bluuebunny/crossref_metadata_2025 huggingface dataset:
Microearthquake crustal reflections, Socorro, New Mexico
Authors: Sanford A. R. , Long L. T.
Published: July 2021

About 25 per cent of the microearthquakes which originate close to Socorro (S minus P ≦ 2.5 secs) produce seismograms with two sharp arrivals about 2.5 and 5.0 seconds after the direct S-phase arrival. The best interpretation of the time-distance data for these late phases is that they are the SxP and the SxS reflections from a crustal discontinuity at a depth of 18 km. The amplitudes of the reflections, which average 0.21 and 0.32 of the direct S-phase amplitude on vertical-component seismograms, are for greater than theoretical amplitudes found by assuming uniform radiation from the focus and reflection of a plane SV wave from a horizontal discontinuity.
The effect of boundaries on wave propagation in a liquid-filled porous solid: IX. Love waves in a porous internal stratum
Authors: Deresiewicz H.
Published: July 2021

abstract

The dispersion equation in the second approximation for small porosity is deduced for the problem of transverse waves in a porous layer separating two impermeable elastic half-spaces, and the expression for the dissipation per cycle displayed. It is shown that Love waves can be propagated in a porous layer only with phase velocities smaller than that of body shear waves in either of the half-spaces, whereas in an impermeable layer Love waves may exist having phase velocity equal to that of body shear waves in one of the half-spaces.
A note on the existence of relative maxima and minima on phase velocity curves
Authors: Thrower E. N. , Harkrider D. G.
Published: July 2021

Phase and group velocity dispersion curves for fundamental Rayleigh waves have been computed with more precision than previously attempted. The new curves show a relative minimum in phase velocity at periods near 50 sec for four perturbed Gutenberg continental models.
Seismicity of Hawaii
Authors: Furumoto Augustine S.
Published: July 2021

abstract

One aspect of the seismicity of Hawaii is investigated by attempting to answer two questions: (1) the possibility of occurrence of major earthquakes in Hawaii, and (2) the probability of occurrence. The first question was answered when a study of the distribution of intensity of the Hawaiian earthquake of 1868 indicated that the earthquake had a magnitude between 714and734. An answer to the second question was approached by studying the occurrence-magnitude relation log N = A − bM for earthquakes of magnitudes 2.5 to 4.9 during a period of 314 years. Extrapolation from the results suggests that major earthquakes may be expected in Hawaii as often as in other seismically active areas. A study of strain release showed that the pattern in Hawaii is consistent with that worldwide.
Routine location of T-phase sources in the Pacific
Authors: Johnson Rockne H.
Published: July 2021

abstract

A program for the routine location of T-phase sources in the Pacific is described. Data for this program are supplied principally by the Pacific Missile Range hydrophone network. Arrival times and power levels are read at Honolulu for processing by an IBM 7040 computer. The solution for location and origin time is the least-squares fit to all hydrophone arrivals which are weighted according to their distribution in azimuth and their distance from the T-phase source. The velocities for the program are obtained from algebraic functions of latitude and longitude which are based upon shot calibrations and upon hydrographic measurements. A T-phase strength is computed from readings of peak power level in a manner analogous to earthquake magnitude. Early results for the r.m.s. difference between T-phase source locations and the corresponding earthquake epicenters were 0.6° in the Aleutians and 1.1° in the Kuril Islands.
Difference between aftershocks and foreshocks in the relationship of magnitude to frequency of occurrence for the great Chilean earthquake of 1960
Authors: Suyehiro Shigeji
Published: July 2021

abstract

When a relatively small perceptible earthquake occurred near a tripartite net of high sensitivity in central Japan, a substantial difference was found between its 25 foreshocks and 173 aftershocks in the relation of frequency of occurrence and magnitude. For that study the coefficient “b” in the magnitude versus frequency equation is 0.35 for the former and 0.76 for the latter.

A similar investigation has been carried out on the great Chilean earthquake of 1960, also accompanied by many foreshocks and aftershocks. Using four sensitive and suitably located U.S.C.G.S. stations, Eureka, Tucson, South Pole, and Byrd, foreshocks and aftershocks were located in addition to those reported by U.S.C.G.S. or B.C.I.S. Forty-five foreshocks and 250 aftershocks were found in a period of 33 hours before and 33 hours after the main shock. The same characteristic found for the Japanese earthquake was also found for the Chilean earthquake; i.e. the foreshocks showed a different picture from the aftershocks for the frequency of occurrence, and an appreciably smaller value seems to be valid for “b” of the foreshocks.
Azimuth and slowness anomalies of seismic waves measured on the central California seismographic array. Part I. Observations
Authors: Otsuka Michio
Published: July 2021

abstract

Arrays of seismographs are usually considered to be detectors which give enhanced signals from distant earthquakes. They also provide, however, a new way of learning more about the structure of the crust and upper mantle. The deviation of the seismic-wave surface from its expected configuration may be regarded as a consequence of non-homogeneous and anisotropic conditions in the earth.

