Showing papers in "Bulletin of the Seismological Society of America in 1976"

Dynamics of an expanding circular fault

Abstract: We study a plane circular model of a frictional fault using numerical methods. The model is dynamic since we specify the effective stress at the fault. In one model we assume that the fault appears instantaneously in the medium; in another, that the rupture nucleates at the center and that rupture proceeds at constant subsonic velocity until it suddenly stops. The total source slip is larger at the center and the rise time is also longer at the center of the fault. The dynamic slip overshoots the static slip by 15 to 35 per cent. As a consequence, the stress drop is larger than the effective stress and the apparent stress is less than one half the effective stress. The far-field radiation is discussed in detail. We distinguish three spectral regions. First, the usual constant low-frequency level. Second, an intermediate region controlled by the fault size and, finally, the high-frequency asymptote. The central region includes the corner frequency and is quite complicated. The corner frequency is shown to be inversely proportional to the width of the far-field displacement pulse which, in turn, is related to the time lag between the stopping phases. The average corner frequency of S waves v s is related to the final source radius, a , by v s = 0.21 β/α . The corner frequency of P waves is larger than v s by an average factor of 1.5.

Scaling relations for earthquake source parameters and magnitudes

Abstract: A data set of 41 moderate and large earthquakes has been used to derive scaling rules for kinematic fault parameters. If effective stress and static stress drop are equal, then fault rise time, τ, and fault area, S, are related by τ = 16S^(1/2)/(7π^(3/2)β), where β is shear velocity. Fault length (parallel to strike) and width (parallel to dip) are empirically related by L=2W. Scatter for both scaling rules is about a factor of two. These scaling laws combine to give width and rise time in terms of fault length. Length is then used as the sole free parameter in a Haskell type fault model to derive scaling laws relating seismic moment to M_S (20-sec surface-wave magnitude), M_S to S and m_b (1-sec body-wave magnitude) to M_S. Observed data agree well with the predicted scaling relation. The “source spectrum” depends on both azimuth and apparent velocity of the phase or mode, so there is a different “source spectrum” for each mode, rather than a single spectrum for all modes. Furthermore, fault width (i.e., the two dimensionality of faults) must not be neglected. Inclusion of width leads to different average source spectra for surface waves and body waves. These spectra in turn imply that m_b and M_S reach maximum values regardless of further increases in L and seismic moment. The m_b versus M_S relation from this study differs significantly from the Gutenberg-Richter (G-R) relation, because the G-R equation was derived for body waves with a predominant period of about 5 sec and thus does not apply to modern 1-sec m_b determinations. Previous investigators who assumed that the G-R relation was derived from 1-sec data were in error. Finally, averaging reported rupture velocities yields the relation v_R = 0.72β.

Site-dependent spectra for earthquake-resistant design

Abstract: The paper presents the results of a statistical analysis of the spectral shapes of 104 ground-motion records obtained from 23 earthquakes, mostly in the western part of the United States. The analysis shows clear differences in spectral shapes for different soil and geological conditions, indicating the need for consideration of these effects in selecting earthquake-resistant design criteria.

Effects of local geological conditions in the San Francisco Bay region on ground motions and the intensities of the 1906 earthquake

