Showing papers in "Bulletin of the Seismological Society of America in 1980"

Estimation of the earthquake recurrence parameters for unequal observation periods for different magnitudes

Abstract: Maximum likelihood estimation of the earthquake parameters N o and β in the relation N = N o exp (− βm ) is extended to the case of events grouped in magnitude with each group observed over individual time periods. Asymptotic forms of the equation for β reduce to the estimators given for different special cases by Aki (1965), Utsu (1965, 1966), and Page (1968). The estimates of β are only approximately chi-square distributed. For sufficiently large numbers of events, they can be estimated from the curvature of the log-likelihood function. Sample calculations for three earthquake source zones in western Canada indicate that for well-constrained data sets, the often-used, least-squares estimation procedures lead to compatible results, but for less well-defined data sets, the effect of subjective plotting and weighting methods used for least-squares fitting leads to appreciably different parameters.

A spectral theory for circular seismic sources; simple estimates of source dimension, dynamic stress drop, and radiated seismic energy

Abstract: The far-field body wave radiation from a class of circular rupture models is investigated as a function of takeoff angle, rupture velocity, and stopping behavior. In particular, the variation of spectral shape, pulse shape, and energy flux over the focal sphere is quantified. These results provide two new methods for estimating the source dimension, the first through the inversion of a characteristic frequency, and the second using the rise time of the displacement pulse shape. The class of kinematic rupture models which has been constructed also allows a direct estimate of the dynamic stress drop using the velocity pulse shape: as an example, this technique is applied to an SMA1 recording of a high stress drop Aleutian earthquake, Finally, the introduction of a new spectral parameter, the integral of the square of the ground velocity, is shown to considerably simplify calculation of the total radiated energy. Together with a re-evaluation of corner frequency and spectral falloff, these new results constitute a spectral theory for circular seismic sources which includes directivity and is valid for a range of subsonic rupture velocities.

The seismic response of sediment-filled valleys. Part 2. The case of incident P and SV waves

Abstract: We present the extension to incident P and SV waves of our previous study (Bard and Bouchon, 1980) concerning the seismic response of sediment-filled bidimensional valleys to incident SH transient signals. The reliability of the Aki-Larner method is briefly discussed and the domain is estimated within which it provides accurate results. Then we investigate the response of three different valleys, having various geometrical and elastic parameters, to vertically incident P and SV waves, in both the frequency and time domains. The behavior of the valleys is shown to be qualitatively similar to their behavior for SH waves: the nonplanar interface causes surface waves (here Rayleigh waves) to be generated on valley edges, and to propagate laterally inside the basin. The amplitude of these Rayleigh waves depends greatly on the velocity contrast, the valley shape, and the incident wave type ( P or SV ), but it may be significantly higher than the disturbance associated with the direct incident signal. The frequency and direction of incident motion determine partly whether the fundamental or first higher mode will be predominantly excited, depending on the main component (vertical or horizontal) of the Rayleigh mode motion. Although the reflections of these Rayleigh waves on valley edges do not appear as clearly as in the SH case, a very long duration of the ground shaking inside the valley is still observed. In deep valleys, these laterally propagating Rayleigh waves may degenerate into a lateral resonance pattern, involving high-amplitude surface motion. These latter resonance modes, however, begin to appear in shallower valleys for incident SV waves than for incident P ones.

Historical seismicity in the near and Middle East, North Africa, and Spain from Arabic documents (VIIth-XVIIIth century)

Abstract: A catalog of historical seismicity from the VIIth to the XVIIIth century A.D. was compiled from Arabic documents (many of which are unpublished manuscripts) for the near and Middle East, and in a lesser measure, for North Africa and Spain. In most cases, detailed descriptions have allowed us to assign Modified Mercalli intensities to the shocks for the localities where they were reported. The detailed chronology of a protracted episode of seismicity in northern Syria between 1156 and 1159 is given shock by shock from the relation of a Damascus chronicler.

