Showing papers in "Bulletin of the Seismological Society of America in 1978"

Automatic earthquake recognition and timing from single traces

Abstract: A computer program has been developed for the automatic detection and timing of earthquakes on a single seismic trace. The program operates on line and is sufficiently simple that it is expected to work in inexpensive low-power microprocessors in field applications. In tests with analog tapes of earthquakes, the program correctly identified and timed to within 0.05 sec about 70 per cent of the events which would normally be timed in operation of a network. The program evaluates the accuracy of its picks, and its estimates appear to be quite reliable. The algorithm is working at present in a 16-bit minicomputer and appears to be compatible with presently available microprocessors.

The use of the coda for determination of the earthquake source spectrum

Abstract: We studied the spectral content and variations in time of the coda of seismic oscillations following the body and surface waves of local earthquakes. Within narrow frequency bands, the form of the envelope of the coda is remarkably stable—independent of epicenter (and therefore epicentral distance), depth of focus, and all other parameters of the source. Only the absolute amplitude of the coda differs from event to event. Similarly, the forms of the coda at two stations from the same earthquake overlap one another, differing only in absolute amplitude by a factor that is the same for all events. Hence given the form of the coda, its amplitude in any frequency band may be parameterized by one number—the amplitude at a certain time. Therefore, the spectrum of the coda as a function of time can be described as the product of two factors—one, independent of time, is dependent only on the source, and the other, reflecting the effects of the medium and the same for all sources, gives the time dependence for each frequency band. Segments of the envelopes with time can be matched by simple theories of scattering. Using the theoretical relationships, estimates of Q can be made and show that for any time interval, Q increases with frequency, approximately proportional to the square root of frequency. As longer elapsed times are considered, the estimates of Q increase, suggesting greater penetration of seismic energy into the higher Q parts of the Earth. The spectra of different events can be compared directly by comparing the spectra of the codas at the same elapsed time. Such a comparison reveals a wide variety of different source spectra. By using empirical relations among coda spectra, observed S -wave spectra, and theoretical constraints, an estimate of the spectrum radiated by the source can be calculated from the coda spectrum. Source parameters (seismic moment, corner frequency of the radiated spectrum, calculated stress drop, etc.) can be determined from coda spectra of events with many different moments and in different regions, with the same station. The results show several interesting features dependent on the seismic moments and on the regions in which the earthquakes occurred.

Slip along the San Andreas fault associated with the great 1857 earthquake

Abstract: Historical records indicate that several meters of lateral slip along the San Andreas fault accompanied the great 1857 earthquake in central and southern California. These records, together with dendrochronological evidence, suggest that the rupture occurred along 360 to 400+ km of the fault, including several tens of kilometers of the currently creeping reach in central California. Geomorphic expressions of late Holocene right-lateral offsets are abundant along the 1857 rupture. Along 300 kilometers of the 1857 rupture, between Cholame and Wrightwood, the youngest discernible offset ranges from 3 to 9 1/2 meters. Dormancy of the fault since 1857 almost certainly indicates that this latest offset was created in 1857. Fault slip apparently associated with the 1857 earthquake varies in a broadly systematic way along the trace of the fault. It is relatively uniform along each of several long segments, but changes rather abruptly in value between these segments. This nonuniform displacement pattern may imply that some segments of the fault rupture more frequently or experience a slower long-term slip rate than others. The 1857 offsets indicate a seismic moment, m_o, between 5.3 and 8.7 × 10^(27) dyne-cm, assuming a 10- to 15-km depth of rupture and relatively uniform slip as a function of depth. A comparison with the rupture length, average slip value, and tectonic setting of the California earthquake of 1906 (M_s = 8 1/4) indicates a value of M = 8 1/4 + for the 1857 event.

Detailed spectral analysis of two small New York state earthquakes

Abstract: A statistically complete methodology for the analysis of SMA-1 data, including a direct inversion of the amplitude spectra, is developed and applied to the records of two small earthquakes ( M L = 1.5 in Attica and M L = 2.2 in Blue Mountain Lake) obtained at epicentral distances of ≈1 km. Using ray dependent corner frequency-radius relations, the fault radii are determined to be a = 80 ± 15 m and 25 ± 10 m. The stress drops vary by an order of magnitude; Δσ = 6 ± 4 bars and 85 ± 60 bars. Using hydrofracture data to estimate σ ¯ , the average stress, the seismic efficiency of the Attica event is determined to be ≈0.08. Because of a high estimate of the fracture energy associated with its rupture, the Blue Mountain Lake event is interpreted as having fractured fresh rock. The stopping character of the two events, inferred from their spectral shapes, is shown to differ considerably. These differences are used to contrast the stress and frictional regimes of the two source regions.

