Showing papers in "Bulletin of the Seismological Society of America in 1970"

Effects of local geology on ground motion near San Francisco Bay

Abstract: Measurements of ground motion generated by nuclear explosions in Nevada were made for 37 locations near San Francisco Bay, California. The results were compared with the San Francisco 1906 earthquake intensities and the strong-motion recordings of the San Francisco earthquake of March 22, 1957. The recordings show marked amplitude variations which are related consistently to the geologic setting of the recording site. For sites underlain by a layer of younger bay mud or artificial fill, maximum horizontal ground velocities generally increased with thickness of the layer and were as much as ten times greater than those recorded on nearby bedrock. The maximum vertical velocities for these sites were between 1 and 3.5 times greater. Spectral amplification curves clearly define a “dominant ground period” of about 1 second for sites underlain by younger bay mud. For sites underlain by older, more consolidated sediments, no clearly defined “dominant ground period” was found. Maximum ground velocities for the older bay sediment sites were about twice those recorded on bedrock. Consistent correlations of the results from the nuclear recordings with the 1906 earthquake intensities and the spectral amplification curves for the 1957 earthquake suggest that areas of high amplification determined from small ground motions may also be areas of high intensity in future earthquakes.

Aftershocks of the 1966 Parkfield-Cholame, California, earthquake: A detailed study

Abstract: Hypocenters and magnitudes of more than 600 aftershocks of the 1966 Parkfield-Cholame earthquake were determined from recordings of a dense network of portable seismograph stations operated in the epicentral region from 3 to 82 days after the main shock. Hypocenters were virtually confined to a nearly vertical zone extending downward from the zone of visible ground fracturing at the Earth9s surface to a depth of 12 to 14 km. Aftershocks were concentrated in patches on the slip surface, with large numbers of events in depth ranges of 2 to 4 km and 8 to 10 km and relatively few at depths of 5 to 7 km. First-motion patterns suggest that simple, nearly horizontal right-lateral strike-slip displacement was the source for an overwhelming majority of the aftershocks. The coefficient “ b ” in the frequency versus magnitude equation appears to be depth-dependent: it is about − 0.6 for events between 8- and 10-km depth but averages about −0.95 for events at other depths. Individual stations with consistently late P -wave arrivals were also found to record abnormally large amplitudes, with both anomalies increasing, apparently, with increasing thickness of sediments beneath the site.

Surface waves in multilayered elastic media. Part II. Higher mode spectra and spectral ratios from point sources in plane layered Earth models

Abstract: Phase and amplitude spectra of Rayleigh and Love waves are presented for two Earth models, one oceanic and one continental shield. The spectra of the first three Rayleigh modes and the first four Love modes are tabulated for point sources at selected depths. These tables along with computer algorithms described here allow one to estimate the amplitude spectra at nontabulated source depths. The use of spectral ratios as a means of determining source depth is investigated. A source depth of 20 km is obtained for the Fallon earthquake of July 20 1962. This depth agrees with previous estimates but the technique requires a fault-plane orientation which differs from radiation pattern solutions.

Lumped mass method for Rayleigh waves

Abstract: A simple numerical method is developed for the analysis of generalized Rayleigh waves in multilayered elastic media. The method which completely avoids the use of displacement potentials leads to a simple eigenvalue problem which may be solved by generally available effective computer codes. Numerical results obtained by the method show excellent agreement with previously published solutions obtained by other theories.

Complexity of energy release during the Imperial Valley, California, earthquake of 1940

Abstract: The pattern of energy release during the Imperial Valley, California, earthquake of 1940 is studied by analyzing the El Centro strong motion seismograph record and records from the Tinemaha seismograph station, 546 km from the epicenter. The earthquake was a multiple event sequence with at least 4 events recorded at El Centro in the first 25 seconds, followed by 9 events recorded in the next 5 minutes. Clear P, S, and surface waves were observed on the strong motion record. Although the main part of the earthquake energy was released during the first 15 seconds, some of the later events were as large as M = 5.8 and thus are important for earthquake engineering studies. The moment calculated using Fourier analysis of surface waves agrees with the moment estimated from field measurements of fault offset after the earthquake. The earthquake engineering significance of the complex pattern of energy release is discussed. It is concluded that a cumulative increase in amplitudes of building vibration resulting from the present sequence of shocks would be significant only for structures with relatively long natural period of vibration. However, progressive weakening effects may also lead to greater damage for multiple event earthquakes.