Abstract: Measurements of ground motion generated by nuclear explosions in Nevada have been completed for 99 locations in the San Francisco Bay region, California. The recordings show marked amplitude variations in the frequency band 0.25 to 3.0 Hz that are consistently related to the local geological conditions of the recording site. The average spectral amplifications observed for vertical and horizontal ground motions are, respectively: (1, 1) for granite, (1.5, 1.6) for the Franciscan Formation, (3.0, 2.7) for the Santa Clara Formation, (3.3, 4.4) for alluvium, and (3.7, 11.3) for bay mud. Spectral amplification curves define predominant ground frequencies in the band 0.25 to 3.0 E for bay mud sites and for some alluvial sites. Amplitude spectra computed from recordings of seismic background noise at 50 sites do not generally define predominant ground frequencies. The intensities ascribed to various sites in the San Francisco Bay region for the California earthquake of April 18, 1906, are strongly dependent on distance from the zone of surface faulting and the geological character of the ground. Considering only those sites (approximately one square city block in size) for which there is good evidence for the degree of ascribed intensity, the intensities for 917 sites on Franciscan rocks generally decrease with the logarithm of distance as Intensity = 2 . 6 9 - 1 . 9 0 log ( Distance in kilometers ) . ( 1 ) For sites on other geological units, intensity increments, derived from this empirical relation, correlate strongly with the Average Horizontal Spectral Amplifications (AHSA) according to the empirical relation Intensity Increment = 0 . 2 7 + 2 . 7 0 log ( AHSA ) . ( 2 ) Average intensity increments predicted for the various geological units are −0.3 for granite, 0.2 for the Franciscan Formation, 0.6 for the Great Valley sequence, 0.8 for the Santa Clara Formation, 1.3 for alluvium, and 2.4 for bay mud. The maximum intensity map predicted on the basis of these data delineates areas in the San Francisco Bay region of potentially high intensity for large earthquakes on either the San Andreas fault or the Hayward fault. The map provides a crude form of seismic zonation for the region and may be useful for certain general types of land-use zonation.

The mechanics of locating earthquakes

Abstract: A complete reexamination of Geiger9s method in the light of modern numerical analysis indicates that numerical stability can be insured by use of the QR algorithm and the convergence domain considerably enlarged by the introduction of step-length damping. In order to make the maximum use of all data, the method is developed assuming a priori estimates of the statistics of the random errors at each station. Numerical experiments indicate that the bulk of the joint probability density of the location parameters is in the linear region allowing simple estimates of the standard errors of the parameters. The location parameters are found to be distributed as one minus chi squared with m degrees of freedom, where m is the number of parameters, allowing the simple construction of confidence levels. The use of the chi-squared test with n-m degrees of freedom, where n is the number of data, is introduced as a means of qualitatively evaluating the correctness of the earth model.

Seismicity of Northern Anatolia

Abstract: Earthquakes of magnitude 5.0 and greater that occurred in 1930-1972 in northern Anatolia have been relocated in order to define more accurately the characteristics of recent seismicity. The revised epicenters were determined either by joint epicenter determination (JED) or singly, with travel-times modified by JED-calculated source-station adjustments. Calibration epicenters were assigned on the basis of published field studies of the earthquakes. Many characteristics of the occurrence of magnitude 5.0 and greater earthquakes on the North Anatolian fault are similar to characteristics of small-earthquake seismicity on California9s San Andreas fault. Earthquakes tend to be concentrated on or near particular sections of the North Anatolian fault, suggesting intrinsic differences in mechanical properties along the fault. The relocated epicenters support the hypothesis that fault rupture in large and great earthquakes will begin in regions of small and moderate earthquakes; the rupture of the large earthquake then propagates into sections of the fault that normally have a low level of activity. From 1939 through 1967, seven earthquakes of magnitude 6.8 or greater ruptured the North Anatolian fault from east to west for a distance of 800 km. Several sections of the fault zone were active before the occurrence of the large earthquakes of 1939-1967. Foreshock activity also extended tens of kilometers away from the fault zone. The time intervals between successive magnitude 6.0 or greater earthquakes on the fault are not consistent with a constant velocity of source migration; a model is proposed here in which these time intervals are equal to the duration of nonelastic effects precursory to the earthquakes. In western Turkey, the burst of normal-fault earthquakes in 1969-1970 was concentrated in distinctly separated source areas. The distribution of aftershocks to the earthquake of March 28, 1970 suggests that the surface fault scarps accompanying this earthquake are a distorted representation of the normal fault plane at depth.

