Showing papers in "Pure and Applied Geophysics in 1975"
TL;DR: In this article, two different physical models are used to explain precursory phenomena before earthquakes in the United States, the Soviet Union, Japan, and China: the dilatancy diffusion model and the dry model.
Abstract: Similar precursory phenomena have been observed before earthquakes in the United States, the Soviet Union, Japan, and China. Two quite different physical models are used to explain these phenomena. According to a model developed by US seismologists, the so-called dilatancy diffusion model, the earthquake occurs near maximum stress, following a period of dilatant crack expansion. Diffusion of water in and out of the dilatant volume is required to explain the recovery of seismic velocity before the earthquake. According to a model developed by Soviet scientists growth of cracks is also involved but diffusion of water in and out of the focal region is not required. With this model, the earthquake is assumed to occur during a period of falling stress and recovery of velocity here is due to crack closure as stress relaxes. In general, the dilatancy diffusion model gives a peaked precursor form, whereas the dry model gives a bay form, in which recovery is well under way before the earthquake. A number of field observations should help to distinguish between the two models: study of post-earthquake recovery, time variation of stress and pore pressure in the focal region, the occurrence of pre-existing faults, and any changes in direction of precursory phenomena during the anomalous period.
205 citations
TL;DR: In this article, three types of triaxial compression experiments were used to characterize the frictional processes during sliding on quartz gouge and the results indicated that at effective confining pressures of less than 2.0 kb, if a fault zone contains quartz gouges, laboratory-type stick-slip can be an earthquake-source mechanism only if a planar sliding-surface develops, and then only when the effective pressure is less than 0.7 kb.
Abstract: Three types of triaxial compression experiments are used to characterize the frictional processes during sliding on quartz gouge. They are: 1) pre-cut Tennessee Sandstone sliding on an artificial layer of quartz gouge; 2) fractured Coconino Sandstone sliding along experimentally produced shear fractures; and 3) a fine-grained quartz aggregate deformed in compression. The specimens were deformed to 2.0 kb confining pressure at room temperature and displacement rates from 10−2 to 10−5 cm/sec dry and with water. There is a transition in sliding mode from stick-slip at confining pressures 0.7 kb. This transition is accompanied by a change from sliding at the sandstone-gouge contact (stick-slip) to riding on a layer of cataclastically flowing gouge (stable sliding). Quartz gouge between the pre-cut surfaces of Tennessee Sandstone lowers both the kinetic coefficient of friction and the magnitude of the stick-slip stress drops compared to those for a pre-cut surface alone. Stick-slip stress drops are preceded by stable sliding at displacements of 10−5 cm/sec. For a decrease in displacement rate between 10−3 and 10−5 cm/sec, stress-drops magnitudes increase from 25 to 50 bars. Tests on saturated quartz gouge show sufficient permeability to permit fluidpressure equilibrium within compacted gouge in 10 to 30 seconds; thus the principle of effective stress should hold for the fault zone with quartz gouge. Our results suggest that at effective confining pressures of less than 2.0 kb, if a fault zone contains quartz gouge, laboratory-type stick-slip can be an earthquake-source mechanism only if a planar sliding-surface develops, and then only when the effective confining pressure is less than 0.7 kb.
118 citations
TL;DR: In this paper, the authors used a disposable pick-up placed directly on the explosive to determine the shot time up to 10 msec by using a fast paper speed (typically 1 cm/sec) and the WWVB radio signals superposed on the seismic trace.
Abstract: About twenty blasts are used to determine crustal structure and to monitor temporal seismic velocity changes in southern California. The shot time is determined up to 10 msec by using a disposable pick-up placed directly on the explosive. About 17 permanent stations and 20 temporary stations are used for the recordings. With a fast paper speed (typically 1 cm/sec) and the WWVB radio signals superposed on the seismic trace, absolute timing accuracy of up to 10 msec is achieved. A representative structure thus determined consists of a 4 km thick 5.5 km/sec layer underlain successively by 23.4 km thick 6.3 km/sec layer, 5.0 km thick 6.8 km/sec layer and 7.8 km/sec half space. The details of the lower crust are somewhat uncertain. This structure can explain the travel time data, corrected for the station and source elevations and for the station delays, to ±0.15 sec. Small but systematic temporal velocity changes up to 3% have been found for some of the profiles. If the effect of the migration of the shot point is small enough, these changes are larger than experimental errors and represent real temporal change in the material property between the shot point and the stations.
111 citations
TL;DR: In the northern and central sections of the San Andreas Fault Zone, and along Calaveras and Hayward faults, clay gouges have been found to occur on the surface and at shallow depths.
Abstract: In the northern and central sections of the San Andreas Fault Zone, and along Calaveras and Hayward faults, clay gouges have been found to occur on the surface and at shallow depths.
97 citations
TL;DR: In this paper, the main scientific result of the two-year study is the statement that it is possible to predict earthquakes of magnitude ≥ 5.5 some days in advance; eight earthquakes had been predicted from a total number of twelve.