Earthquake mechanisms and island arc tectonics in the Indonesian-Philippine region

Abstract: Forty-four new focal mechanisms from shallow-focus earthquakes in the Indonesian-Philippine region have been determined from P and S waves recorded on long-period instruments. The dominant modes of deformation are thrust and normal faulting rather than strike-slip faulting. Along the Sunda and Philippine arcs the most active zone of shallow focus activity occurs between the ocean trench and the line of active volcanoes. Mechanism solutions from earthquakes in this zone are all of the thrust type with the sense of motion on the shallower dipping of the two nodal planes consistent with underthrusting beneath the island arc. Seismic slip vectors strike in a northeasterly direction along the convex side of the western Sunda arc and strike in a westerly direction along the Philippine arc and its extension along the eastern margin of the Celebes Sea. Normal faulting mechanisms from one earthquake under the Java Trench and another under the Philippine Trench may result from extension in the upper surface of the lithosphere as it bends beneath the island arc. The curvature of the Sunda arc may have caused the inclined seismic zone beneath the arc to shoal toward the northwest. There is a correlation between negative isostatic gravity anomalies, maximum water depth in the trench and the length and shape of the seismic zone beneath the Sunda arc. Shoaling of the inclined seismic zone beneath the nearly linear Phillippine arc can be explained by a decrease in the slip rates from south to north along the arc. Complex regions such as western New Guinea, the Sulu Spur and lesser Philippine islands may include a number of small plates of lithosphere.

Surface-wave dispersion computations

Abstract: Fundamental-mode Love- and Rayleigh-wave dispersion computations for multilayered, perfectly-elastic media were studied. The speed of these computations was improved, and the accuracy brought under full control. With sixteen decimal digits employed in these computations, fifteen significant-figure accuracy was found possible with Love waves and twelve to thirteen figure accuracy with Rayleigh waves. In order to ensure that the computed dispersion is correct to a specified accuracy, say σ significant figures, ( σ + 1)/4 wavelengths of layered structure must be retained above a homogeneous half-space. To this accuracy, the homogeneous half-space is a sufficient model of the true layering it replaces. Using this result, it was possible to refine the usual layer-reduction technique so as to ensure retention of the specified accuracy while employing reduction. With this reduction technique in effect, and with σ specified below single-precision accuracy, the program can be run entirely in single precision; the specified accuracy is maintained without overflow or loss-of-precision problems being encountered during calculations.

Seismic wave velocities and earth structure on the African continent

Abstract: Phase and group velocities of seismic surface waves were determined for the African continent. Rayleigh wave phase velocities range from approximately 3.90 to 4.20 km/sec in the period range of 30.0 to 63.0 sec. Group velocities of fundamental mode Love and Rayleigh waves in the period range of 10.0 to 60.0 sec range from 3.35 to 4.15 km/sec and 3.00 to 3.85 km/sec respectively. As expected, for the shorter periods the velocities depend strongly upon regional structure. Group velocities of sedimentary Rayleigh waves and of higher mode Rayleigh waves have also been determined. Short period body-wave velocities were determined for this region as follows: Pn = 8.06 km/sec, Sn = 4.55-4.72 km/sec, and Lg = 3.48-3.60 km/sec. Due to insufficient observations, a Pg velocity was not determined. The observed surface wave velocities are similar to those reported for the Baltic Shield and differ somewhat from those velocities found for the Canadian Shield. A theoretical model that accounts for the observed velocities is derived for the African continent. Regional variations in Sn propagation indicated structural differences which may be explained by the existence of a gap in the mantle portion of the lithosphere beneath the northern part of the African rift zone.