Q estimates using the coda of local earthquake

Abstract: An extension of the Aki (1969) coda model takes into account the instrument and Earth Q -filter effects on the dispersion of the coda. The coda dispersion and coda shapes are easily derived and plotted as a function of t * = t/Q . Observed data can be overlaid on master curves to directly estimate shear-wave Q and to normalize for earthquake moment to estimate the attenuation coefficient of γ. The method works well when the earthquakes studied have corner frequencies greater than the combined peak response of the instrument and Earth Q -filters. Comparing results from central California and the southeastern United States, there is a slight indication that the excitation of the coda is inversely proportional to Q .

Strong-motion duration and RMS amplitude of earthquake records

Abstract: A simple procedure is proposed for estimating the strong-motion duration s o and the rms strong-motion acceleration tσ o of earthquake ground motion records. The strong-motion duration is found to be nearly proportional to the quantity I o / a 2 max where a max is the maximum ground acceleration, and I o is the integral over time of the squared accelerations. A less important factor influencing the relationship between s o and I o / a 2 max is the predominant period T o of the strong phase of the accelerogram. The following simplified expression is also derived for the strong-motion duration: s o = 7.5 I o / a 2 max . Duration-related statistics are presented for a set of 140 horizontal components of earthquake ground motion recorded in California. The dependence of duration upon magnitude, epicentral distance, and local condition is examined. The proposed measure of duration provides an important link between the major spectral representations of earthquake ground motion: the Fourier amplitude of acceleration, the acceleration spectral density function, and the response spectrum.

A numerical study of P-, SV-, and SH-wave generation in a plane layered medium

Abstract: A solution for the surface displacements due to buried dislocation sources in a multi-layered elastic medium is found using the Haskell (1964) paper as a starting point and more importantly, for notation. Through the introduction of some simple matrix operations, the Haskell (1964) solution is made simultaneously more compact and computationally stable. Time histories are computed for a perfectly elastic medium by performing classical contour integration in the complex wavenumber plane. A new aspect in the evaluation of those contours is introduced because of the recognition of nonzero singularity contributions of the Hankel and modified Bessel functions at k = 0. Theoretical ground motion time histories are presented to show the usefulness of the formulation. The overall objective of this paper is to incorporate the modifications made since 1964 to the Haskell (1964) paper in an easily understandable, step-by-step development.

Recurrence of moderate to large magnitude earthquakes produced by surface faulting on the Wasatch fault zone, Utah

Abstract: No historical earthquake associated with surface faulting have occurred along the Wasatch fault zone during at least the past 133 yr. Exploratory trenching, analysis of fault scarp morphology and scarp-derived colluvial deposits, and detailed mapping at two locations along the fault zone indicated there have been repeated moderate to large magnitude earthquakes ( M 6 1 2 to 7 1 2 ) associated with surface faulting during the late Pleistocene and Holocene. These detailed geological studies provide new data on the style of deformation associated with normal-slip faulting, amount of cumulative displacement, amount of displacement per event, and the recurrence intervals of surface faulting earthquakes. At the Kaysville site, 30 km north of Salt Lake City, at least three surface faulting events have produced 10 to 11 m of cumulative net vertical tectonic displacement since the middle Holocene; estimates of the net tectonic displacement for individual faulting events range from 1.7 to 3.7 m. The two most recent events occurred within the past 1,580 ± 150 yr and interval between these events was from 500 to 1,000 yr. Along this segment of the fault, the average recurrence interval is probably close to 1,000 yr. The middle to late Holocene slip rate is 1.8 (+1.0; −0.6) nm/yr. At the Hobble Creek site, 46 km south of Salt Lake City, six or seven surface faulting events have produced 11.5 to 13.5 m of cumulative net vertical tectonic displacement during the past 12,000 to 13,000 yr; the average net tectonic displacement per event is 0.8 to 2.8 m. The average Holocene recurrence interval at this location is 1,500 to 2,600 yr and the slip rate is 1.0 ± 0.1 mm/yr. If the recurrence intervals on these segments of the Wasatch fault zone are typical of the other segments of the zone, the recurrence interval of moderate to large magnitude earthquakes for the entire Wasatch fault zone may be 50 to 430 yr.