Duration characteristics of horizontal components of strong-motion earthquake records

Abstract: The duration characteristics of horizontal components of strong ground motion records obtained in the western U.S. were examined for engineering applications. A total of 84 accelerograms were used in the study, corresponding to a range of earthquake magnitudes, M , between 4.7 and 7.6, to distances to the source between 0.1 and 130 km, and to local subsurface conditions ranging from rock to soft clay. The definition of significant duration, D (sec), used was the time needed to build up between 5 and 95 per cent of the total Arias intensity of the record I A = π 2 g ∫ 0 t f a 2 ( t ) d t ( t f = total length of record ) . For rock sites, a consistent correlation was obtained between D and M : log D = 0.432 M 1.83. Values of D at soil sites show a much larger scatter, with the duration on rock being a lower bound. Detailed analyses were conducted on the accelerograms and their associated Husid plots, I A (t)/I A (t f ) versus t . It was found that a main parameter controlling the duration of strong ground motion was the total duration of rupture at the fault, d . Rock records are dominated by a strong part, having a duration, Δ, which is essentially a straight line in the Husid plot, and which is Δ ≲ D . This strong part coincides approximately with the arrivals of S waves which followed a direct path between source and station (direct S waves). The values of both Δ and D are mainly controlled by the duration of rupture at the fault, d , for M ≲ 7. Many soil records have, in addition to the strong part, another part which is moderately strong, has longer periods and appears after the direct S wave arrives. This additional part is responsible for the increased values of D observed for soil, and also for the curved shape of the Husid plot observed in many soil records. This part also contributes significantly to the observed richer content of long periods in soil records, reported by several authors. Some evidence suggests this part of the record may be mostly associated with the amplification by the soil of indirect, multipath body-wave arrivals, and with surface wave effects.

A numerical boundary integral equation method for elastodynamics. I

Abstract: The boundary initial value problems of elastodynamics are formulated as boundary integral equations. It is shown that these integral equations may be solved by time-stepping numerical methods for the unknown boundary values. A specific numerical scheme is presented for antiplane strain problems and a numerical example is given.

Determination of local magnitude, ML, from strong-motion accelerograms

Abstract: A technique is presented for determination of local magnitude, M_L, from strong-motion accelerograms. The accelerograph records are used as an acceleration input to the equation of motion of the Wood-Anderson torsion seismograph to produce a synthetic seismogram which is then read in the standard manner. When applied to 14 records from the San Fernando earthquake, the resulting M_L is 6.35, with a standard deviation of 0.26. This is in good agreement with the previously reported value of 6.3. The technique is also applied to other earthquakes in the western United States for which strong-motion records are available. An average value of M_L = 7.2 is obtained for the 1952 Kern County earthquake; this number is significantly smaller than the commonly used value of 7.7, which is more nearly a surface-wave magnitude. The method presented broadens the base from which M_L can be found and allows M_L to be determined in large earthquakes for which no standard assessment of local magnitude is possible. In addition, in instances where a large number of accelerograms are available, reliable values of M_L can be determined by averaging.

Regional variation of the scaling law of earthquake source spectra

Abstract: The dependence of the seismic source spectrum on earthquake magnitude was determined with improved accuracy in the frequency range from 1 to 25 Hz using coda waves from local earthquakes. Separation of the source effects from the effects of scattering and attenuation was achieved by the method of Aki and Chouet (1975) assuming that the earth is a randomly heterogeneous medium with a uniform statistical property. Scaling laws constructed for seismic regions in Japan, California, and Hawaii show marked variations which may be attributed to the differences in the scale length of inhomogeneity and strength of the earth's crust among these regions. In the magnitude range from 1 to about 5, the stress drop increases in most areas from roughly 1 bar to about 1 kbar. The exception is Hawaii where the stress drop is very low and almost constant. The dependence of the stress drop on earthquake magnitude reflects the heterogeneous material properties in the seismic zone and is explained by a fault plane with barriers which may be skipped, unbroken, when the tectonic stress is low.