Accumulation of tectonic strain in California

Abstract: The shear-strain accumulation in five tectonically-active areas of California has been calculated from triangulation data supplied by the U. S. Coast and Geodetic Survey. Three of the areas lie along active sections of the San Andreas fault. Near Hollister, no appreciable strain accumulation was detected for the period 1930 to 1962. The movement of the fault blocks there appears to be accommodated by slip on the San Andreas and Calaveras faults. Near Cholame, the only appreciable accumulation of shear strain in the period 1932 to 1962 appears to be associated directly with slip on the San Andreas north of Cholame, slip which probably occurred during the 1934 Parkfield earthquake. Significant strain accumulation was confined to a zone centered on the San Andreas fault and extending 10 km on either side. Some of this strain was released in the 1966 Parkfield earthquake. In Imperial Valley, an average accumulation of γ = 0.4 μstrain per year right-lateral (referred to vertical planes parallel to the Imperial fault) shear strain extends over a zone perhaps 100 km wide centered on the Imperial fault. It appears that this shear pattern may be resolved into two zones of shear, one concentrated near the Imperial fault and the other near the San Andreas fault. No appreciable shear-strain accumulation was detected in the two areas that do not lie on the San Andreas fault—Santa Barbara channel for the period 1880 to 1923 and Owens Valley for the period 1934 to 1956.

Analysis of rayleigh-wave multipath propagation at lasa*

Abstract: An investigation has been made of the multipath propagation of Rayleigh waves by using data obtained from the large aperture seismic array (LASA). The use of the LASA in conjunction with a high-resolution analysis technique provides a greater angular resolution and accuracy than was previously possible for the analysis of the multipath propagation. Measurements have been made of this phenomenon for the Rayleigh waves of 26 events distributed at various azimuths and distances from LASA. On the bas of these measurements reasonably good conjectures are made concerning the actual propagation paths for groups in the 20- to 40-sec period range. It is shown that in almost all cases these propagation paths can be associated with refractions and reflections at the continental margins.

The travel times of S and SKS

Abstract: Observations are reported of the travel times of S and SKS for arc distances of 30° to 126°. It was found that for S the new times do not differ significantly from the Jeffreys-Bullen tables up to 80° and are up to 4 seconds later between 80° and 100°. The apparent velocity found for the diffracted S phase was 1/8.941 degrees per second agreeing well with that reported by Cleary. The S station anomalies found in this study were about four times those for P in agreement with the earlier analysis of Doyle and Hales. As was the case for P the travel times for S can be fitted to within experimental error by a quadratic in Δ. However, in this case, the values of dT/d Δ found from the raw data suggest that there is a discontinuity in dt/d Δ at about 42°.

A clustering model for earthquake occurrences

Abstract: A statistical analysis of the time occurrence of the earthquakes listed in the USCGS catalog was performed. Except for the deep earthquakes, goodness of fit tests have rejected the simple Poisson process as a model of earthquake occurrences at a very low significance level. It was inferred that another parameter related to the Poisson index of dispersion is required to describe earthquake occurrences. Attempts to fit the distribution of the number of days with n earthquakes with standard two-parameter distributions like the lognormal and the γ distribution were not very successful. The data were tested with respect to the generalized Poisson process, a model in which there are finite probabilities for more than one event occurring at the same time instant. It was deduced that the distribution of the number of earthquakes generated in an aftershock sequence is reasonably approximated by an inverse power law distribution with exponent E being between 2.5 and 4.0. Good fits were obtained with respect to the generalized Poisson distribution, and regional variations in the parameter E could be detected. Estimates of the autocovariance function and power spectra failed to detect definite periodicities of earthquake occurrences in the range of 2 to 256 days. Positive correlations of earthquake activity were observed for time intervals as long as 10 days.