The operations of the University of California network of telemetry stations in the Coast Ranges of California provides an opportunity to discover the practicality of this approach. The situation of this network near the continental margin gives the study particular interest.

The differences in arrival-times between array elements of coherent peaks or troughs of P and pP phases from 28 teleseisms in the period of 1963-1964 were read from the telemetry records of the central California seismographic array. The direction of approach and velocities of the wave fronts were then determined and compared with the great circle azimuths and with the apparent velocities calculated from the Jeffreys-Bullen tables.

The observed anomalies in direction of approach and apparent velocites are found to be cyclic functions of the direction of the source. The amplitudes of these functions are almost 10 degrees in azimuth anomaly and 1.0 sec/deg in slowness anomaly. Error analyses show that the anomaly functions cannot be attributed to the measurement errors.

The derived anomaly functions provide a powerful means of examining crustal and upper mantle structure under the array and perhaps at the source. Variations between subsets of the array indicate significant differences in structure between portions of the Coast Ranges to the north and to the south of Hollister.
Deformation of railroad tracks by slippage on the Hayward fault in the Niles District of Fremont, California
Authors: Bonilla M. G.
Published: July 2021

abstract

Slippage (also called creep) on the Hayward fault has produced a right-lateral shift of about 0.7 feet in three sets of railroad tracks that were built 55 and 56 years ago. The average annual rate of 0.15 inch per year is about one-third the average rate of slippage on the San Andreas fault, but the rate has been much greater during short intervals. The zone of acute deformation of the tracks is 20 to 80 feet wide; the tracks are underlain by unconsolidated deposits more than 300 feet thick. Surface faulting at this locality in the 1868, and probably in the 1836, earthquake suggests that the processes of fault rupture and fault slippage can alternate at the same locality.
The determination of seismometer damping
Authors: Morencos Julio
Published: July 2021

a4bstract

Eaton's method of determining the seismometer damping is extended, in order to make it more easily applicable to any value of period and damping.

The theory of the seismometer response to the release test is developed, showing that, for the application of this method, both release and tapping tests are equivalent.

As a summary of the work, curves are plotted corresponding to the function β = f (τ, Q), where τ is the “reduced time” during which the deflection decreases from its initial value ϕ0 (release test) or first maximum one ϕm (tapping test) to Qϕ0 or Qϕm, respectively, Q, being an arbitrary parameter smaller than unity. The graph contains seven curves, for values of Q from 0.1 to 0.5. The higher values are applicable to cases of large period and damping.
Velocity filtering of seismic core phases
Authors: Hannon W. J. , Kovach R. L.
Published: July 2021

abstract

Recent studies have proposed complexities in the velocity-depth function for the region surrounding the inner core which require additional branches in the travel time curve for PKP in the epicentral range of 125° to 160°. The proposed PKP arrivals can be separated on the basis of their apparent velocities, which range from 24 km/sec to 100 km/sec. Using the Tonto Forest array in Arizona coupled with adjoining LRSM stations in the western United States, an effective linear array of 400 km in size is attained. Data from several events in the distance range from 130° to 160° recorded on this array have been velocity filtered and show some evidence of two precursors to PKP in the distance range from 135° to 143° and at least one intermediate branch between PKP1 and PKP2 at distances greater than 143°. The results appear to support the velocity solution for the core proposed by Adams and Randall, although more data are required before a conclusive discrimination can be made between competing velocity models.
The San Salvador earthquake of May 3, 1965
Authors: Lomnitz Cinna , Schultz Rudolf
Published: July 2021

abstract

The San Salvador earthquake of May 3, 1965 was preceded by a local seismic swarm of three months duration. The main shock was destructive in a densely populated area of not more than 15 km in radius; the same general area was damaged in the earthquakes of 1576, 1659, 1798, 1839, 1854, 1873, 1880, 1917, and 1919. Over 120 casualties were reported.

The epicenter has been located on the south rim of the Median Trough, a post-Pliocene structure which accounts for the high seismic and volcanic activity in the region. The observed intensity is attributable to shallow focal depth and to the presence of thick inhomogeneous beds of fluviatile pumice. The tectonic setting and shallow subsurface factors should be recognized in future building codes and zoning regulations.
Seismic parameter φ: Computation at very high pressure from laboratory data
Authors: Anderson Orson L.
Published: July 2021

abstract

By using the accuracy inherent in ultrasonic velocity measurements taken at pressures less than 10 kb, the seismic parameter φ=vp2−(43)vS2 can be computed at very high pressures. The equation used requires the assumption that the second derivative with respect to pressure of the bulk modulus be negligible at all pressures considered. This assumption is checked by computing the compression (V/V0) in the pressure range by equations of state using the assumption, and comparing the resulting values with measured compression. Illustrations are given for MgO and Al2O3.
Regional strain release characteristics for Indian regions
Authors: Chouhan R. K. S.
Published: July 2021

abstract

The strain accumulation and release curves for shallow and deep focus earthquakes of Indian origin have been constructed for a span of sixty years, from 1905 to 1964. For shallow focus earthquakes, magnitudes 7.2 and above have been considered; for deep focus shocks, magnitudes 6.7 and above are used. Strain rebound characteristics yield a number of very interesting features; for example, the curve for shallow focus earthquakes shows two linear segments of strain accumulation. Deep focus shocks show a single cycle of strain accumulation. Comparison of these curves with similar curves from other regions given by Benioff are made.
High Rayleigh wave phase velocities for the New Delhi, India-Lahore, Pakistan profile
Authors: Gabriel V. G. , Kuo John T.
Published: July 2021