Slip on the San Andreas fault in central California from alinement array surveys

Abstract: Lines of monuments ranging between 30 and 200 meters (m) in length that extend across the traces of several strike-slip faults in California have been surveyed periodically by the U.S. Geological Survey to determine local slip rates, widths of slip zones, and patterns of near-field deformation. Ten monument lines, or alinement arrays, were established during the years 1966 to 1968 along a 180-km-long segment of the San Andreas fault in central California from Cholame Valley northwestward to Watsonville. Data from subsequent alinement array surveys and from other sources show that this segment of the fault can be partitioned into three distinct sections. A 55-km-long central section northeast of King City shows the highest rates of fault creep observed (∼32 mm/yr), essentially matching the long-term slip rate measured geodetically across the entire fault system. The two sections flanking the central section show only partial strain release and are characterized by regional decreases in creep rates outward toward the limits of surface ruptures associated with the great 1857 and 1906 earthquakes. The data show that creep does not occur at present along the zones of surface rupture associated with the two great historic earthquakes, except for minor amounts at scattered sites along the 1906 break. The fall-off in creep rate from the ends of the central section outward along the flanking sections shown by alinement array data is characterized by distinct asymmetry: the gradient of outward creep rate decline along the northwest section is only about half that of the southeast section, implying a similar contrast in rates of strain accumulation. The northwest section characteristically produces frequent microearthquakes and occasional moderate earthquakes as large as magnitude 5 1 4 , while the southeast section is characterized by comparatively lower background seismicity and less frequent moderate shocks which may, however, be as great as magnitude 5 3 4 .

The ray series method and dynamic ray tracing system for three-dimensional inhomogeneous media

Abstract: The ray series method is used to study propagation of seismic waves in the three-dimensional media consisting of generally inhomogeneous layers separated by curved interfaces. The investigation is carried out with the help of the so-called ray centered corrdinate system which was proposed by Popov and Psencik (1978a). It is shown that in this system the principal components of the amplitude coefficients in the ray series for S waves do not rotate about the ray with respect to the basis vectors when the wave progresses, even though they rotate with respect to the unit vectors n^ and b^ along the direction of the normal and binormal to the ray, respectively. This considerably simplifies the final expressions for the amplitude coefficients for S waves, whose two principal components are decoupled in the ray-centered coordinate system. The ray-centered coordinate system is also applied to the eikonal equation in order to produce a dynamic ray tracing system consisting of three nonlinear ordinary differential equations of the first order determining the second derivatives of the time field and, in this manner, even the basic geometrical properties of the wave fronts (e.g., principal curvatures and geometrical spreading) along the ray. Several different modifications of the dynamic ray tracing system are presented. It is demonstrated that in the case of generally inhomogeneous two-dimensional media the dynamic ray tracing system reduces, under certain not too restrictive conditions, to the single first order differential equation of the Riccati type. Finally, the phase matching method is used to determine discontinuities of individual quantities in the dynamic ray tracing system when the wave is impinging on a curved interface separating two generally inhomogeneous media. Since all basic equations are presented in a computationally convenient matrix formulation, they can be readily employed for any numberical evaluation of dynamic properties of seismic waves propagating through structurally complicated media. As the paper describes all basic features of asymptotic ray theory (the name under which the ray series method is known on this continent), it can serve as a starting point for anyone wishing to develop computer programs for the computation of synthetic seismograms.