Preliminary empirical model for scaling Fourier Amplitude Spectra of strong ground acceleration in terms of earthquake magnitude, source-to-station distance, and recording site conditions

Abstract: An empirical model for scaling Fourier Amplitude Spectra of strong earthquake ground acceleration in terms of magnitude, M, epicentral distance, R, and recording site conditions has been presented. The analysis based on this model implies that: 1.(a) the Fourier amplitude spectra of strong-motion accelerations are characterized by greater energy content and relatively larger amplitudes for long-period waves corresponding to larger magnitudes M, 2.(b) the shape of Fourier amplitude spectra does not vary appreciably for the distance range between about 10 and 100 km, and 3.(c) long-period spectral amplitudes (T > 1 sec) recorded on alluvium are on the average 2.5 times greater than amplitudes recorded on basement rocks, whereas short-period (T < 0.2 sec) spectral amplitudes tend to be larger on basement rocks. It has been shown that the uncertainties which are associated with the forecasting of Fourier amplitude spectra in terms of magnitude, epicentral distance, site conditions, and component direction are considerable and lead to the range of spectral amplitudes which for an 80 per cent confidence interval exceed one order of magnitude. A model has been presented which empirically approximates the distribution of Fourier spectrum amplitudes and enables one to estimate the spectral shapes which are not exceeded by the presently available data more than 100 (1 - p) per cent of time where p represents the desired confidence level (0 < p <1).

Seismotectonic provinces of Iran

Abstract: Over 600 earthquakes that occurred in Iran between 1920 and 1972 have been relocated. The new epicentral positions delineate many active faults and seismically active tectonic zones in Iran. The Ferdows fault, the Kuhbanan fault, the Nayband fault, various segments of the Shahrud fault, and a fault off the Coast of Makran are seismically active; there are other seismic trends which are not clearly related to corresponding surface faults. The Zagros thrust separates a more active folded series from the rest of Iran. The folded series of the Zagros itself shows various seismicity levels and is thus divided into several seismotectonic provinces. Older Arabian north-south trends in some areas of the folded series are marked by subcrustal earthquakes. The east-west trend near Lar consists of crustal earthquakes. The epicentral locations present a generally diffuse pattern, best explained in terms of relative motions between the Arabian plate and the Persian plate. The relocated epicenters, together with geological information, regional geomorphology, distribution of salt domes, structural trends, and active faults, are used to construct a seismotectonic map of Iran which includes 23 seismotectonic provinces. The seismicity and boundaries of each province are discussed.

A two-dimensional analysis of surface deformation due to dip-slip faulting

Abstract: A two-dimensional model of dip-slip faulting is developed on the basis of elastic dislocation theory. General relationships among the fault-slip distribution, the stress-drop distribution, and the crustal surface deformation are derived for this model for arbitrary dip angle. The relationships are valid for arbitrary distribution of slip on the fault plane, except for the requirement that all stress components be finite at the boundaries of the slip zone. In order to illustrate some of the features of the derived relationships, a sample calculation based on surface deformation data obtained following the 1964 Alaska earthquake is performed, and the calculated values of various fault parameters appear to fall within accepted limits. For purposes of direct comparison, the same calculations are performed assuming the slip distribution to be uniform over the slip zone.

Simplified procedures for estimating the fundamental period of a soil profile

Abstract: A study was made of simplified procedures for estimating the fundamental period, T , of a linear or equivalent linear model of a soil profile. Closed form solutions and approximate methods for computing the first period of an elastic shear-beam representation of the profile were examined. Closed form solutions are presented in chart form for the following cases: (a) shear-wave velocity increasing as a power of depth, (b) shear modulus increasing or decreasing linearly with depth, (c) two-layer profile, and (d) overconsolidated or uniform crust on layer with modulus increasing with depth. Seven approximate methods developed to estimate the period of a layered soil profile without a computer were also discussed and evaluated. For this evaluation, the periods of 76 representative soil profiles were estimated by each one of the approximate methods, and the results compared with the exact values. Methods 6 (Successive Use of Two-Layer Solution) and 7 (Simplified Version of the Rayleigh Procedure) gave errors less than 10 per cent for the period in all cases considered, and these methods are recommended for practical use. The approximate methods and the closed form solution charts may be used to estimate the characteristic site period, T s , included in the proposed (1976) modifications to the seismic provisions of the Unified Building Code.