Phase velocities were determined for the New Delhi-Lahore profile by using data from these two stations and earthquakes located approximately on the great circle path through the stations. The phase velocities were found to be higher than those expected for normal continental structures and somewhat similar to those found by Brune and Dorman (1963) for the Canadian shield. A structural model, based on the phase velocity values of the CANSD model given by Brune and Dorman (1963) and consistent with the local geology, was evaluated and is presented herewith as the INDSD model.

It is postulated that high phase velocity values in the Lahore-New Delhi profile indicate the shield character of the crustal structure along the profile, as an extension of the Indian shield located south and southeast of it.
Preliminary results of an investigation of crustal structure in Southeastern Europe
Authors: Papazachos Basil C. , Comninakis Peter E. , Drakopoulos John C.
Published: July 2021

Travel-time curves for the most prominent phases of the longitudinal and shear waves for epicentral distances between zero and 140 km and between 200 km and 1000 km have been constructed for Southeastern Europe. Both refraction and reflection methods indicate a three-layered crust. The average thickness is approximately 16 km for the top layer, 15 for the middle layer and 11 for the lower layer. The crustal thickness varies from about 47 to 32 km from place to place. The extremely low value 0.21 for Poisson's ratio was found in the top as well as in the middle layer.
The 1962 Kaoiki, Hawaii, earthquake and its aftershocks
Authors: Koyanagi Robert Y. , Krivoy Harold L. , Okamura Arnold T.
Published: July 2021

At 18:27:14.3 on June 27, 1962 an earthquake of moderate intensity occurred near the Kaoiki fault system between the volcanoes Kilauea and Mauna Loa on the Island of Hawaii. It was felt strongly on Hawaii, where it caused minor property damage, and was perceptible as far as Oahu to the north-west.

Intense aftershock activity continued for several weeks; 1540 aftershocks were recorded during the initial 60 hours. A hyperbolic decay in the number of aftershocks with increasing time was evident. The epicentral zone covered about 150 square km and focal depths ranged from 3 to 12 km.
Spectra of S-waves radiated from bilateral fracture
Authors: Savage J. C.
Published: July 2021

abstract

The phase and amplitude spectra of the S waves radiated from five different bilateral, tensile fractures in two-dimensional models are discussed. Two S waves, radiated in approximately opposite directions, were recorded for each fracture. The amplitude spectra display the regular sequence of holes first explained by Ben-Menahem. The phase spectra display a phase jump of π radians associated with each hole in the corresponding amplitude spectrum. The phase of the source-time function was extracted from the phase spectra; it is consistent with a representation of the source-time function by a ramp function of 14 μsec duration. An explanation of the observed spectra is given in terms of interference between the initial and stopping phases of the radiated signal. From this viewpoint Ben-Menahem's directivity method is seen as a procedure for detecting the stopping phase in dispersed wave trains. Finally, it is suggested that the analyses by Ben-Menahem and Toksöz of the phase spectra of the Kamchatka 1952 and Alaska 1958 earthquakes have not properly taken account of the phase jumps in the spectra.
Crustal structure in the Himalayan and Tibet Plateau region from surface wave dispersion
Authors: Gupta Harsh K. , Narain Hari
Published: July 2021

abstract

Dispersion of Rayleigh and Love waves recorded at Seoul, Hong Kong, Shillong, New Delhi and Quetta by similar long-period seismographs has been studied for the Arctic region earthquake of 25th August 1964. The entire paths from the epicenter to the recording stations are continental. Waves recorded at Shillong, New Delhi, and Quetta pass through the high mountain regions of the Himalayas and Tibet Plateau while those recorded at Seoul and Hong Kong do not pass through these high mountain regions. Observed dispersion has been compared with theoretical dispersion curves computed for a three-layer earth model. This gives an average crustal thickness of about 45 km between the epicenter and Shillong, New Delhi and Quetta, 35-37 km between the epicenter and Seoul and Hong Kong and 65-70 km in the Himalayan and Tibet Plateau regions.
Short-period seismic energy radiation patterns from underground nuclear explosions and small-magnitude earthquakes
Authors: Sutton George H. , Mitronovas Walter , Pomeroy Paul W.
Published: July 2021