Asymptotic ray theory and synthetic seismograms for laterally varying structures: theory and application to the imperial valley, California

Abstract: The interpretation of refraction profiles that traverse laterally varying velocity structures has been hindered by lack of a practical algorithm for computing synthetic seismograms to compare with observations. However, a modification of zero-order asymptotic ray theory to incorporate the amplitudes of rays that turn in a velocity gradient, as well as reflected waves, allows the computation of high-frequency synthetic seismograms for laterally varying velocity structures. The method is general in that synthetic seismograms may be computed for any structure through which rays can be traced. We have modeled seismic refraction data from the Imperial Valley, California, by applying this method. A major feature of our model is a sedimentary column that thickens from -4 km at the Salton Sea to -5.5 km at the United StatesMexico border. To approximate the observed amplitude behavior, a dipping velocity discontinuity was modeled near 13-km depth, but velocity gradients were found to be more appropriate than sharp boundaries in the rest of the model.

RMS accelerations and spectral amplitudes of strong ground motion during the San Fernando, California earthquake

Abstract: An estimate of the Fourier amplitude spectrum of horizontal shear-wave motion is obtained using the Brune seismic source model in the presence of anelastic attenuation, from which the root-mean-square acceleration ( a rms) can be calculated. Predicted spectral amplitudes and values of a rms are compared to observations for 160 free-field or basement-level components of horizontal acceleration of the San Fernando earthquake. For a stress drop of 50 bars and a faulting duration of 10 sec, observed and predicted spectral amplitudes and values of a rms are in reasonable agreement: the observations conform to the predicted attenuation with distance (which is frequency-dependent for spectral amplitudes and proportional to R −3/2 for a rms), but exceed predicted values by an average factor of about 3. Some of this difference is attributable to azimuthal variation in source excitation and the highly nonuniform distribution of stations with azimuth; the remainder results from the high-stress drop associated with the initial rupture for this earthquake. Predictions made by the model are nonetheless in agreement, both in terms of attenuation and amplitude, with spectral amplitudes estimated by empirical models calibrated with records from a wide variety of earthquakes.

A probability model for regional focal mechanism solutions

Abstract: The increase in the number of determinations of fault-plane solutions and their use for inferences on regional and local tectonics and stress fields makes it advisable to treat them in a standard way in groups on a regional basis. Probability models for joint group estimation of the shear dislocations, involving data from I earthquakes and J observations of the same region, are presented. The general model gives the probability π ij of reading a compression ( Y ij = 1) for the P wave from event i at station j as Prob { Y i j = 1 } = γ i + ( 1 − 2 γ i ) Φ ( ρ i A i j ) where γ and ρ are precision parameters and Φ is the normal cumulative. This probability depends on A ij (θ), the theoretical amplitudes from a particular orientation of the greatest and least axes of principal stress for the regional solution. Estimates of the principal stress axes T and P and their variances are computed by constructing a loss function based on the likelihood. The algorithm allows for the estimation of a mean regional solution and one for each event. A treatment of measures of fit and the winnowing of clusters of solutions is given. The resulting program has been applied to the determination of the regional solution and exceptional mechanisms for the main shock and 25 aftershocks of the great 1964 Alaskan earthquake and 19 small earthquakes in Bear Valley, California.

Solving two-point seismic-ray tracing problems in a heterogeneous medium. Part 1. A general adaptive finite difference method

Abstract: A study of two-point seismic-ray tracing problems in a heterogeneous isotropic medium and how to solve them numerically will be presented in a series of papers. In this Part 1, it is shown how a variety of two-point seismic-ray tracing problems can be formulated mathematically as systems of first-order nonlinear ordinary differential equations subject to nonlinear boundary conditions. A general numerical method to solve such systems in general is presented and a computer program based upon it is described. High accuracy and efficiency are achieved by using variable order finite difference methods on nonuniform meshes which are selected automatically by the program as the computation proceeds. The variable mesh technique adapts itself to the particular problem at hand, producing more detailed computations where they are needed, as in tracing highly curved seismic rays. A complete package of programs has been produced which use this method to solve two- and three-dimensional ray-tracing problems for continuous or piecewise continuous media, with the velocity of propagation given either analytically or only at a finite number of points. These programs are all based on the same core program, PASVA3, and therefore provide a compact and flexible tool for attacking ray-tracing problems in seismology. In Part 2 of this work, the numerical method is applied to two- and three-dimensional velocity models, including models with jump discontinuities across interfaces.