A study of earthquake response spectra for different geological conditions

Abstract: The current earthquake design spectra are based mainly on response spectra from recording stations located on alluvium deposits. A limited number of studies have shown that shape and the magnitude of response spectra for stations located on rock deposits are different from those located on alluvium deposits. This study examines the effects of geological conditions on the response spectra and the ground-motion parameters such as peak ground acceleration, velocity, and displacement. Design spectra are presented for various sites such as alluvium deposits, rock deposits, and alluvium layers underlain by rock deposits. The study shows that the current design spectra are too conservative for most structures located on competent rock deposits.

Inversion of the body waves from the Borrego Mountain earthquake to the source mechanism

Abstract: The generalized linear inverse technique has been adapted to the problem of determining an earthquake source model from body-wave data. The technique has been successfully applied to the Borrego Mountain earthquake of April 9, 1968. Synthetic seismograms computed from the resulting model match in close detail the first 25 sec of long-period seismograms from a wide range of azimuths. The main shock source-time function has been determined by a new simultaneous short period-long period deconvolution technique as well as by the inversion technique. The duration and shape of this time function indicate that most of the body-wave energy was radiated from a surface with effective radius of only 8 km. This is much smaller than the total surface rupture length or the length of the aftershock zone. Along with the moment determination of M_o = 11.2 × 10^(25) dyne-cm, this radius implies a high stress drop of about 96 bars. Evidence in the amplitude data indicates that the polarization angle of shear waves is very sensitive to lateral structure.

Relationships of maximum acceleration, maximum velocity, distance from source, and local site conditions for moderately strong earthquakes

Abstract: The paper presents the results of a study to determine the influence of local geological conditions on the attenuation of peak accelerations and peak velocities, with increasing distance from the source of energy release, for earthquakes with a magnitude of about 6.5 occurring in the western part of the United States. While the results can only be considered to be strictly applicable to these conditions, it is hoped that they can also serve as a guide to possible relationships between peak accelerations and peak velocities which may be expected for earthquakes of higher magnitudes occurring in different locations and thereby serve a useful purpose in the difficult task of predicting the likely characteristics of earthquake ground motions in different geological settings.

Preliminary analysis of the peaks of strong earthquake ground motion--dependence of peaks on earthquake magnitude, epicentral distance, and recording site conditions

Abstract: Analyses of peak amplitudes of strong earthquake ground motion have been carried out with the emphasis on their dependence on earthquake magnitude, epicentral distance, and geological conditions at the recording site. Approximate empirical scaling functions have been developed which, for a selected confidence level, yield an estimate of an upper bound of peak accelerations, velocities, and displacements. The parameters in these scaling functions have been computed by least-squares fitting of the recorded data on peak amplitudes which are now available for a range of epicentral distances between about 20 and 200 km and are representative for the period from 1933 to 1971 in the Western United States. The possibility of extrapolating the derived scaling laws to small epicentral distances where no strong-motion data are currently available has been tested by comparing predicted peak amplitudes with related parameters at the earthquake source. These source parameters (average dislocation and stress drop) can be derived from other independent studies and do not contradict the inferences presented in this paper. It has been found that for an approximate 90 per cent confidence level the presently available data suggest that peak accelerations, velocities, and displacements at the fault and for the frequency band between 0.07 and 25 Hz probably do not exceed about 3 to 5 g, 400 to 700 cm/sec, and 200 to 400 cm, respectively. The logarithms of the peaks of strong ground motion seem to depend in a linear manner on earthquake magnitude only for small shocks. For large magnitudes this dependence disappears gradually and maximum amplitudes may be achieved for M ≈ 7.5. The influence of geological conditions at the recording site appears to be insignificant for peak accelerations but becomes progressively more important for peaks of strong-motion velocity and displacement.