Preliminary analysis of the spectral content of P and S waves from local earthquakes in the Garm, Tadjikistan region

Abstract: This paper reports preliminary results of an analysis of the spectral content of seismic waves from over 1,000 local earthquakes in the Garm, Tadjikistan region. Very low values of Q (∼100) were obtained for the Mesozoic and Cenozoic sedimentary rocks of the Peter I Range compared with those in the crystalline rock of the South Tien Shan ( Q ≳ 500). For events with the same low frequency spectra, earthquakes in the South Tien Shan are recorded with more energy at higher frequencies than earthquakes in the Peter I Range, at least in part, because of the greater attenuation of the sedimentary rock in the latter region. There is a wide variety of spectra radiated by earthquakes throughout the Garm region, but a marked difference between spectra radiated by earthquakes from these two regions is not apparent. Nevertheless although calculated stress drops of earthquakes appear to vary considerably throughout the region, there appears to be a dependence on the type of rock in which the earthquakes occurred. The variation in calculated stress drops, however, is greater within either region than between the two. For most regions and for most of the range of seismic moments studied the shape of the spectrum is relatively independent of seismic moment. Consequently, calculated stress drops increase with seismic moment, with the functional dependence varying from region to region. For the largest events, the spectrum shifts to lower frequencies with increasing moment and stress drops become essentially independent of moment. Preliminary results suggest that the stress drops are not detectably different for earthquakes with thrust or normal faulting fault-plane solutions. A limited amount of data are consistent with a change in spectral content and stress drop of earthquakes in the vicinity of and before stronger earthquakes but a clear, consistent pattern is not yet evident. Toward high frequencies, S -wave spectra both begin to decrease at lower frequencies and seem to decrease more rapidly than P -wave spectra, in contrast with predictions of published dislocation models of earthquakes.

Observation of 1- to 5-second microtremors and their application to earthquake engineering. Part I: Comparison with long-period accelerations at the Tokachi-oki earthquake of 1968

Abstract: A study on elucidation of possible amplification characteristic of strong motions due to deep situated deposit was made by means of 1 to 5 sec microtremors observation. At the Tokachi-oki earthquake of 1968 ( M =7.9) several accelerograms were obtained, among which some are dominant but others are not significant in longer periods than 1 sec. To understand whether these differences are from source and path effects or site conditions is important for estimating seismic input motions to high-rise buildings. A long-period microtremors observation was introduced to pursue this problem. Observations were carried out in three cities where the typical acceleration records had been obtained, employing a specially designed instrument good for the microtremors with periods ranging from 0.5 to 6 sec. Each observation line was chosen so as to traverse the accelerograph site along which a remarkable geological change of the underground structure is expected, for example, from the outcrop of bedrock to the alluvial deposit. Through comparison of the obtained spectra and their peaks with those derived from the strong-motion records, it was derived that their predominancy and predominant period in the long-period range are clearly responsible to the presence of deep situated deposit. A formulation of observation and analysing procedures of the long-period microtremors was also proposed, paying attention to overcome the defects in the well-known technique for the short-period microtremors.

The February 9, 1971 San Fernando earthquake: A study of source finiteness in teleseismic body waves

Abstract: Teleseismic P, SV, and SH waves recorded by the WWSS and Canadian networks from the 1971 San Fernando, California earthquake (ML = 6.6) are modeled in the time domain to determine detailed features of the source as a prelude to studying the near and local field strong-motion observations. Synthetic seismograms are computed from the model of a propagating finite dislocation line source embedded in layered elastic media. The effects of source geometry and directivity are shown to be important features of the long-period observations. The most dramatic feature of the model is the requirement that the fault, which initially ruptured at a depth of 13 km as determined from pP-P times, continuously propagated toward the free surface, first on a plane dipping 53°NE, then broke over to a 29°NE dipping fault segment. This effect is clearly shown in the azimuthal variation of both long period P- and SH-wave forms. Although attenuation and interference with radiation from the remainder of the fault are possible complications, comparison of long- and short-period P and short-period pP and P waves suggest that rupture was initially bilateral, or, possibly, strongly unilateral downward, propagating to about 15 km depth. The average rupture velocity of 1.8 km/sec is well constrained from the shape of the long-period wave forms. Total seismic moment is 0.86 × 10^(26) dyne-cm. Implications for near-field modeling are drawn from these results.