Modeling the long-period body waves from shallow earthquakes at regional ranges

Abstract: A procedure for modeling P and PL waves recorded on long-period WWSSN instruments at ranges 1° to 12° is presented. Following the experience gained by modeling explosions (Helmberger, 1972), we demonstrate that these long-period phases are adequately treated by a single crustal layer for most of Western United States. After generating the Green's functions at the various ranges for the three fundamental dislocation types, we need only construct linear combinations of these vectors to represent any arbitrary oriented earthquake. The waveform patterns produced from the various fault types are quite diagnostic with the dip-slip orientations showing a strong ringing nature which is caused by the vertical SV lobes. To test the usefulness of this technique, we construct synthetics for some well-studied west coast earthquakes where the orientation, time history, and moment have been determined independently. Comparing the predicted seismograms with observations, we find good agreement in waveshapes and amplitudes.

Rapid solution of ray tracing problems in heterogeneous media

Abstract: The determination of local earthquake hypocenters and orgin times from first- P -arrival times by Geiger9s method requires a technique for finding the minimum travel time (and derivatives) between the source and the station. Sophisticated ray tracing techniques have been developed for this purpose for use in complex velocity structures. Unfortunately, the two common techniques, shooting and bending, are generally prohibitively expensive for routine use in data analysis. The bending method is also particularly vulnerable to the problem of local minima in travel time. A method has been developed known as the ray initializer, which can be used to circumvent these problems in many cases. First, the technique can find a reasonable estimate of the minimum-time ray path in a quick and efficient manner. The velocity in a region local to the source and receiver is laterally averaged to yield an approximate layered velocity model. One-dimensional ray tracing techniques are used to find the minimum-time path for this layered structure. The ray path estimate can then be used as the starting path in a bending routine, a procedure resulting in more rapid convergence and the avoidance of local minima. Second, the travel time found by numerical integration along the estimated ray path is an excellent approximation to the actual travel time. Thus, in many cases, the ray initializer can be substituted for a three-dimensional ray tracing routine with a tremendous increase in efficiency and only a small loss in accuracy. It is found that the location of an explosion, derived using the ray initializer, is nearly identical to a complete ray tracing solution, even for a highly complex velocity structure.

Finite difference analysis of Rayleigh wave scattering at a trench

Abstract: A finite difference scheme with the improved treatment of boundary conditions at corners and with the introduction of the absorbing boundary conditions at computational edges is applied to the analysis of Rayleigh wave scattering at a trench ditched on the surface of an elastic half-space. Complex transmission and reflection coefficients are calculated from the surface waveforms at a given moment and are compared with the experimental and theoretical predictions. In order to investigate the effect of converted waves, the complex transmission coefficient along a step is calculated from the time series waveforms at observation points with a fixed incident angle to the free surface.

Spectra from high-dynamic range digital recordings of Oroville, California aftershocks and their source parameters

Abstract: Twenty-six small aftershocks of the Oroville, California earthquake were recorded on three orthogonal components of a digitally recording seismograph with a 15-bit analog to digital converter at a sample rate of 400 samples/sec/channel. The magnitude of the events ranged from less than 1 to about 2.6, and hypocentral distance varied from about 3 to 14 km; depths were 3 to 10 km. For 14 of these events, both P and S waveforms were recovered. Fourier displacement spectra were calculated for the body waves and analyzed with respect to the Brune (1970, 1971) source model. Most spectra have a constant long-period level and falloff above a corner frequency at a rate proportional to ω −γ . γ for these earthquakes is usually greater than 2 and frequently as high as 4 or 5 for spectra that have corner frequencies between 10 and 70 Hz. For 12 of the 14 events that recorded both P and S waves, the ratio of the P -corner to S -corner frequency is about 1.7, as predicted by Hanks and Wyss (1972). Stress drops vary from a few tenths of a bar to 27 bars and appear to increase with depth. One earthquake that fell significantly above the stress drop-depth curve (a large stress drop event) was followed by a M L = 3.4 earthquake in 2.5 to 3 days after a period of quiescence.