Exact earth-flattening calculation for Love waves

Abstract: The calculation of the periods of the free modes of torsional oscillations of an inhomogeneous earth, as well as the phase and group velocities of Love waves on an inhomogeneous sphere, can be performed by evaluating the properties of an isotropic flat-earth structure derived from the spherical structure. The differences between spherical- and flat-earth phase and group velocities can be expressed as polynomials in the periods up to 600 sec.

Lateral variation of attenuation of P and S waves beneath the United States

Abstract: New observations of the differential attenuation of long period S waves, measured using a spectral ratio technique, and a re-examination of reported attenuation measurements for P waves indicate significant lateral variations in attenuation beneath the United States. High attentuation is observed at stations between the Rocky Mountains and the Sierra Nevada-Cascade ranges and in the northeast United States. Low attenuation is seen at stations in the central and eastern parts of the country and along the Pacific coast. Relative magnitudes of S - and P -differential attenuation are consistent with P -wave absorption being due entirely to losses in shear. A large azimuthal dependence for near-source attenuation is suggested for an earthquake on a mid-ocean ridge.

A note on fast computation of Rayleigh wave dispersion in the multilayered elastic half-space

Abstract: Matrix formulas for modal solutions in the layered, elastic half-space are modified to give faster machine computation. Accuracy at high frequencies is also included. Computing time is reduced by 30 per cent from the fastest program previously reported.

Study of regional seismicity and associated problems

Abstract: This paper constitutes a compilation of seismicity data available in the literature plus regional and worldwide data obtained from the USCGS Preliminary Determination of Epicenter (PDE) lists. Data are presented in the form of the recurrence relationship of log N versus magnitude, where N is either cumulative or incremental number of earthquakes, and magnitude is either m b or M S . Relative shapes of these recurrence curves as regards m b versus M S , large magnitude versus small magnitude, shallow focus versus deep focus, etc. are discussed. Conclusions on variability of seismicity with time and on worldwide level of seismic activity are included.

Evolutionary power spectral density of strong-motion earthquakes

Abstract: This paper presents the evolutionary (time-dependent) power spectral density curves of six strong-motion earthquakes. The earthquake accelerograms are treated as piecewise-separable and the frequency-independent modulating function is estimated by applying the mean square minimization criterion. The results show that, as the time increases, the power spectral densities of earthquakes can vary both in magnitude and in peak location over the frequency axis. The analysis demonstrates that the nonstationary characteristics of earthquakes can be reasonably assessed by a simple computation procedure and the information thus obtained can be helpful in developing improved stochastic models for earthquake simulation.

Surfa ce-wave dispersion computations: Knopoff's method

Abstract: The optimization of Knopoff's method computer computation of surface-wave dispersion, and the comparison of this method with the Thomson-Haskell technic and its extensions are studied. Of the various versions of the Thomson-Haskell formulation for Rayleigh-wave dispersion computations, the reduced-δ-matrix extension is the most powerful, i.e., the fastest which contains the feature controlling the loss-of-precision problem occasionally encountered by the original formulation. It is shown that the results of the δ-matrix extensions are actually contained in Knopoff's work, which appeared earlier than these extensions, and that these results are obtainable directly from his formulation without recourse to δ-matrix theory. The flexibility of Knopoff's method is used to devise a new representation of the Rayleigh-wave dispersion function which is more powerful than the most powerful of the Thomson-Haskell versions, i.e., it contains the loss-of-precision control feature and is about 38 per cent faster than the reduced-δ-matrix extension; in fact, it is about 12 per cent faster than the fastest of the Thomson-Haskell versions. Explicit forms of the new representation are given for the layered-half-space analogs of continental and oceanic models of the Earth, which can be terminated by a homogeneous half-space beginning in the mantle, a liquid outer core, or a solid inner core. The representations for symmetric and antisymmetric modes of the symmetric-plate analog of the Earth are also given. The Rayleigh-wave dispersion functions for the product form of the original Thomson-Haskell formulation, the δ-matrix and reduced-δ-matrix extensions of this formulation, and the representations developed from Knopoff's method, are identical when written out fully. The various technics differ only in their matrix representations of the dispersion function, which, of course, govern the speeds of computation of the various methods. The Knopoff and Thomson-Haskell technics for computing Love-wave dispersion are compared. Although the matrix representations of the dispersion function differ for the two technics, the functions are identical, except for a possible sign reversal, when written out fully. The speeds of computation are also identical. The representations derived from Knopoff's method are given for the two possible flat-layer analogs of the Earth, which are applicable to Love-wave propagation.