The influence of rupture incoherence on seismic directivity

Abstract: As has long been recognized in teleseismic studies, smooth rupture propagation significantly modifies the azimuthal variation in elastic wave radiation and introduces a dependence of peak motion on the ratio of rupture velocity to wave propagation velocity. Rupture propagation also has a first-order effect on the ground motions close to faults as calculated from models of coherent rupture. For engineering purposes, it is important to know whether the effect occurs only with coherent ruptures, or whether it is a more general phenomena of propagating faults. This question was examined by both analytical and Monte Carlo studies of models of nonuniform ruptures. The principal models were defined by ruptures moving continuously in time along the fault with random variations in rupture velocity or in slip amplitude. These models were richer in high frequencies than the corresponding smooth ruptures. The randomness introduced a new corner into the spectrum at a frequency that is simply related to the coherence length of the random variations and to the azimuth between the fault and station. The lower frequency corner due to the overall rupture was preserved. For the model with varying rupture velocity the azimuthal variation in spectral amplitude was enhanced over that for the smooth rupture. For the model with varying slip the azimuthal variation was the same as for a smooth rupture. These models showed directivity effects as strong or stronger than the corresponding smooth rupture, providing that the average rupture velocity was the same. Monte Carlo simulations with statistical models gave peak amplitudes with the same general dependence on rupture velocity as the peak amplitudes from smooth ruptures although in the mean the peak motions were enhanced in the incoherent model. An analytic expression was also derived for the mean spectrum of an extreme model in which rupture occurred in little patches distributed with complete randomness over the fault surface and in time. Even this model showed some effects of directivity. The results of our study are consistent with the interpretation that rupture propagation produces destructive interference in the radiated motion; incoherence reduces this interference and in general leads to higher peak motions and spectral levels.

Lateral variation of surface-wave anelastic attenuation across the pacific

Abstract: Seismograms from island stations within or near the edge of the Pacific plate were used to obtain surface-wave attenuation coefficients in the period range 18 to 110 sec for Rayleigh waves and 20 to 110 sec for Love waves. The average Rayleigh-wave attenuation coefficient values range from a maximum of 1.64 × 10 −4 km −1 at short periods to a minimum of 0.72 × 10 −4 km −1 at longer periods. Corresponding extreme values for Love waves are 3.30 × 10 −4 km −1 and 0.60 × 10 −4 km −1 . The data are characterized by relatively large standard deviations which reflect departures from an ideal medium having laterally homogeneous elastic and anelastic properties. The possibility of regional variations in anelastic properties was examined by dividing the Pacific into three regions according to age (0 to 50 m.y., 50 to 100 m.y., >100 m.y.). A systematic decrease in attenuation coefficient values over most of the period range is readily apparent from the data. Q β −1 models obtained by stochastic inversion of the attenuation data suggest that the observed differences are produced by increasing values of Q β in the low Q zone (at depths of 40 to 200 km), and possibly also by increasing values of Q β in the lithosphere, as the age of the Pacific sea floor increases.

Central California foreshocks of the great 1857 earthquake

Abstract: Analysis of contemporary accounts indicates that several small to moderate central California earthquakes preceded the great 1857 earthquake by 1 to 9 hr. The earliest events apparently were felt only in the San Francisco area or the Sacramento and Sierran Foothills region. Two later and much more widely felt foreshocks were experienced within the region bounded by San Francisco, Visalia, Fort Tejon, and Santa Barbara. A comparison with felt areas and intensity distributions of modern events of known source and magnitude indicates that these later two shocks were 5 ≦M ≲ 6 and probably originated at some point within an area of radius ≈60 km that includes the southeastern 100 km of the historically creeping segment of the San Andreas fault. The northwestern terminus of the 1857 rupture is probably located along this segment. If the location of these foreshocks is indicative of the epicenter of the main event, then the several-hundred-kilometer main-event rupture propagated principally in a unilateral fashion toward the southeast. This implies that, like many great earthquakes, the 1857 rupture originated on a fault segment historically characterized by moderate activity and propagated into an historically quiet segment. There is a strong possibility that the foreshock activity represents a moderate Parkfield-Cholame sequence similar to those of 1901, 1922, 1934, and 1966. To the extent that such premonitory activity is characteristic of the failure of the 1857 segment of the fault, studies of the creeping segment of the fault may be relevant to the prediction of large earthquakes in central and southern California.