Three-dimensional seismic structure of the lithosphere under Montana Lasa

Abstract: Using P -wave residuals for teleseismic events observed at the Montana Large Aperture Seismic Array (LASA), we have determined the three-dimensional seismic structure of the lithosphere under the array to a depth of 140 km. The root-mean-square velocity fluctuation was found to be at least 3.2 per cent which may be compared to estimate of ca. 2 per cent based on the Chernov random medium theory. The solutions are given by both the generalized inverse and stochastic inverse methods in order to demonstrate the relative merit of different inversion techniques. The most conspicuous feature of the lithosphere under LASA is a low-velocity anomaly in the central and northeast part of the array siting area with the N60°E trend and persisting from the upper crust to depths greater than 100 km. We interpret this low-velocity anomaly as a zone of weakness caused by faulting and shearing associated with the building of the Rocky Mountains.

Dynamic motions near an earthquake fault: a three dimensional solution

Abstract: The rupture process occurring during an earthquake is described here in terms of one of the few known solutions to a problem of brittle fracture, in which rupture originates at a point, and spreads over a fault plane, initiating shearing motions. Using a stress-relaxation model with a particular geometry of rupture growth, stresses and displacements can readily be found throughout the medium in which rupture is taking place. The qualitative and quantitative properties of this fracture solution can give considerable assistance in interpreting records of strong ground motion, and provides insight into the processes taking place at an earthquake source. It is shown that rupture speeds are likely to lie between the Rayleigh-wave and shear-wave speeds, that temperatures can be raised substantially by faulting, and that the ratio of particle velocity to stress drop is approximately proportional to rupture velocity. In order to obtain the rupture velocity in rock mechanics experiments, measurement of displacement normal to the fracture surface is recommended.

Tectonic implications of the Brawley earthquake swarm, Imperial Valley, California, January 1975

Abstract: The Brawley earthquake swarm provided a unique opportunity for studying a highly interesting tectonic region. The swarm was most intense for a period of 4 days including 75 events with M_L between 3.0 and 4.7 with a spatial extent of 12 km. Precise relative hypocenters were obtained for 264 earthquakes (M_L ≧ 1.5) using a master event method to calibrate the USGS Imperial Valley array. These locations together with well-constrained focal mechanisms for 16 of the largest events suggest faulting on at least three distinct structures. Hypocentral depths ranged from 4 to 8 km, compared to a basement depth of about 6 km for this part of the Imperial Valley. The swarm began on a nearly vertical right-lateral fault striking N8°W (Brawley fault) about 8 km southeast of Brawley at a point which had experienced enhanced shallow seismicity during the preceding 4 days. The seismicity migrated bilaterally north and south from this point at a constant velocity of 0.5 km/hr terminating to the north on a steeply south dipping, N50°E-striking fault. This structure is on trend with splays associated with the northern end of ground breakage of the 1940 Imperial Valley earthquake. To the south the seismicity ended near the northern end of the 10 km of surface rupture mapped by R. V. Sharp, which continues on strike to a point near the Imperial fault. Tectonic interpretations include the transfer of right-lateral offset from the Imperial fault to the Brawley fault associated with the formation of a closed depression bounded on the west and east by these two faults.

Analysis of seismograms from a downhole array in sediments near San Francisco Bay

Abstract: A four-level downhole array of three-component instruments was established on the southwest shore of San Francisco Bay to monitor the effect of the sediments on low-amplitude seismic ground motion. The deepest instrument is at a depth of 186 m—2 m below the top of the Franciscan bedrock. Earthquake data from regional distances (29 km ≦ Δ ≦ 485 km) over a wide range of azimuths are compared with the predictions of a simple plane-layered model with material properties independently determined. Spectral ratios between the surface and bedrock, computed for the one horizontal component of motion that was analyzed, agree rather well with the model predictions; the model predicts the frequencies of the first three peaks within 10 per cent in most cases and the height of the peaks within 50 per cent in most cases. Surface time histories computed from the theoretical model predict the time variations of amplitude and frequency content reasonably well, but correlations of individual cycles cannot be made between observed and predicted traces.