Evaluation of S to P amplitude rations for determining focal mechanisms from regional network observations

Abstract: The ratios of the amplitudes of SV to P , as recorded on vertical component seismographs near the earthquake, provide a means of determining the focal mechanism The observed ratios are compared with values calculated on the basis of dislocation theory, with the effects of transmission across boundaries and incidence on the free surface approximately accounted for by plane-wave coefficients A search is made for the strike and dip of the fault that provides a fit to as many of the stations as possible, for the direction of fault slip assumped The direction of slip may also be treated as a free parameter to be determined, but the theoretical results are especially simple for pure strike-slip or pure dip-slip faulting The technique has been applied to the analysis of events in three settings, in which pure thrusting, pure strike-slip, and pure normal faulting could be assumed The utility and the limitations of the method are illustrated by these examples The important limitations are that the method cannot distinguish between conjugate mechanisms or show the sense of slip on the fault It also has a weak resolution of strike for some commonly occurring mechanisms, so that the solution may depend on the readings at only a few stations The method has been used to pick out a group of foreshocks to an Aleutian Islands earthquake and to provide a focal mechanism for an event in the Rhine graben, at a place where such information is lacking and needed A study of 15 central California earthquakes revealed no changes in fault plane orientation or in relative attenuation of P and S waves during 1 yr prior to a magnitude 5 earthquake

The excitation and attenuation of seismic crustal phases in Iran

Abstract: The most prominent crustal phases of Iranian earthquakes recorded by stations in Iran are the first P ( Pn or refracted P ), Pg , Sn , and Lg . Pg , Sn , and Lg appear as trains of waves whose attenuation behaves as that of dispersed surface waves. For 1-sec period Pg , Sn and Lg , the coefficient of anelastic attenuation has an average value of 0.0045 km −1 , similar to that for California but much greater than that for eastern North America. For 3-sec period Lg waves, the average value is 0.003 km −1 . The excitation of 1-sec Pg and Lg waves is approximately equal. The amplitude of their vertical components for an m b = 5.0 earthquake at 10-km epicentral distance is 270 μm. The 10-km amplitude of vertical-component, 3-sec period Lg waves is 800 μm for an m b = 5.0 earthquake. For all epicentral distances the resultant horizontal, 1-sec period Lg motion is twice that of the vertical component. The 10-km value of the resultant horizontal Sn motion for an m b = 5.0 earthquake is 240 μm, slightly less than half that of Lg . The amplitude of the first P motion is, in general, one or more orders of magnitude smaller than that of the other crustal phases to distances of 500 km. However, at distances of 1000 to 1500 km the amplitude of first P is comparable to that of the other phases. Formulas are given to compute body-wave magnitude of Iranian earthquakes from the amplitudes of 1-sec period, vertical component Pg and Lg waves as well as amplitudes of the first arrival.