Abstract: Tentative forecasts of strong earthquakes on Kamchatka have been carried out continuously since 1972 with scientific purpose. Three methods are being used: electric, ratio V P /V S and seismostatistical. The main scientific result of the two-year study is the statement that it is possible to predict earthquakes of magnitude ≥5.5 some days in advance; eight earthquakes had been predicted from a total number of twelve.
74 citations
TL;DR: In this article, the scale independent inclusion theory of rock failure is applied to the problem of crustal earthquakes and, in particular, to the premonitory phenomena reported to precede such earthquakes.
Abstract: The scale independent inclusion theory of rock failure developed in Part I is applied to the problem of crustal earthquakes and, in particular, to the problem of premonitory phenomena reported to precede such earthquakes. Several well-known premonitory effects such as anomalous variation in the ratio of longitudinal (V p ) and shear (V s ) seismic velocities, V P /V S , tilt, regional and local crustal movements and stress axis rotation, to mention a few, are shown to be a natural consequence of the physical processes leading to failure in dry rock. The effects of fluids on failure in the focal region of a potential earthquake are considered in terms of the scale independent inclusion theory.
70 citations
TL;DR: In this article, various models that have been proposed for describing the dynamical development of small cumuli are discussed in terms of how well they predict the observed distribution of liquid water.
Abstract: The various models that have been proposed for describing the dynamical development of small cumuli are discussed in terms of how well they predict the observed distribution of liquid water. The entrainment concept is examined and shown to involve contradictions when applied to growing blobs. An alternative process is offered in which the dominating factor in the cloud dynamics is the mixing in of drier overlying air down through the cloud until temporary static equilibrium is established. Consideration of the mixing process and calculations of the required dilution show this process can lead to the observed liquid water distributions.
61 citations
TL;DR: This article showed that the resistivity of typical pore fluids changes very little down the transition so that changes in rock resistivity will be almost solely due to dilatant volume change.
Abstract: Dilatancy may lower pore pressure at mid-crustal depths to near the liquid-vapor transition of water. However, the resistivity of typical pore fluids changes very little down the transition so that changes in rock resistivity will be almost solely due to dilatant volume change. Experiments with partly saturated rocks at room temperature suggest that rocks containing mixtures of fluid and vapor, along the transition, will generally behave electrically during dilatancy as though they were fluid saturated. Observed resistivity changes before earthquakes give the dilatant volume change directly and thus may be used to further constrain the dilatancy-diffusion model, independent of observed uplift or velocity changes.
59 citations
TL;DR: In a series of triaxial experiments, this article measured V p, V s and volumetric strain simultaneously in dilating dry and saturated rocks and showed that dilatancy and fluid flow are responsible for seismic travel time anomalies prior to earthquakes, but only in regions where pore fluid source to sink dimensions are of the order of 10 km or more.
Abstract: In a series of triaxial experiments we have measured V p , V s and volumetric strain simultaneously in dilating dry and saturated rocks. For the first time these data permit quantitative comparison of seismic velocities or their ratio and dilatant volumetric strain. In air-dry samples V p /V s decreases by a few per cent at strains of 10−3; in saturated materials with high pore pressure, V p /V s increases by a comparable amount. Decreases in seismic velocity ratio are difficult to generate in initially saturated rocks even with low pore pressures and at strain rates of 10−4/sec. A liquid-vapor transition will not produce a significant drop in V p /V s . If dilatancy and fluid flow are responsible for seismic travel time anomalies prior to earthquakes, our results suggest that such anomalies will occur only in regions where pore fluid source to sink dimensions are of the order of 10 km or more, or in regions where the rocks are not saturated to begin with.
54 citations
TL;DR: In this paper, the authors studied the possibility of delayed strain release by aseismic faulting following a main shock in relation to the recent earthquake off Nemuro peninsula, Hokkaido, Japan 17 June, 1973.
Abstract: The possibility of delayed strain release by aseismic faulting following a main shock is studied in relation to the recent earthquake off Nemuro peninsula, Hokkaido, Japan 17 June, 1973. The main purpose of this paper is to obtain a comprehensive view of the land movement in eastern Hokkaido with respect to the seismic cycle there This region, particularly the Nemuro peninsula area, has been subject to extensive land subsidence in the past several decades. At Hanasaki, for example, it amounted to −60 cm in the past 70 years. The long-term accumulation of subsidence in the same area, as read from remaining marks of the ancient sea level, is only −2 m in the past 5000–6000 years. There-fore, the rate of the recent land movement is twenty times or more as high as the long-tern one, which is known from the geomorphological evidence. Such disagreement between the two kinds of rates leads us to expect a future uplift phase which will compensate the previous subsidence.
45 citations
TL;DR: More than 20 coseismic resistivity steps were observed by a resistivity variometer at a station about 60 km south of Tokyo during a six-year period of observation as discussed by the authors.
Abstract: More than 20 coseismic resistivity steps were observed by a resistivity variometer at a station about 60 km south of Tokyo during a six-year period of observation. The logarithmic magnitude of steps normalized to an epicentral distance (δ) of 100 km is linearly correlated to earthquake magnitude (M). The larger the earthquake magnitude is, the longer is the time required for completing a resistivity step.