Microearthquake seismicity and tectonics of Coastal Northern California

Abstract: This paper demonstrates that high-gain, high-frequency portable seismographs operated for short intervals can provide unique data on the details of the current tectonic activity in a very small area. Five high-frequency, high-gain seismographs were operated at 25 sites along the coast of northern California during the summer of 1968. Eighty per cent of 160 microearthquakes located in the Cape Mendocino area occurred at depths between 15 and 35 km in a well-defined, horizontal seismic layer. These depths are significantly greater than those reported for other areas along the San Andreas fault system in California. Many of the earthquakes of the Cape Mendocino area occurred in sequences that have approximately the same magnitude versus length of faulting characteristics as other California earthquakes. Consistent first-motion directions are recorded from microearthquakes located within suitably chosen subdivisions of the active area. Composite fault plane solutions indicate that right-lateral movement prevails on strike-slip faults that radiate from Cape Mendocino northwest toward the Gorda basin. This is evidence that the Gorda basin is undergoing internal deformation. Inland, east of Cape Mendocino, a significant component of thrust faulting prevails for all the composite fault plane solutions. Thrusting is predominant in the fault plane solution of the June 26 1968 earthquake located along the Gorda escarpement. In general, the pattern of slip is consistent with a north-south crustal shortening. The Gorda escarpment, the Mattole River Valley, and the 1906 fault break northwest of Shelter Cove define a sharp bend that forms a possible connection between the Mendocino escarpment and the San Andreas fault. The distribution of hypocenters, relative travel times of P waves, and focal mechanisms strongly indicate that the above three features are surface expressions of an important structural boundary. The sharp bend in this boundary, which is concave toward the southwest, would tend to lock the dextral slip along the San Andreas fault and thus cause the regional north-south compression observed at Cape Mendocino. The above conclusions support the hypothesis that dextral strike-slip motion along the San Andreas fault is currently being taken up by slip along the Mendocino escarpment as well as by slip along northwest trending faults in the Gorda basin.

Calculation of surface motions of a layered half-space

Abstract: A new method is presented for computing the transient response of a set of horizontally stratified, linearly elastic layers overlying a uniform half-space and excited by vertically incident, transient plane waves. In addition, a simple approximate method of satisfactory accuracy is developed that reduces the computing time required. Calculated responses are compared with motions recorded under Union Bay in Seattle to evaluate the agreement between recorded and calculated motions.

Analyses of ground motions at Union Bay, Seattle during earthquakes and distant nuclear blasts

Abstract: Recordings of accelerations at three different depths within the soil profile at Union Bay were obtained during a mild nearby earthquake and three distant nuclear blast events. These recordings were made by instruments which had been placed in peat 10 feet below the ground surface, in clay 61 feet below the ground surface and in glacial till 105 feet below the ground surface. Accelerations in the EW, NS and vertical directions were recorded simultaneously at these three depths. Using an equivalent linear variable damping lumped-mass solution, incorporating strain-dependent material properties for the peat and clay layers, the motions throughout the deposits were computed. The recorded accelerations in the glacial till were used as input base motion. Comparisons of the time histories of accelerations, maximum accelerations and spectral values of the motions recorded during the nearby earthquake with the corresponding computed motions indicated a high degree of agreement. The maximum accelerations recorded during the distant nuclear blast events also compared well with the computed accelerations.

