. Two large, deep earthquakes are examined using this technique. The source parameters determined compare favorably with those obtained previously using different techniques. In the second section a method is proposed for discrimination between underground explosions and earthquakes. The technique utilizes the ratio of the spectrums of the two classes of events where the path of propagation is common to both. On the basis of the analysis of the SHOAL event and a nearby shallow earthquake it appears that the duration as determined from the spectral ratio is almost 10 times smaller for an explosion than it is for a comparable earthquake.
Crustal characteristics from short-period P waves
Authors: Ellis R. M. , Basham P. W.
Published: July 2021

Thirty-four teleseismic events, recorded on the deep horizontal sediments of central Alberta, using one fixed and one movable station, have been analyzed as a test of the Haskell matrix formulation applied to short period P waves. Only limited agreement is obtained between averaged experimental vertical-horizontal spectral ratio curves and those calculated theoretically using known layer thicknesses and velocities.

Scattering in the crust and upper mantle is indicated by large transverse amplitudes including distict phases and by lower coherency for smaller epicentral distances where the travel path is confined to the crust and upper mantle. Anomalous SV/P ratios are believed to contribute to the difficulties. A study of 20 events in the azimuth range 285° to 310° indicates an apparent azimuth approximately 18° more northerly than the true azimuth. Localized dips of approximately 15° on the crustal boundaries are required to explain this deviation.

It is concluded that this region for which the sediments are horizontally layered does not fulfill the requirements of the Haskell matrix theory due to scattering and anomalous PS conversions in the crust and upper mantle.
Seismic source characteristics from explosion-generatedPwaves
Authors: Trembly Lynn D. , Berg Joseph W.
Published: July 2021

Signals from nuclear explosions were used to calibrate seismograph stations (near-regional, regional, and teleseismic ranges) by determining the transfer functions of the lumped source-propagation path-receiver systems. Recorded signals other than those used for calibration purposes were used to derive the characteristics of the sources. It was found that source functions could be derived from distant signals when the frequencies of the output signals were reliably related to the source. For the output data used in this research, the low-frequency cutoff was 0.2 cps at all stations and the high-frequency cutoffs were 4.0, 3.0, and 2.0 cps at the near-regional, regional, and teleseismic distances, respectively. The low-frequency cutoff was due mainly to the recording instruments, and the high-frequency cutoffs were due to the attentuation of the seismic energy by the Earth. The most reliable results were obtained when three half-cycles of the observed output signals (first arrivals) were used. When explosions in granite were used as calibration sources, the energies derived for explosions in tuff, alluvium, and dolomite media were 88, 65, and 12 per cent of the respective “observed” source energies.
P-wave travel times from shallow earthquakes recorded in India and inferred upper mantle structure
Authors: Kaila K. L. , Reddy P. R. , Narain Hari
Published: July 2021

P-wave travel times of 39 shallow earthquakes and three nuclear explosions with epicenters in the North in Himalayas, Tibet, China and USSR as recorded in Indian observatories have been analyzed statistically by the method of weighting observations. The travel times from Δ = 2° to 50° can be represented by four straight line segments indicating abrupt velocity changes around 19°, 22° and 33° respectively. The P-wave velocity at the top of the mantle has been found to be 8.31 ± 0.02 km/sec. Inferred upper mantle structure reveals three velocity discontinuities in the upper mantle at depths (below the crust) of 380 ± 20, 580 ± 50 and 1000 ± 120 km with velocities below the discontinuities as 9.47 ± 0.06, 10.15 ± 0.07 and 11.40 ± 0.08 km/sec respectively.

The J-B residuals up to Δ = 19° are mostly negative varying from 1 to 10 seconds with a dependence on Δ values indicating a different upper mantle velocity in the Himalayan region as compared to that used by Jeffreys-Bullen in their tables (1940). Between 19° to 33° there is a reasonably good agreement between the J-B curve and the observation points. From Δ = 33° to 50° the J-B residuals are mostly positive with an average excess value of about 4 sec.
Large scale processing of seismic data in search of regional and global stress patterns
Authors: Ben-Menahem A. , Jarosch H. , Rosenman M.
Published: July 2021

A composite compute program has been devised for a fast reduction of multistation seismic data in the period range 50–500 sec for mantle surface waves and 20–100 sec for body waves.