Change in b-values during movement on cut surfaces in granite

Abstract: A large granite sample containing a saw cut, modeling a natural fault, was triaxially loaded at confining pressures up to 1000 bars. Fourteen violent slip events accompanied by foreshock and aftershock sequences occurred under constant strain rate loading. From digitally recorded acoustic emission, locations and amplitudes were determined for nearly 8,000 microseismic events. Plots of log amplitude versus log frequency of microseismic events were drawn for three periods between each slip event, termed foreshock, aftershock, and background. These plots indicate that the b -value is lower during foreshocks than for periods between events, implying increased average amplitude of microseismic activity just before slip. These experimental results suggest that it may be possible to devise an earthquake warning system based on changes in b -values in active tectonic regions. It has been suggested that according to the dilatancy-diffusion model, b -value would decrease prior to earthquakes. In our experiment, however, the rock was dry.

Three-dimensional numerical simulations of dynamic faulting in a half-space

Abstract: A method is presented for the computation of near-field particle displacements and particle velocities resulting from a dynamic propagating, stress relaxation occurring on a finite fault plane embedded within a three-dimensional semiinfinite medium To check our numerical procedure we compare our results for a circular fault in a full space with Kostrov's (1964) analytic solution for a self-similar propagating stress relaxation We have simulated two bilateral strike-slip earthquakes differing only in hypocentral location and examined the particle motion on the traction-free surface and on the rupture surface Focusing of energy is evident in both ruptures The static displacement on the rupture surface overshoots the theoretical static value by approximately 25 per cent For the rupture that nucleated at depth the free surface almost doubled the particle velocities along the fault trace as compared with the rupture that nucleated at the free surface Our numerical results indicate that for an earthquake occurring on a semi-circular fault with radius of 10 km, an effective stress of 100 bars and a rupture velocity of 09 β in a medium characterized by β = 3 km/sec, α = α = 3 β and a density of 27 gm/cm3 particle velocities can reach 400 cm/sec and displacements 250 cm We also compare our numerical results with the observations made by Archuleta and Brune (1975) for a spontaneous stress relaxation on a semi-circular crack in a prestressed foam rubber block

Focal mechanism studies in the New Madrid seismic zone

Abstract: A recent study of seismicity in the New Madrid seismic zone by Stauder et al. (1976) has shown the existence of linear micro-earthquake patterns of up to 120 km in length. This study presents the results of composite microearthquake focal mechanism studies along these trends together with focal mechanisms obtained using long-period surface-wave data from larger events. Due to the present microearthquake array geometry, the composite focal mechanism studies do not indicate a complete picture of the nature of the earthquake processes for all the trends. However, the motion on the major 120-km long trend into northeastern Arkansas has significant components of right lateral fault motion. The consistency of surface-wave focal mechanisms and the composite focal mechanism along this trend indicates that it should be considered as a single tectonic unit.

A documentary study of the felt effects of the great California earthquake of 1857

Abstract: We have collected over 60 hitherto unpublished accounts of the California earthquake of January 9, 1857. We have used them, together with those already known, to estimate felt intensities and prepare an isoseismal map which roughly indicates the level of short-period ground motion experienced during this earthquake. Modified Mercalli intensities of VI to VII occurred in the modern metropolitan areas of southern California, and VI to VIII in the southern San Joaquin Valley. The intensity along the fault was IX or more. Instances of seiching, fissuring, sandblows, and hydrologic changes were reported from Sacramento to the Colorado River delta. Most reports say that shaking lasted between one and three minutes. At least two large aftershocks occurred within a week of the main event.

Glorified optics and wave propagation in nonplanar structure

Abstract: Waves propagating in varying nonplanar structure can produce many interesting phenomena, such as focusing, caustics, and triplications. A high-frequency technique based on the first-motion approximation, referred to as glorified optics, has been developed to generate synthetic seismograms for these types of problems. The technique, in its simplest form, uses the spreading rate of a beam with transmission and reflection coefficients along each possible ray path. The time behavior of each arrival is either that of the original pulse or its Hilbert transform depending on the position of caustics. The geophysically interesting structure of a soft basin over a half-space is investigated in detail by this method. Synthetic seismograms appropriate for various locations are compared with the results of finite difference and finite element methods. The technique appears rich in insight and should prove very useful in modeling problems.