A catalog of historical earthquakes in China compiled from recent Chinese publications

Abstract: A catalog of historical earthquakes in China from 1177 B.C. to 1899 A.D. has been compiled in a form suitable for computers. The data include the date, epicenter, magnitude, and epicentral intensity of the earthquake as well as the province where the earthquake occurred. The source materials are publications in Chinese of the Institute of Geophysics, Academia Sinica. Some of the historical events are evidently related to large faults that are easily discernible from satellite images. Comparing the historical seismicity map to epicenters located by the World Wide Standardized Seismograph Network stations since 1962, we may see the influence of population distribution on the historical data.

Body waves in a layered anelastic solid

Abstract: A formulation extending the Haskell-Thompson matrix method to include the effects of anelastic attenuation is presented. The formulation is exact in that no low-loss approximations are made. Consideration is given to nonparallel propagation and attenuation directions with corresponding velocity anisotropy. Examples are presented for models representing soils, the crust, and the core-mantle boundary.

Body-wave spectra of central California earthquakes

Abstract: PZ and SH spectra (0.5 to 20 + Hz) of 26 events (0.9 ≦ M ≦ 4.1) in the January 15, 1973, earthquake sequence on the San Andreas fault near Bear Valley have been obtained from seismograms written at a three-component short-period ( T o = 1 sec) station (Δ = 12 to 18 km) located 4 km west of the San Andreas fault trace in the Gabilan Range. For each of the larger events ( M ≧ 3.3), the observed SH spectrum has greater high-frequency to low-frequency content than the observed PZ spectrum. For the smaller events, the observed SH and PZ spectra are similar. At high frequencies ( ƒ ⪞ 10 Hz), the observed SH -displacement spectra decrease more rapidly with frequency than do the observed PZ displacement spectra—indicating more attenuation for SH than for PZ for these propagation paths. For any reasonable propagation-path correction, SH source displacement spectra corner frequencies are systematically larger than PZ source displacement corner frequencies for these events. Ratios of spectra of large-to-small events in the sequence imply an equivalent total path Q α of 175 to 250 and an equivalent total path Q β of 100 to 150 for these paths.

On the adequacy of plane-wave reflection/transmission coefficients in the analysis of seismic body waves

Abstract: In order to estimate the effect (on body waves) of discontinuities within the Earth, it is common practice to use the theory for plane waves incident upon the plane boundary between two homogeneous half-spaces. The resulting reflection/P~ V conversion/transmission coefficients are shown here to he inaccurate for many problems of current interest. Corrected coefficients are needed, in particular, for cases where the discontinuity (upon which boundary conditions are to be applied) is near a turning point of the P- or S-wave rays, or if one of these rays intersects the discontinuity at a near-grazing angle. Adequate corrections, based upon the Langer approximation to a full wave theory, are shown to be easily derived in practice. The method is first to write out the plane-wave coefficients as a rational polynomial, in sines and cosines of the angles of incidence upon the boundary, and second to introduce a multiplicative factor for each cosine. The new factors depart from unity only when the associated cosine tends to zero; i.e., when a turning point is approached. They incorporate all the corrections required for curvature of the boundary, frequency dependence, and Earth structure (velocity gradients) near the boundary.

An analysis of tidal strain observations from the United States of America II. The inhomogeneous tide

Abstract: In Beaumont and Berger (1975), strain observations from seven sites in the continental United States were analyzed and compared with the homogeneous tides predicted for a radially stratified earth including the effects of ocean-tide loading. In this work the effects of lateral inhomogeneities in structure are examined using two- and three-dimensional finite element models. The results demonstrate that the topographic and geologic corrections are as large as ±25 per cent of the homogeneous strain tide for some of the strainmenter sites examined. Cavity effects are normally less than 5 per cent unless the strainmeters are installed transversely in tunnels. Contour plots of the topographic effects show that they may exceed ±50 per cent of the homogeneous strain in extreme cases. It is concluded from an error analysis that a theoretical model including local perturbations due to lateral inhomogeneities as well as the load and earth tides explains the observations. However, the errors in the observations, and uncertainties in our knowledge of the deep ocean tides in particular, prevent strain observations from providing useful measures of whole earth elastic properties (Love numbers) or anelasticity. In addition to earth tides, the results demonstrate that secular strain, in situ stress, and seismic strain must all be corrected for site effects.