abstract

Azimuthal radiation patterns of short-period (0.5-2.0 cps) seismic energy obtained from integrals of the seismograms from two underground nuclear explosions and two earthquakes are used to study the propagation and source characteristics of the Pg and Lg phases in the United States. In addition, the energy spectrum is divided into two bands, greater than and less than 1.4 cps, and the ratio of higher-to-lower-frequency energy is mapped to study the nature of propagation as a function of frequency. Both the total energy and the ratio show large fluctuations with azimuth and distance. However, a general correlation is found between the energy and ratio contours and the major tectonic provinces of the United States. This correlation is attributed to focusing, resulting from lateral variations in velocity and to regional differences in attenuation of the seismic energy. The range in the Q values across the United States, based on the assumption of symmetrical surface wave propagation, is from 200 to 1000, about a factor of 5. The transverse (T) component shows about the same total energy and ratio contour patterns as the vertical (Z) and longitudinal (L) components. Also, energy contour maps are similar to maps obtained using the maximum amplitudes of the Pg and Lg phases. For the events examined, it seems that the nature and distribution of tectonic features along the propagation path are more important in detemining the resultant radiation patterns than the initial conditions at the source. The particle motion at most stations does not give direct proof for the surface wave nature of the Pg and Lg phases, except that Pg tends to be longitudinal or mixed and Lg transverse or mixed.
Automated epicenter locations from a quadripartite array
Authors: Sokolowski T. J. , Miller G. R.
Published: July 2021

abstract

A quadripartite seismic array has been installed on the island of Oahu, Hawaii, to supply additional data for the Tsunami Warning System of the USCGS. The nuclear explosion LONGSHOT on October 29, 1965, on Amchitka Island served in calibrating the epicenter location system. A moving cross-correlation determined the P wave time delays between the various station pairs. Time delays were then used in a least-squares method to obtain azimuth and emergence angles. The emergence angle is used with the Jeffreys-Bullen travel-time curve to obtain the distance to the source. The emergence angles obtained by using the observed station-pair time delays should be corrected to the emergence angles obtained by using the station-pair time delays as computed from the Jeffreys-Bullen travel-time curve. This comparison will then compensate for the anomalies of the Pacific area. With the values for azimuth and distance, one can then arrive at an approximate epicenter location for earthquakes of normal depth. This method can be used for those situations which require an epicenter location within minutes after the earthquake has been recorded. Methods developed here are designed for an on-line computer.
A micro-earthquake survey of the San Andreas fault system in southern California
Authors: Brune James N. , Allen Clarence R.
Published: July 2021

abstract

Micro-earthquakes have been systematically recorded with magnitudes down to -1.3 at more than 60 sites along the San Andreas fault system in southern California during intervals of 2 days to 1 year, representing more than 35,000 hours of usable records. Eight trailer-mounted instruments were operated with peak gains of 4-8 million at 20 cps with noise levels averaging about 0.1 mu amplitude of ground motion.

Observed micro-earthquake activity varies from virtually nil along the central section of the San Andreas fault to more than 75 shocks daily in the Imperial Valley. Quietest is the 300-km segment between Cholame and Valyermo; more than one year of recording at Lake Hughes indicates an average of only one micro-earthquake within 24 km every nine days. Activity increases northward from Cholame toward Hollister, and southward it increases abruptly near Valyermo and continues high along major branches of the fault southeast into Mexico, with the exception of the Banning-Mission Creek fault southeast of Desert Hot Springs. Most areas where regional strain or fault creep have been demonstrated by geodetic measurements are also areas of high micro-earthquake activity. Existence of an area of minimal micro-earthquake activity within a broad region of active tectonism, and indeed along the very segment of the fault that broke in the great 1857 earthquake, suggests that short-term micro-earthquake activity is not necessarily positively correlated with long-term activity and with earthquake hazard, and in some areas the relationship may be inverse. However, areal distribution of micro-earthquake activity is grossly similar to that of larger earthquakes (M ≧ 3) during the past 29 years, and in many areas micro-earthquake activity can be approximately predicted by extrapolation of 29-year recurrence curves based solely on larger earthquakes.
Reflection and transmission of elastic waves in a system of corrugated layers
Authors: Levy A. , Deresiewicz H.
Published: July 2021

abstract

The scattered field generated by normally incident body waves in a system of layers having small, but otherwise arbitrary, periodic deviations from plane parallel boundaries is shown to consist of superposed plane body and surfacetype waves. Results of numerical computations for two like half-spaces separated by a sinusoidally corrugated single layer, and by two layers, reveal the variation of the amplitude of the field with ratios of velocities, densities, impedances, and with those of depth of layers and wavelength of the boundary corrugations to the wavelength of the incident wave.
A search for small, deep earthquakes using quadripartite stations in the Andes
Authors: Suyehiro Shigeji
Published: July 2021

abstract

To identify small, deep earthquakes in South America quadripartite nets of about one km in size were set up in Toconce, Chile and Cuzco, Peru. During the period of observation the U. S. Coast and Geodetic Survey identified two fairly large, deep earthquakes in northern Argentina within about a month by teleseismic observations, but neither foreshocks nor aftershocks associated with these earthquakes during the period of observation may indicate that there are comparatively few small, deep earthquakes as compared with shallow earthquakes.