Uniform risk functionals for characterization of strong earthquake ground motion

Abstract: A uniform risk functional (e.g., Fourier spectrum, response spectrum, duration, etc.) is defined so that the probability that it is exceeded by some earthquake during a selected period of time is independent of the frequency of the seismic waves. Such a functional is derived by an independent calculation, at each frequency, for the probability that the quantity being considered will be exceeded. Different aspects of the seismicity can control the amplitude of a uniform risk functional in different frequency ranges, and a uniform risk functional does not necessarily describe the strong shaking from any single earthquake. To be useful for calculating uniform risk functionals, a scaling relationship must provide an independent estimate of amplitudes of the functional in several frequency bands. The scaling relationship of Trifunac (1976) for Fourier spectra satisfies this requirement and further describes the distribution of spectral amplitudes about the mean trend; here, it is applied to find uniform risk Fourier amplitude spectra. In an application to finding the uniform risk spectra at a realistic site, this method is quite sensitive to the description of seismicity. Distinct models of seismicity, all consistent with our current level of knowledge of an area, can give significantly different risk estimates.

The foreshock activity of the 1971 San Fernando earthquake, California

Abstract: All of the earthquakes which occurred in the epicentral area of the 1971 San Fernando earthquake during the period from 1960 to 1970 were relocated by using the master-event method. Five events from 1969 to 1970 are located within a small area around the main shock epicenter. This cluster of activity is clearly separated spatially from the activity in the surrounding area, so these five events are considered foreshocks. The wave forms of these foreshocks recorded at Pasadena are, without exception, very complex, yet they are remarkably similar from event to event. The events which occurred in the same area prior to 1969 have less complex wave forms with a greater variation among them. The complexity is most likely the effect of the propagation path. A well located aftershock which occurred in the immediate vicinity of the main shock of the San Fernando earthquake has a wave form similar to that of the foreshocks, which suggests that the foreshocks are also located very close to the main shock. This complexity is probably caused by a structural heterogeneity in the fault zone near the hypocenter. The seismic rays from the foreshocks in the inferred heterogeneous zone are interpreted as multiple-reflected near the source region which yielded the complex wave form. The mechanisms of the five foreshocks are similar to each other but different from either the main shock or the aftershocks, suggesting that the foreshocks originated from a small area of stress concentration where the stress field is locally distorted from the regional field. The number of small events with S-P times between 3.8 to 6 sec recorded at Mt. Wilson each month suggests only a slight increase in activity of small earthquakes near the epicentral area during the 2-month period immediately before the main shock. However, because of our inability to locate these events, the evidence is not definitive. Since the change in the wave forms is definite the present result suggests that detailed analyses of wave forms, spectra, and mechanism can provide a powerful diagnostic method for identifying a foreshock sequence.

Crustal structures inferred from Rayleigh-wave signatures of NTS explosions

Abstract: An improved method for determining plane-layered earth models that accurately represent the important features controlling the amplitude and wave form of surface waves is presented. The method includes a formal inversion of phase and group velocity data determined from observed seismograms and is applied to the Rayleigh waves from Nevada Test Site (NTS) explosions recorded at Albuquerque, New Mexico and Tucson, Arizona. For both paths the observed dispersion agrees with that from the models with a maximum residual of only 0.01 km/sec. Further, the models are consistent with other available information about these paths (e.g., from refraction surveys). To properly account for local differences in the material at the source, an approximate theory is constructed in which the amplitude excitation is computed in a source structure and the dispersion in a separate path structure. Using this theory and the crustal models from the inversion, synthetic seismograms are computed that match the observed seismograms remarkably well.

The effects of boundary friction on the propagation of elastic waves

Abstract: The effects of a frictional boundary plane on the propagation of normally incident plane harmonic shear waves is investigated by an approximate analytical technique. Three nonlinear models for sliding boundary friction are considered, including a new model which accounts for kinematic locking effects at large slip displacements. It is shown that maximum energy absorption at the boundary occurs in each case for an intermediate level of boundary friction.