The analysis aims at the reconstruction of the seismic source from the spectrum of its far radiation field and the correlation of its parameters with its depth, size and regional environment.

The capability of the computational procedure has been demonstated in two studies of WWNSS records: one includes a spectral analysis of surface waves from a shallow shock in the Kurile Islands; the other includes a spectral analysis of P waves from 9 shocks in the depth range 550–700 km at Fiji, Mariana, Java, Japan, Peru and Brazil.

Other applications of the proposed data processing routine are foreseen; a tsunami warning system and focal depth determination from spectral modal ratios.

It is believed that a persistent search for stress patterns, based on the processing of a sufficiently large sample of seismic events, is essential to any future program of earthquake prediction.
An orthonormality relation for elastic body waves
Authors: Alsop L. E.
Published: July 2021

Herrera's orthonormality relation for elastic surface waves is generalized for elastic body waves. As an example, the normalization of a P-wave incident on the surface of a half space is considered. The Green's function for body waves and surface waves is obtained.
An analysis of ground motions during the 1957 San Francisco earthquake
Authors: Idriss I. M. , Seed H. Bolton
Published: July 2021

The seismic responses of soil deposits are calculated at four sites where recordings were obtained during the 1957 San Francisco earthquake. The analyses are made utilizing an equivalent linear lumped-mass method of analysis incorporating the soil characteristics at the four sites and estimates of the rock motions at each site based on the record obtained on rock during this earthquake. Comparisons of the maximum amplitudes of ground motions, the time history of ground accelerations, velocity and acceleration spectra and spectral intensities for both computed and recorded ground accelerations indicate a reasonably high degree of agreement. The influence of the amplitude of base rock accelerations on the response of a soil deposit is also discussed and illustrated.
Effect of a fault on explosion-generated ground shock spectra
Authors: Galbraith Frank W.
Published: July 2021

Shock spectrum measurements were made for an underground nuclear explosion at the Nevada Test Site of the U. S. Atomic Energy Commission using 29 reed gages. The objective of the measurements was to determine the effect of a fault zone on shock spectra measured at the ground surface and generated by the explosion. The reed gages were located along two gage lines; one extended to the west and crossed the fault at right angles. The other gage line extended to the north and was generally parallel to the fault. The results showed that the fault zone had an effect on vertical spectra but not on horizontal radial spectra. The vertical gage directly on the fault showed significantly lower displacements for all frequencies above 3 cycles per second but showed no apparent effect on the 3 cps displacement. For gages beyond the fault the vertical spectra for frequencies higher than 3 cps were greater than for corresponding gages along the north gage line. It is believed that this was caused by the upthrust base rock on the side of the fault away from the explosion with a resultant decreased thickness of tuff and alluvium. This resulted in less attenuation of high frequency response than was expected.
Ray theory for an anisotropic inhomogeneous elastic medium
Authors: Vlaar N. J.
Published: July 2021

A ray theory is developed for an inhomogeneous, anisotropic medium, based on the concept of a wave front as the carrier of a discontinuity in particle velocity. The discontinuity conditions of several field quantities involved are formulated and serve to cast the problem in terms of a partial differential equation. The characteristics of this equation subsequently are identified in terms of rays, represented in parametric form as space curves. The energy transport along the rays is formulated by means of transport equations.

The theory is applied in particular to the case of the transversely (horizontally) isotropic, vertically inhomogeneous medium. Equations for rays and travel times are obtained in the form of integrals, which are suited for serving the purpose of numerical computations.
Spectral response to ground displacement at Hattiesburg resulting from nuclear event SALMON
Authors: Blume John A.
Published: July 2021

Spectral response is developed from principal portions of a record of ground displacement in Hattiesburg, Mississippi, resulting from the SALMON event, Project DRIBBLE, an underground nuclear explosion near Baxterville, Mississippi in October 1964. No acceleration records are available for Hattiesburg. As a part of the study it was found that peak response in the important short-period range could be reasonably estimated using as little as one second of selected motion from the entire record, and that a simple stochastic model of this key portion of the ground motion essentially duplicated the peak response.