In Toconce, at the foot of the Andes, there was a systematic azimuthal distortion in apparent velocity and in direction of propagation, whereas in Cuzco, on the Altiplano, such a distortion was not found.
Washington State earthquakes 1840 through 1965
Authors: Rasmussen Norman
Published: July 2021

abstract

A tabulation has been made of approximately 850 earthquakes, in Washington State and near its borders, between 1841 through 1965. This list was compiled from previously published scientific reports, newspaper accounts, and unpublished epicenter computations from the University of Washington Seismograph Station. Shocks of intensity V or less are not complete prior to about 1900 because of sparse population making reporting incomplete. There appears to be an increase of larger intensity and magnitude earthquakes since 1936 in Washington State, and especially in the Puget Sound area since 1949.
Magnitude determination at regional and near-regional distances in the United States
Authors: Evernden J. F.
Published: July 2021

abstract

This report deals with the problems of variation of Pn amplitudes in the regional and near-regional distance ranges (200-2100 kilometers). The data used were recorded by Long Range Seismic Measurement vans of the VELA Seismological Center as a result of earthquakes throughout the United States and numberous nuclear and chemical explosions in the same region.

It is shown that the patterns of Pn amplitudes versus Δ in Western United States and Eastern United States are markedly different and that these differences are related to different velocity structures in the two regions. These differences extend to at least 150 kilometers depth.

Neither the Western United States nor Eastern United States amplitude patterns conform even approximately to that predicted or suggested by Gutenberg and Richter in the 1000-2000 kilometer range. The predicted zone of low-amplitude signals in that distance range does not exist and overestimation of magnitude by as much as 1.5 magnitude units is frequently done because of failure to properly understand the patterns of radiation.

By proper calibration of the Western United States by use of numerous events, it is now possible to make consistent estimates of magnitude at all distance ranges for most explosions and earthquakes. Obtaining consistent estimates of amplitude as a function of distance for a particular event requires a knowledge of the energy distribution between the several refracted phases used between 200 and 2500 kilometers distance. Data on hand show that the energy partition function is reasonably uniform throughout the regions investigated but that locally it may vary radically, resulting in a ten-fold change in relative excitation of two refracted phases.

Patterns of energy radiation and decay are probably approximately determinable from knowledge of velocity structure and vice versa. Both are best determinable from explosion data where origin time and focus are accurately known.
Truncated crustal transfer functions and fine crustal structure determination
Authors: Leblanc Gabriel
Published: July 2021

abstract

A seismic pulse is influenced by the various media through which it has travelled. The crust is known to have a considerable influence. With the use of the Thomson-Haskell matrix formulation, the crustal effect can be easily calculated. Because a time window is imposed on the signal, it is necessary to take into account this truncation in calculating the theoretical crustal effects. A method to obtain truncated crustal transfer functions is presented.

It is suggested that the frequency analysis of short-period P waves can yield detailed information on the crustal structure at a given station when selected truncated transfer functions are used as crustal inverse filters. The basis for deciding upon a particular model is the optimum match between one of a series of spectra corrected for a series of crustal models and the approximated spectrum of the incident pulse obtained through the averaging of many well grouped spectra.
The earthquake P phases which penetrate the earth's core
Authors: Shurbet D. H.
Published: July 2021

abstract

Travel times of all earthquake phases which enter the core as P and emerge as P are explained in terms of a core model containing two velocity discontinuities. Amplitudes of PKIKP and PKP change little with changes in distance from the source except in the rangeΔ = 142° to Δ = 152°. In this range large amplitubes are observed which result from the simultaneous arrival of two phases as well as large amplitudes which represent the PKP1 - PKP2 caustic.

Amplitudes in the caustic allow detection of events perhaps smaller than magnitude 4. In all distance ranges PKIKP and PKP are useful in magnitude determination, but added care must be taken if they are used in the distance range where amplitudes are large. Magnitudes determined from PKIKP and PKP are very consistent as are focal depths. The PKJKP phase is small if present at all.
Spatial and temporal clustering of deep and shallow earthquakes in the Fiji-Tonga-Kermadec region
Authors: Isacks Bryan L. , Sykes Lynn R. , Oliver Jack
Published: July 2021

abstract

A study of the tendency of deep and shallow earthquakes in the Fiji-Tonga-Kermadec region to cluster in space and time revealed that (a) in general, deep earthquakes do not form either aftershock sequences of swarms of the types commonly observed in series of shallow shocks throughout the world; (b) a small percentage of the deep earthquakes cluster in the form of multiplets, i.e., small numbers of events closely grouped in space and time; and (c) during the 7-year interval studied, clustering of the shallow events in the Kermadec region was markedly greater than that of shallow events in the Tongan region; clustering of shallow events in the Tongan area was, in turn, greater than that of the deep shocks in the Fijian region. The data provide new constraints for hypotheses of focal mechanisms.