A second look at the British Columbia earthquake of June 23, 1946

Abstract: Available near- and far-field data have been used to reassess and reevaluate the focal parameters of the June 23, 1946 British Columbia earthquake. The preferred epicenter (49.76°N, 125.34°W) is located on Vancouver Island, inland from the population centers along the east coast. This location is consistent with observed intensities, water disturbances, and calculated ground deformation. The hypocentral depth is near 30 km, making surface rupture a distinct possibility. A revised fault-plane solution indicating strike-slip faulting (probably right lateral on a northwest striking plane) though not a unique interpretation, is the most consistent with observed intensities, water disturbances, and calculated ground deformation. A new surface-wave magnitude calculation of 7.2 ± 0.1 agrees with the previously published value of 7.3. Calculated source parameters are as follows: seismic energy release of 5.6 × 10 22 ergs; seismic moment of 2.5 × 10 27 dyne-cm for the preferred (strike-slip) solution; an apparent stress of 10 bars for the preferred solution. The lack and relatively small size of aftershocks may be indicative of a high stress drop but a reliable evaluation of stress drop is not possible because of uncertainties in estimates of fault dimensions. The epicentral location favors an intraplate setting because it is away from the continental-oceanic boundary and appears to lie within the continental crust of Vancouver Island, which overlies subducted oceanic lithosphere. However, tectonic forces causing the earthquake probably result from the interplate dynamics of the subduction zone.

A dynamic source model for the San Fernando earthquake

Abstract: We model the San Fernando earthquake as a propagating rupture in a half-space, using for the slip-time-history on the fault plane analytical expressions which approximate the slip functions of dynamic crack models obtained by Das and Aki (1977a, b). We synthesize the strong ground motions and accelerations at the Pacoima Dam site and compute the teleseismic signals for different models of cracks. Three major featuras of the data–the strong pulse associated with the beginning of the rupture, the high acceleration phase on the Pacoima Dam records, and the presence of ripples on the teleseismic seismograms–which are not compatible with a smooth rupture process, are well explained by a crack with barriers model where the rupture encounters, along the fault plane, barriers or obstacles of high strength materials which may remain unbroken after the passage of the rupture front. A high-stress drop (400 to 500 bars) is required in the hypocentral area to explain the high-amplitude short-duration first pulse of the teleseismic records. This indicates a high level of tectonic stress in the area. A study of the earthquake series following the main shock shows that the aftershocks which took place in the region where major slipping occurred during the earthquake may represent the release of some of the barriers.

Geological control on the three-component spectra of Rayleigh-wave microseisms

Abstract: Microseisms in the band 1 to 15 Hz propagate principally as multi-mode Rayleigh waves. Comparison of some previously published spectra from sites over unconsolidated sediments, with theoretical Rayleigh-wave dispersion curves shows a high correlation between observed broad spectral maxima and theoretical group-velocity minima. This gives practical support to a recent prediction of this relationship. Comparison of three-component spectra with theoretical Rayleigh-wave dispersion curves and particle motion figures shows that fine structure in the observed spectra can be correlated with changes in the particle motion figures for different Rayleigh modes. Thus both broad and fine spectral features are affected by local geology and can give useful control when inverting microseism data to obtain a seismic model. The Rayleigh wave nature of microseisms implies that direct interpretation of spectra in terms of body-wave seismic resonances of the earth is incorrect. However, since an approximate correspondence exists between theoretical group velocity minima and body-wave resonant frequencies, some spectral maxima do occur near such frequencies.

Numerical study of diffraction of plane elastic waves by a finite crack with application to location of a magma lens

Abstract: Location, shape, and size of near-surface heterogeneities in the Earth may be determined by active or passive seismic diffraction experiments. An array of geophones would be set up to record near-field wave motion resulting from diffraction by the inhomogeneity. As an example of such inhomogeneities, we study the diffraction of plane P and S waves by a finite two-dimensional crack as a function of wavelength and direction of incident wave. We develop a method which can be used to solve for diffraction effects by empty and fluid-filled (zero viscosity) cracks. The magnitude of the effect of the fluid in the crack is governed by a parameter called the crack stiffness factor which is the ratio of bulk modulus of the fluid in the crack to the rigidity of the solid divided by the crack aspect ratio. The diffraction problem is solved using the finite difference scheme developed by Madariaga (1976) to study dynamic crack propagation problems. It is found that a crack has little effect on wave motion for incident wave with wavelengths greater than 10 times the crack length. For wavelengths less than 5 times the crack length there is a complicated diffraction pattern. The amplitude and phase of both components of motion are affected by the crack. We find that the diffraction effect decreases as the crack stiffness factor increases. For wavelengths less than 2 times the crack length there is a region of reduced amplitude (shadow zone) behind an empty crack. Relative phase of two components of particle motion varies rapidly with position inside the shadow zone. The results of the theoretical calculation are used to try to locate the edge of a magma lens in Kilauea Iki Crater, Hawaii. It is found that the ratio of vertical-to-horizontal displacement and the direction of rotation of initial particle motion measured by a seismic array located on the floor of the crater agree qualitatively with what the theory predicts. The edge of the magma found by this method shows excellent agreement with the one inferred from the distribution of seismic events originating in the crust of the lava lake (Aki et al. , 1977).