Regional variation of QScS

Abstract: The ScS n phase-equalization and stacking algorithm of Jordan and Sipkin (1977) has been applied to an extensive set of HGLP and ASRO data to obtain regionalized estimates of Q ScS . Tests of the algorithm using synthetic data reveal no significant sources of bias. The low value of Q ScS previously obtained for the western Pacific (156 ± 13) is corroborated by additional data, and Q ScS observations in other regions correlate with variations in crustal age and tectonic type. A representative value for the ocean basins sampled by our data is 150, with the best estimates being somewhat lower (135 to 142) for younger oceanic regions and somewhat higher (155 to 184) for older regions. The two subduction zones sampled here, Kuril-Japan and western South America, are characterized by larger Q ScS estimates than the ocean basins (197 ± 31 and 266 ± 57, respectively), and the difference between them is qualitatively consistent wit the contrasts in upper-mantle attenuation structure proposed by Sacks and Okada (1974). Continental regions are poorly sampled in this study because the signal-generated noise in the vicinity of the ScS n phases is generally larger for continental paths, but a representative value is inferred to be Q ScS = 225. For paths crossing China, Q ScS is observed to be lower (∼180), providing additional evidence for a high-temperature upper mantle previously inferred from surface-wave and travel-time measurements. Our best estimate for the average Earth is Q ScS = 170 (±20 per cent), which appears to be significantly lower than that predicted by normal mode data, suggesting some frequency dependence. Q − 1 ScS correlates with ScS n - ScS n −1 travel time along a line given by Q S c S − 1 = ( 4.4 × 10 − 4 ) Δ T S c S + 4.88 × 10 − 3 , where Δ T ScS is the JB residual in seconds; this correlation favors a thermal control on the Δ T ScS variations. It is inferred from the tectonic correlations that much, if not most, of the heterogeneity expressed in the Q ScS and Δ T ScS variations is confined to the upper mantle. Substantial differences in the attenuation structures underlying continents and oceans are implied. In fact, the average quality factor for the upper mantle beneath stable cratons may not be much less than that for the lower mantle.

Simulation of strong ground motions

Abstract: The estimation of potential strong ground motions at short epicentral distances (Δ = 10 to 25 km) resulting from large earthquakes, M ≧ 6.5, generally requires extrapolation of a limited data set. The goal of this project has been to quantify the extrapolation through a simulation technique that relies heavily upon the more extensive data set from smaller magnitude earthquakes. The simulation utilizes the smaller events as Green's functions for the elements of a larger fault. Comparison of the simulated peak acceleration and duration with the data from the Parkfield earthquake is very good. Simulation of three earthquakes, M = 5.5, 6.5, and 7.0 indicate that the slope of the peak acceleration versus distance curve (Δ = 5 to 25 km) flattens, for strike-slip earthquakes, as the magnitude increases.

Implication of seismicity for failure of a section of the San Andreas Fault

Abstract: On January 15, 1973, a magnitude M L 4.1 earthquake occurred near Cienega Road on the San Andreas Fault about 20 km south of Hollister, California. A 3-km-long segment of the fault southeast of the earthquake was aseismic for the 7 weeks preceding the event, although microearthquakes occurred at both its ends. The first day9s aftershocks occurred at the northwest end of the aseismic segment; later aftershock activity migrated to the southeast, filling the remainder of the segment. If the discontinuous surface trace of the fault can be extrapolated to the focal region of the earthquakes to define fault geometry at depth, then aftershocks occurred primarily on one continuous segment of the fault and epicenter locations and direction of rupture propagation (inferred from the azimuthal pattern of P -wave radiation) of the precursory shocks correlate with the discontinuities in the trace that terminate the segment. The 1970 to 1976 deficit in seismic slip within the segment suggests that fault creep accounts for a significant part of cumulative slip within the segment. The pattern of seismicity is consistent with the hypothesis that creep on the segment before the main shock caused a buildup of stress at the ends of the segment or at the ends of adjacent offset segments. Correlation of seismicity and discontinuities or bends in the mapped fault trace are the basis for an extension and refinement of the “stuck” and “creeping” patch model of the San Andreas Fault in central California. Patch boundaries extend from the free surface down through the seismogenic zone. Creeping patches lie beneath smooth continuous segments of the fault trace. Stuck patches lie beneath discontinuities or bends in the fault trace.

Dynamic rupture in a layered medium: the 1966 parkfield earthquake

Abstract: A method for computing ground motion from a propagating stress relaxation in a vertically heterogeneous medium was developed wherein computational efficiency is enhanced by separating the source, a three-dimensional calculation, from the wave propagation, a two-dimensional calculation. As an application of this technique, displacement-time histories were computed corresponding to those determined from accelerograms recorded during the 1966 Parkfield, California earthquake. On the basis of these comparisons, an effective stress of 25 bars, an average slip of 43 cm, and a moment of 2.32 x 1025 dyne-cm were determined for the Parkfield earthquake.