Distant motions from a building vibration test

Abstract: Horizontal ground motion generated by vibration tests of the nine-story Millikan Library Building on the Caltech campus was recorded on the surface of the ground in the Pasadena area at distances up to 3 miles from the building. Later it was learned that the vertical component of the motion also was recorded by the seismograph on Mt. Wilson, 6.7 miles from the Library and 4,800 ft higher in elevation. The magnitude of the acceleration varied from 2.04 × 10-^(2)g at the excitation level on the ninth floor of the building to 3.2 × 10^(-7)g at Mt. Wilson. Simple calculations show that multistory buildings are particularly well-suited for inducing large dynamic forces in the ground with relatively small equipment.

Fast evaluation of source parameters from isolated surface-wave signals. Part I. Universal tables

Abstract: Tables for spectral displacements of seismic surface waves from shear dislocations in flat multilayered earth models were prepared. Earth response functions for seven modes (R_(11), R_(21), R_(12), L_0, L_1, L_2, L_3) at six periods (300 sec, 250 sec, 200 sec, 150 sec, 100 sec, 50 sec) and three paths (continental, oceanic, shield) were calculated for the source-depth range of 10 to 600 km at intervals of 5 km until 200 km, and thereafter at intervals of 10 km. Ground motion is given in micron-seconds for the three fundamental shear dislocations, each of strength U_0dS = 10^3 (m × km^2) and a delta-function time-dependence. The tables provide the means for rapid evaluation of source parameters from spectral radiation patterns of amplitudes and initial phases.

Seismic studies: South central Illinois earthquake of November 9, 1968

Abstract: The largest earthquake to occur in the central Mississippi seismic region this century took place in south central Illinois on November 9, 1968. The hypocenter and origin time based on observations from twelve regional stations varying in epicentral distance from 171 to 549 km, are 37.95°N, 88.48°W, h = 25 km, 0 = 17 h 01 m 42.0 s ± 0.2 s . Travel times of P at stations distant less than 2600 km indicate regional mantle variations, corresponding to rays bottoming at depths down to 650 km. Beyond this point travel times show a much smaller dependence, if any, on region. For stations in the central United States P times may be fitted by two straight line branches which intersect at about 600 km. The first branch corresponds to P n , the second to rays refracted from a surface at depth 97 km with a velocity below it of 8.37 km/sec. At larger distances (48°-100°) there are non-azimuth dependent residuals with respect to the Herrin Tables averaging about −1.5 sec, indicating a source-region correction with respect to these tables. Body wave magnitude was determined to be m b = 5.54 ± 0.44 for stations for which Δ > 25°, and m b = 5.44 ± 0.29, using Evernden9s formula, for P n in eastern North America. Surface waves give a value M s = 5.2. The fault plane solution determines two nodal planes each striking approximately north-south and dipping 45° to the east and to the west, respectively. This corresponds to dip slip, reverse motion, and to a horizontal east-west axis of compressional stress. While there are no mapped faults in the immediate epicentral region, the motion indicated is in conformity to that along the Wabash Valley Fault System 10 miles to the east.

Shear velocities in the lower mantle and the radius of the core

Abstract: Lower earth mantle shear velocity models derivation and core radius estimation based on S travel times observation

Ocean microseism measurements with a neutral bouyancy free-floating midwater seismometer

Abstract: Seismometers in spherical aluminum pressure housings have been weighted to float stably at midwater depths in the ocean, and thus record water motions in a frequency band of 0.02 to 5 cps. Simultaneous records made with a midwater instrument at 1.2-km depth and a bottom instrument at 4.6-km depth showed coherence at spectral power peaks of leaky organ-pipe frequencies and additional coherence peaks at frequencies down to 0.025 cps. Twenty organ-pipe modes can be tentatively identified. The spectral power can be attributed almost entirely to microseismic motions in wave-guide modes. We conclude that the forcing functions for microseisms are broad enough so that deep ocean-bottom and midwater microseism spectral peak frequencies are characteristic of local bathymmetry.