Most deep multiplets were doublets, and in no case were more than 12 events observed per multiplet. Resolution of the analysis was sufficient to show that, in the case of at least one multiplet, separation between events was smaller than 2 km, but that in other cases it was larger than 5 km. Separations between events in time were sometimes smaller than 2 seconds, but in no case was the relation in time and space such that a later event of a multiplet could not have been dependent upon an earlier event, i.e., the time between events was greater than that required for seismic wave propagation between the two foci.

A narrow linear feature about 60 km long is defined by a spatially progressive sequence of deep multiplets that were observed during 1965. These deep multiplets are also confined to a narrow range of depths between about 625 and 660 km. This feature seems to form a portion of the lower edge of the narrow inclined zone of deep earthquakes. Patterns of first motions radiated from earthquakes in the linear feature are generally similar to one another and are in agreement with a model of the source corresponding to a double-couple or shear dislocation. Variations among the radiation patterns, although minor, are distinct and indicate some heterogeneity in the detailed distribution of stress. The data of this study do not support the hypothesis of a volumetric phase change as the mechanism of the deep-focus earthquakes. Although there is no existing hypothesis that does predict completely the results determined here, the data appear to agree best with the mechanism proposed by Orowan.

A significant source of data for this work is a new network of five seismograph stations now operating in Fiji and Tonga. This study, under the U.S. Upper Mantle Program, is part of a more comprehensive investigation of deep earthquakes in the region.
Analysis of accelerograms—Parkfield earthquake
Authors: Housner G. W. , Trifunac M. D.
Published: July 2021

Integrated velocities and displacements show that near the fault at Cholame the surface motion exhibited a transient horizontal displacement pulse of approximately ten inches amplitude and one and one-half seconds duration, normal to the fault. Although 50 per cent of g ground acceleration was recorded at the fault, the ground motion attenuated rapidly with distance and at ten miles from the fault the maximum acceleration was reduced to one-tenth of its near-fault value. The ground motion also changed its character with distance, losing its pulse-like directional characteristic and becoming isotropic. Computed response spectra are presented and the large spectrum ordinates for this shock of relatively small magnitude and moderate destructiveness indicate that in an engineering sense the Parkfield ground motion is in a different class from such large destructive ground motions as El Centro 1940, Tehachapi 1952, and Olympia 1949.
Wave propagation in inhomogeneous elastic media
Authors: Eason G.
Published: July 2021

The propagation of waves in an inhomogeneous elastic solid whose elastic parameters depend on one space coordinate only is considered. The stress and displacement components are assumed to depend on this same space coordinate and time alone. It is shown that the wave equation, with variable wave speed, is the basic equation governing the propagation of shear waves in spherical or cylindrical coordinates as well as the propagation of longitudinal waves in a rod. The values of the elastic parameters which permit a particular simple solution are determined and some problems are then discussed for these solids.
Strain steps associated with earthquakes
Authors: Wideman C. J. , Major M. W.
Published: July 2021

Strain steps have been observed following earthquakes ranging in magnitude from 3.0 to 8.5. An empirical relationship is derived which indicates that the magnitude, M, of the smallest earthquake from which a strain step of the order of 10−9 may be expected at a distance of Δkm is:

M = 1.1 + 1.7 log ⁡ Δ 10 .

Fault length is related empirically to earthquake magnitude by the equation

M = 3.3 + 1.7 log ⁡ L 10

Where L is fault length in km. Comparison of these two equations shows that strains of the order of 10−9 may be expected at distances of approximately 20 fault lengths.

The fault length versus magnitude equation presented is shown to be compatible with a previously published energy-magnitude relationship

log ⁡ 10 E = A + 1.8 M .

Strain steps are shown to have two characteristics which are similar to those of surface waves. The speed of propagation is nearly constant over a wide range of epicentral distances. Depending upon whether the travel paths are continental or oceanic, the speeds are, respectively, 3.0 ± 0.3 km/sec or near 3.6 km/sec. The strain step amplitude dependence upon distance is like R -3/2.
Stress drop in earthquakes
Authors: King Chi-Yu , Knopoff Leon
Published: July 2021

abstract

A correlation is made between earthquake magnitude and parameters of fault trace on the basis of dislocation theory. For earthquakes with magnitudes M between 5.5 and 8.5, the correlation with fault length L and the maximum horizontal or vertical offset D (both in cm) is approximately

log L D 2 = 2.24 M − 4.99.

Combining this result with a magnitude-energy formula, it is found that the stress drop is dependent upon magnitude, with the fractional stress drop increasing with magnitude.
High frequency microseisms from a known source
Authors: Sanford Allan R. , Carapetian Ara G. , Long Leland T.
Published: July 2021

abstract

Trains moving through the Rio Grande Valley at Socorro are a known isolated source of short-period microseisms. Power spectra at five different locations indicate that the train noise is confined to a narrow band (< 3 cps) of frequencies. In general, the frequency limits and peaks of the power spectra shift to lower values with an increase in distance from the source. Peaks in the power spectra, which range from 1.80 to 4.75 cps, do not correlate with mechanical interactions of the train with the track.