Direct observation of a p-τ curve in a slant stacked wave field

Abstract: The well known Wiechert-Herglotz technique for computing a velocity-depth profile from refraction seismograms uses data in the form of ray parameter as a function of intercept time (a p-τ curve). The technique of slant stacking used by reflection seismologists automatically produces a p-τ curve from a common shot profile, thereby bypassing arbitrary travel-time picking and several intermediate calculations of previous methods. Successful preliminary results are presented for both synthetic and real data. This method deteriorates for widely separated geophones and where a number of apparent sources with low inter-source coherency coexist in a single profile, but is capable of completely sorting out triplications in data with less extreme contrasts without human intervention.

Source dynamics of two great earthquakes of the Indian subcontinent: The Bihar-Nepal earthquake of January 15, 1934 and the Quetta earthquake of May 30, 1935

Abstract: Source mechanisms of two major destructive earthquakes which occurred at the Bihar-Nepal border and in the Quetta region on January 15, 1934 and May 30, 1935, respectively, are determined using the P -wave first motions, S -wave polarization angles, and surface-wave spectral data. The high stress drop and apparent stress associated with these events suggest that high tectonic stresses are prevailing in these regions. A major part of the stresses accumulated before the occurrence of the two earthquakes had been released through the main shock. An investigation of temporal and spatial variation of regional seismicity reveals possible existence of seismic gaps before the occurrence of these two major events.

A semi-Markov model for characterizing recurrence of great earthquakes

Abstract: A semi-Markov model estimating the waiting times and magnitudes of large earthquakes is proposed. The model defines a discrete-time, discrete-state process in which successive state occupancies are governed by the transition probabilities of the Markov process. The stay in any state is described by an integer-valued random variable that depends on the presently occupied state and the state to which the next transition is made. Basic parameters of the model are the transition probabilities for successive states, the holding time distribution, and the initial conditions (the magnitude of the most recent earthquake and the time elapsed since then). The model was tested by examining compatibility with historical seismicity data for large earthquakes in the circum-Pacific belt. The examination showed reasonable agreement between the calculated and actual waiting times and earthquake magnitudes. The proposed procedure provides a more consistent model of the physical process of gradual accumulation of strain and its intermittent, nonuniform release through large earthquakes and can be applied in the evaluation of seismic risk.

Earthquake studies along the Wasatch front, Utah: Network monitoring, seismicity, and seismic hazards

Abstract: The Wasatch front area of north-central Utah occupies an active segment of the Intermountain seismic belt that is characterized by late Quaternary normal faulting and high seismic risk. This paper summarizes new earthquake information for the Wasatch front area, derived from telemetered seismic arrays operated by the University of Utah since October 1974, and implications for the evaluation of earthquake hazards in this area. Revised earthquake data from a skeletal statewide network for 1962 to 1974 substantiate the pattern of recent seismicity. Important features of this pattern include diffuse but locally intense seismicity throughout a 200-km-wide zone, roughly centered on the major N-S Wasatch fault zone, and persistent quiescence along major sectors of the Wasatch Fault.

Crustal structure of the southeast flank of Kilauea Volcano, Hawaii, from seismic refraction measurements

Abstract: In November 1976, the U.S. Geological Survey, in conjunction with the Hawaii Institute of Geophysics, established a 100-km-long seismic refraction line normal to the southeast coast of Hawaii across the submarine flank of Kilauea Volcano. Interpretation of the data suggests that the oceanic crust dips about 2° toward the island underneath the volcanic pile. The unreversed Pn velocity is 7.9 km/ sec with crustal velocities varying strongly along the profile. Profiles across the rift zones of Kilauea suggest that the velocity in the rifts is higher than the velocity in the surrounding extrusive rocks and that the velocity in the southwest rift (∼6.5 km/sec) is lower than the velocity in the east rift (∼7.0 km/sec). The rift boundaries seem to dip away from the rift such that a large part of the volcanic pile is composed of the higher velocity core of riftzone rock.