The dynamic field of Haskell's rectangular dislocation fault model

Abstract: We study the near-field generated by Haskell9s rectangular fault model used extensively to interpret seismic data. By means of the Cagniard-de Hoop method we have been able to find an exact solution for the near-field particle velocities in the case of a step-function source slip history. The results show that there are two distinct regions of radiation. One is a cylindrical region in front of the fault where two-dimensional approximations are valid and result in a substantial reduction of computation. In the rest of space the field is dominated by spherical waves radiated from the corners of the dislocation. These waves are much more complicated to calculate. First motion approximations demonstrate that the cylindrical waves are stronger than the spherical waves; in particular, infinite accelerations at the cylindrical wave fronts are predicted even for ramp sourcetime functions. The stress field on the fault plane generated by this dislocation is also calculated. Strong stress singularities of type r −1 are found all around the edges of the fault. These singularities are a consequence of the assumption of constant dislocation across the fault width. They may only be eliminated by smoothing the dislocation near the fault edges. These singularities are so strong that an infinite averaged stress drop on the fault is predicted independently of any source parameters. As a consequence, the Haskell model is essentially a long-period model of faulting.

The source mechanism of the August 7, 1966 El Golfo earthquake

Abstract: The El Golfo earthquake of August 7, 1966 (m_b = 6.3, M_S = 6.3) occurred near the mouth of the Colorado River at the northern end of the Gulf of California. Synthetic seismograms for this event were computed for both the body waves and the surface waves to determine the source parameters of the earthquake. The body-wave model indicated the source was a right lateral, strike-slip source with a depth of 10 km and a far-field time function 4 sec in duration. The body-wave moment was computed to be 5.0 × 10^(25) dyne-cm. The surface-wave radiation pattern was found to be consistent with that of the body waves with a surface-wave moment of 6.5 × 10^(25) dyne-cm. The agreement of the two different moments indicates that the earthquake had a simple source about 4 sec long. A comparison of this earthquake source with the Borrego Mountain and Truckee events demonstrates that all three of these earthquakes behaved as high stress-drop events. El Golfo was shown to be different from the low stress-drop, plate-boundary events which were located on the Gibbs fracture zone in 1967 and 1974.

Simulation of strong-motion displacements using surface-wave modal superposition

Abstract: Synthetic seisomograms constructed by addition of surface-wave modes in a layered half-space are compared to Cagniard-de Hoop calculations of Heaton and Helmberger (1977, 1978) and to ground displacement recordings near El Centro, California to examine the applicability of modal superposition as a means of simulating ground motion of possible engineering interest. Modal solutions of flat earth problems are desirable because of the modest cost involved and the versatility of the method in simulating extended sources and anelastic damping. P-SV and SH motions can be computed with almost equal ease. The comparisons show that in sedimentary structures surface waves can dominate ground displacement motion at epicentral distances of only a few source depths. Superposition of the higher modes often approximates quite well impulsive arrivals with analogies to refracted and reflected rays. Ground displacement recordings of El Centro from the 1968 Borrego Mountain earthquake are modeled using a multi-layered geological structure and a source model based on independent studies. The gross character of the records appears to be insensitive to the details of the source. Both point sources and propagating sources with horizontal dimensions larger than half the epicentral distance give'reasonable fits to the observed transverse motion. This insensitivity appears to be due to a complex interaction between rupture propagation and the surface-wave dispersion. By using the integrated El Centro accelerogram, which may have more reliable amplitude information than the Carder displacement record used in other studies, the moment is estimated to be 12 x 102s dyne-cm. This is similar to values found from studies of teleseismic data.