Maximum-likelihood estimation of seismic magnitude

Abstract: Seismic networks often tend to overestimate the magnitude of earthquakes, because those stations within the network that do not detect a particular event are ignored in the conventional magnitude-averaging procedure. By assuming a normal distribution of worldwide magnitudes for any given event, it is possible to establish a simple statistical model that includes the additional information that event magnitudes at nondetecting stations must be below a certain threshold value. In this paper, maximum likelihood estimation is applied to determine event magnitude based on this model. The advantages and limitations of the technique are discussed using both simulated and real data. It is found that the maximum likelihood method, when applied properly, has the potential to yield a significant improvement in network magnitude estimates compared to the conventional averaging technique.

Geophysical assessment of peak accelerations

Abstract: Forty of the larger peak accelerations at source-site distances R ≃ 10 km for earthquakes in the magnitude range 3.2 ≦ M ≦ 7.1 are the basis of a geophysical interpretation of peak acceleration data. For 4 1 2 ≲ M ≦ 7.1 these peak acceleration data are essentially independent of magnitude; for 3.2 ≦ M ≲ 4 1 2 these data increase from 0.1 to 0.2 g at M = 3.2 to about 1 2 g at M ≃ 4 1 4 . A qualitative argument is advanced to attribute the observed dependence on magnitude in the range 3.2 ≦ M ≲ 4 1 2 to the effects of faulting duration, anelastic attenation, and instrumental response. If this argument is valid, physical processes in the source region responsible for generating these high-frequency acceleration amplitudes at R ≃ 10 km are independent of magnitude. A simple theoretical argument predicated on the basis that high-frequency ground accelerations reflect isolated and localized bursts of faulting, suggests that this should be the case if the dynamic shear-stress differences gs accompanying localized failure in the source region are magnitude-independent. The peak acceleration data at R ≃ 10 km suggest that σ ˜ ≃ 2 2kb, a value nearly coincident with the maximum shear-stress differences likely to be sustained by active crustal fault zones at depths ≦ 10 km. If 5 kb is a more reasonable limit to the shear strength of crustal rocks at 10-km depth, 1.8 g is a more reasonable limit to ground accelerations caused by sources of faulting at R = 10 km.

Seismicity, gravity, and tectonics of northeast India and northern Burma

Abstract: Practically the whole of northeastern India and northern Burma is characterized as an anomalous gravity field as well as an area of high seismicity. The Bouguer anomaly in the region varies from +44 mgals over Shillong Plateau to −255 mgals near North Lakhimpur in Assam Valley. Isostatic anomaly (Hayford) varies from +100 to −130 mgals in these areas. Over Arakan-Yoma and the Burmese plains, the isostatic anomalies vary from −20 mgals to −100 mgals. Regions of high seismicity in the area include the eastern Himalaya (including Assam syntaxis), Arakan-Yoma including the folded belt of Tripura, Irrawaddy basin, Shillong Plateau, Dauki fault and the northern part of Bengal basin. The abnormal gravity and seismicity are related to large scale tectonic movements that have taken place in the area mostly during the Cretaceous and Cenozoic times, due to interaction of the Indian, Tibetan, and Burmese plates. The high seismicity indicates that the movements are continuing. The seismic zone underlying Burma is approximately V shaped and dips toward the east underneath Arakan-Yoma. Most of the intermediate-focus earthquakes in Burma underlie the area characterized by negative isostatic anomalies, indicating the probable existence of a subduction zone underneath the Arakan-Yoma and the Burmese plains. The Shillong Plateau has a history of vertical uplift since Cretaceous times. Provided this statement is true, the uplift of the plateau preceded Himalayan tectonics starting 20 to 30 m.y. before continental India made solid contact with the Eurasian plate. The plateau is characterized by large positive isostatic anomalies as well as high seismicity. The positive isostatic anomalies may be due to intrusion or incorporation of basic material from the mantle into the crust underlying the Plateau. These intrusions may have taken place through deep seated faults such as the Dauki and could be responsible for its uplift as well.