Particle motions for randomly selected samples of train noise are extremely complex and no dominant component of motion—Rayleigh or Love—can be identified. However, for samples of noise showing the same regular frequency on all components of ground motion, a strong fundamental mode Rayleigh wave can generally be found.

Phase-velocity measurements at a distance of about 17000 feet from the train indicate that a major amount of the train noise at Socorro is being propagated as a fundamental mode Rayleigh wave. The average of over two hundred phase velocity determinations was 1400 ft/sec, a value that corresponds closely to 0.92 of the shear velocity of the surface layer at the recording location. Other observations at the same location indicate normal dispersion of fundamental mode Rayleigh waves, but the change in phase velocity with frequency (in the range of 2 to 5 cps) appears to be quite small.
Seismicity of Iranian Plateau and Hindu Kush region
Authors: Niazi Mansour , Basford Jeffrey R.
Published: July 2021

abstract

Over 1400 earthquakes of magnitude 4 and above which occurred between 20° and 40° North Latitudes and between 40° and 75° East Longitudes during the decade ending on June 30, 1967 are analysed. On the basis of the computed strain release, seismicity maps are constructed for the first and second halves of the period. Three or possibly four seismic zones in the eastern Turkey-Caucasus region, along the marginal heights of the Iranian Plateau, in the Hindu Kush region, and West Pakistan are delineated. The magnitude-frequency relationship of the earthquakes which occurred in Iran during this period is discussed.
A surface wave particle motion discrimination process
Authors: Simons R. S.
Published: July 2021

A digital computer process is described which examines the three-componen particle-motion pattern of seismic data as a function of frequency and uses this information to discriminate against all motion except surface waves arriving from some pre-assigned direction. Application of the process to long-period data has shown that it can substantially improve the signal-to-noise ratios of Love and Rayleigh waves from earthquakes and underground detonations.
A synthesis of short-period P-wave records from distant explosion sources
Authors: Kogeus (Korkman) Kaj
Published: July 2021

A computer program is developed for computation of theoretical seismograms. The program treats explosive sources in layered crustal structures and computes frequency-domain functions for the crust, mantle and instrument transfers. The product of these is Fourier-inverted to give the seismograph trace. A brief study of parameter variation is presented.
Seismicity in the vicinity of the Ramapo fault, New Jersey-New York
Authors: Page Robert A. , Molnar Peter H. , Oliver Jack
Published: July 2021

Since 1962 the Lamont seismograph network in southeastern New York and northern New Jersey has recorded four small earthquakes with epicenters along the Ramapo fault near the New York-New Jersey border. The northeast trending fault separates the Triassic basin to the southeast from the Precambrian highland to the northwest. The proximity of these events to the fault and the previous earthquake history of the area suggest that much of the seismic activity of the area is associated with this major tectonic feature, which has apparently not been highly active since the Triassic.
Parkfield earthquake of June 28, 1966: Magnitude and source mechanism
Authors: Wu Francis T.
Published: July 2021

The Parkfield earthquake of June 28, 1966 (04:26:12.4 GMT) is studied using short-period and long-period teleseismic records. It is found that (1) Mb = 5.8 and Ms = 6.4 as compared to Mb = 5.4 and Ms = 5.4 for the foreshock (04:08:54), (2) both the Rayleigh and Love wave radiation patterns conform to those of a double couple at a depth of about 8.6 km, (3) the main shock can be represented by a series of shocks separated in space and time.

The near-field strong-motion data support the last conclusion. Based on strong-motion seismograms, and the surficial evidences of the dimensions of the fault, the energy is found to be 1021 ergs.
Array design
Authors: Haubrich Richard A.
Published: July 2021

abstract

Arrays of detectors placed at discrete points are often used in problems requiring high resolution in wave number for a limited number of detectors. The resolution performance of an array depends on the positions of detectors as well as the data processing of the array output. The performance can be expressed in terms of the “spectrum window”.

Spectrum windows may be designed by a general least-square fit procedure. An alternate approach is to design the array to obtain the largest uniformly spaced coarray, the set of points which includes all the difference spacings of the array. Some designs obtained from the two methods are given and compared.
Some statistical aspects of the estimation of seismic travel times
Authors: Tucker William , Herrin Eugene , Freedman Helen W.
Published: July 2021

Some of the statistical aspects of estimating travel-time anomalies and station corrections are considered. In order to estimate these quantities using earthquake data the events themselves must first be located. We investigated the use of the Gauss-Newton iterative technique to obtain a least-squares epicenter location employing Monte Carlo methods. Results of these studies indicate that the Gauss-Newton process converges to an absolute minimum and that confidence ellipses computed by linear techniques are reliable for reasonable networks of well-distributed stations. Also the Monte Carlo studies indicate that a least-squares solution may be inaccurate if appreciable travel-time anomalies or station-error means exist. We then expanded the location procedure to include the estimation of travel-time anomalies and station corrections. In order to obtain these estimates data from some 278 large earthquakes were analyzed by using a modified Seidel iterative process.
Multipolar elastic fields in a layered half space
Authors: Ben-Menahem Ari , Jit Singh Sarva
Published: July 2021