Spatial attenuation of intensities for central U.S. earthquakes

Abstract: Attenuation of ground motion in the central United States has to be determined principally using the Modified Mercalli (MM) intensity observations because of the absence of instrumental strong ground-motion data. Nuttli9s previous studies of Mississippi Valley earthquakes indicate that higher-mode surface waves produce the largest ground motion except possibly in the near-field region. Particle velocity rather than acceleration correlates directly with intensity and the coefficient of anelastic attenuation has an average value of 0.10 per degree. Using data from isoseismals of the November 9, 1968, southern Illinois and the December 16, 1811, New Madrid, Missouri earthquakes and assuming a linear relationship between log( A / T ) and MM intensity, attenuation is expressed by the equation, valid for I ( R ) ≧IV (MM), I ( R ) = I 0 + 3.7 − 0.0011 R − 2.7 log ⁡ R ; for R ≧ 20 k m where R is the epicentral distance in kilometers. This relationship shows fairly good agreement with isoseismals of many large earthquakes in the central United States and may therefore be useful in providing realistic estimates of spatial attenuation and hence of design earthquakes for a given site. It can also be sometimes useful in estimating the epicentral intensity of an earthquake whose maximum intensity is not reliably known.

An extension of Frank's method for obtaining crustal shear strains from survey data

Abstract: Frank9s method of obtaining shear strains γ 1 and γ 2 from a repeated observation of the angles of a triangle has been generalized to deal with any collection of angles and an arbitrary number of surveys of each angle. This modification makes it possible to increase the signal-to-noise ratio and thus detect smaller magnitude strain fields. Under the assumption that the strain rate is constant over the space time covered by the survey data, the shear strain components γ 1 and γ 2 are replaced by their rates γ˙ 1 and γ˙ 2 , and a least-squares adjustment of the observations is used in determining most probable values of γ˙ 1 and γ˙ 2 . The nonzero correlation between observations of adjacent angles that have been observed by the method of rounds is taken into consideration in making the adjustment by using the full variance-covariance matrix at each station. Statistical testing of the method indicates that it correctly estimates the dispersion of γ˙ 1 and γ˙ 2 .

The Seismic Research Observatory

Abstract: Thirteen advanced seismograph systems, called Seismic Research Observatories (SRO), are being installed as part of a program to upgrade the worldwide seismic data network. The SRO system was created by combining a recently developed broad-band borehole seismometer and a software-controlled recording system. The seismometers are being installed at a depth of 100 meters to avoid wind-generated noise in the long-period band. A seismometer output that is flat in acceleration between periods of 1 and 50 sec is used to produce both short- and long-period data that are recorded on analog drum recorders and in digital form on magnetic tape. Very-long-period data, obtained from the seismometer mass position output, can be recorded as well. Digital recording of gain-ranged data provides an amplitude of nearly 120 dB. Preliminary evaluation of the SRO data system indicates that major design objectives have been met. The network of SRO stations will be an important new data resource for seismological investigations, especially for those studies that require computer processing of the data.

Crustal structure in central New Mexico interpreted from the gasbuggy explosion

Abstract: Interpretation of a seismic profile extending 548 km southward from the GASBUGGY nuclear test of December 10, 1967 resulted in a crustal model for central New Mexico. The crust is 39.9 km thick below the Paleozoic “basement”. It consists of an upper crust 18.6 km thick having P velocity 6.15 km/sec, and a lower crust 21.3 km thick having P velocity 6.5 km/sec. The apparent upper mantle velocity is 8.12 km/sec. This model applies near the crossover distance, 50 km west of Albuquerque. Additional information from earthquakes and explosions suggests that the upper crustal velocity drops to 5.8 km/sec in the Rio Grande rift, and that the true upper mantle velocity is 7.9 km/sec. The low upper crustal velocity in the Rio Grande rift can be detected on the record section of the GASBUGGY profile.