Hansen's expansion is used to derive integral expressions for the displacement field due to a localized buried source of the mth order in a layered half space. The dipolar case (m ≦ 2) is worked out in detail for arbitrary source-depth in the layer and in the substratum. A new type of representation of the source is used which gives the final results in a concise form. Explicit expressions for the displacements at the free surface are obtained for a center of explosion, a vertical strike-slip fault and a vertical dip-slip fault. The results for a horizontal thrust are found to be the same as for a vertical dip-slip fault.

The relations between the Galerkin vector and the biharmonic eigenvectors are clarified. It is shown that the Galerkin-Boussinesq solution for the elastic half space cannot be extended to structures of higher complexity, except for a few simple sources. On the other hand, the Hansen Solution is valid for a wide class of sources and structures.

Both dynamic and static regimes are considered.
Engineering seismic risk analysis
Authors: Cornell C. Allin
Published: July 2021

This paper introduces a method for the evaluation of the seismic risk at the site of an engineering project. The results are in terms of a ground motion parameter (such as peak acceleration) versus average return period. The method incorporates the influence of all potential sources of earthquakes and the average activity rates assigned to them. Arbitrary geographical relationships between the site and potential point, line, or areal sources can be modeled with computational ease. In the range of interest, the derived distributions of maximum annual ground motions are in the form of Type I or Type II extreme value distributions, if the more commonly assumed magnitude distribution and attenuation laws are used.
A seismic refraction profile and crustal structure in central interior Alaska
Authors: Hanson Kenneth , Berg Eduard , Gedney Larry
Published: July 2021

An unreversed seismic refraction profile was obtained during the winter of 1966-1967 from several small chemical explosions in an open pit coal mine near Suntrana, Alaska. The profile runs from south to north across the Tanana Basin in central interior Alaska. Both P and S waves, generated by the explosions, were observed out to a distance of 217 km from the shot point. A four-layer crustal model is adopted to explain the first and later prominent seismic phases. The P velocities range from 3.67 km/sec in the sedimentary surface layer to an apparent value of 8.83 km/sec in the upper mantle. It is assumed that the Moho is dipping to the south under the Alaska Range, from a depth of 32 km at Fairbanks to approximately 48 km under the shot point.
Source bias in epicenter determinations
Authors: Herrin Eugene , Taggart James
Published: July 2021

Epicenter determinations using data from stations at distances greater than 20° from the source make use of standard travel times based on a spherically symmetrical Earth. Lateral inhomogeneities in the upper mantle result in relative delays with respect to the standard times. Delays associated with the end of the up-traveling ray can be handled through the use of station corrections. A constant delay beneath the source can not be easily corrected, but it will result only in errors in origin time. However, if the delay arising beneath the source changes with azimuth, a consistent error, here called source bias, will be present in the estimate of the epicenter. Studies of explosions within continental masses have revealed no significant source bias; however, events on two linear, volcanic island chains in the Pacific (Rat Islands-Aleutians and Hawaii) show significant source bias. Errors arising from this effect may be as large as 12 degree and are most likely to occur with events near tectonically active island chains.
A microearthquake swarm in 1965 near Mould Bay, N.W.T., Canada
Authors: Smith T. E. T. , Whitham K. , Piché W. T.
Published: July 2021

In a 30-day period beginning March 29, 1965, some 2026 microearthquakes were recorded at MBC and by the array NPNT. About 10 per cent of them were located. The extent of the epicentral region was 3 sq km and of the hypocentral region 8 cu km. The centroid of the hypocenters was 12.6 km from MBC on an azimuth of 121° and at a depth of 6.8 km, very nearly on the strike of an old fault. Although no foreshock-aftershock series could be distinguished, the events were not independent nor randomly distributed in time.

Particle motion diagrams were drawn for 10 events. These identified the longitudinal and transverse waves and confirmed the azimuths and angles of emergence obtained from amplitude data. The array data also confirmed the azimuths and yielded a P-wave velocity of 4.33 km/s near the surface.

The magnitudes ranged from − 1.1 to 2.9 and the frequency-magnitude relation was:

log N = 2.46 ( ± 0.01 ) − 0.68 ( ± 0.01 ) M L .

The coefficient 0.68 suggests tectonic activity. The partition of energy between P and S, and the observed directions of motion (all compressions at MBC) are compatible with the double couple model of focal mechanism and motion on the old fault. However, magma movement cannot be entirely ruled out as there exists geomagnetic evidence of abnormally high upper mantle temperatures.

The following energy-magnitude relation was determined from 100 events in the range 0.1 ≦ ML ≦ 1.6:

log E = 10.1 ( ± 0.3 ) + 1.91 ( ± 0.03 ) M L .