TL;DR: In this article, the vertical eddy diffusion coefficient of CH4 and H2 was derived by comparing the average measured profiles to those predicted by a one-dimensional chemical model, and the limitations of the calculated k z are discussed.
Abstract: New measurements on the stratospheric distribution of H2, CH4, CO and N2O are presented and used to demonstrate the natural variability of the trace gas concentrations. The present CH4 and H2 measurements and data from older balloon flights are combined to give average vertical profiles. These profiles are compared with water vapor data from various authors to see if the vertical decrease in CH4 is matched by a corresponding increase in H2O. By comparing the average measured profiles to those predicted by a one-dimensional chemical model, profiles of the vertical eddy diffusion coefficientk z are deduced. Generally, a barrier in the low stratosphere and increasing transport in middle and upper stratosphere seem required to match theoretical and experimental profiles. The limitations of the calculatedk z are discussed.
TL;DR: In this paper, the distance of stable sliding before sudden slip on fault surfaces in granite decreases rapidly as the confining pressure is increased, and the amount of stable creep is very small or absent.
Abstract: The distance of stable sliding before sudden slip on fault surfaces in granite decreases rapidly as the confining pressure is increased. At a pressure of 6 kb the amount of stable creep is very small or absent. Two orders of magnitude change in strain rate has no effect on the distance of stable sliding. Our results suggest that in the earth, fault creep should predominate in the shallow crust but in the deep crustal layer most of the stresses are probably relieved by sudden earthquake type of motion. Below the crust high temperature would promote stable-slip so in this region creep would once more predominate.
TL;DR: In this article, the authors summarized the migration velocities of convective storms for six situations, with different environmental wind fields, and found that large-diameter (ca. 20-30 km) storms generally deviated to the right, in proportion to their sizes and to the veering of wind with height.
Abstract: Migration velocities of convective storms are summarized for six situations, with different environmental wind fields. Small-and medium-sized storms generally moved to left of the direction of, and at speeds somewhat less than, the vector mean wind in the troposphere. Large-diameter (ca. 20–30 km) storms generally deviated to the right, in proportion to their sizes and to the veering of wind with height. This behavior, and the tendency for large storms to move appreciably slower than the mean wind, are even more pronounced when giant clusters of thunderstorms are considered. An example is analyzed in which a multicellular storm, 80 km wide, moved 55° to right of the mean wind and with half its speed. This behavior results from a characteristic pattern of propagation, in which new cells tend to form on the general upwind side of the cluster, with the larger and more intense cells developing on its right flank. The individual cells move through the cluster, dissipating on approach to its advancing and left flanks. Preferential formation of cells toward the rear side of the cluster is shown to be compatible with the probable origin and trajectories (relative to the moving storm) of air ascending from the lower part of the subcloud layer. The sometimes-observed rapid movement of large multicellular storms to left of the mean wind is partly accounted for by an opposite (left forward flank) pattern of propagation.
TL;DR: For a power law of the form ''gamma = \delta \cdot {\text{I}}_2^{n/2}\) (where γ is dilatant strain, δ is a constant and I 2 is the deviatoric stress invariant) as mentioned in this paper, the ratio of scale to depth of faulting is significantly less than unity.
Abstract: The spatial extent of the dilatant zone and its time, variations associated with preseismic fault creep are very sensitive to the dilatant constitutive law. For a power law of the form \(\gamma = \delta \cdot {\text{I}}_2^{n/2}\) (where γ is dilatant strain, δ is a constant and I 2 is the deviatoric stress invariant) the ratio of dilatant scale to depth of faulting is significantly less than unity for n = 2 (‘microcrack dilatancy’), about unity for n = 1 (‘joint dilatancy’), and much larger than unity for n = 1/2 (’sand pile dilatancy’). Consequently it is necessary to establish the in situ nature of the dilatant process in order to understand the onset, distribution and development of crustal dilatant regions.
TL;DR: In this article, a 2-component tiltmeters have been installed in shallow boreholes along 85 km of the currently most active section of the San Andreas fault in the western United States.
Abstract: The continuous monitoring of surface deformation near active faults is clearly necessary for an understanding of elastic strain accumulation and elastic and anelastic strain release associated with earthquakes. Fourteen 2-component tiltmeters have been installed in shallow boreholes along 85 km of the currently most active section of the San Andreas fault in the western United States. These instruments operate at a sensitivity of 10−8 radians. Five of these tiltmeters, extending along one 35 km section of the fault, have been in operation since June 1973. The results indicate that regional tectonic tilting has occurred before more than ten individual earthquakes or groups of earthquakes with epicenters within ten earthquake source dimensions of one or more instruments. This tilting has a time scale of up to a month depending on earthquake magnitude. The amplitude of these tilts exceeds by almost an order of magnitude that expected from a dislocation model of the source using seismically determined parameters. No indication of rapid or accelerated tilt just prior to these earthquakes has been seen.
TL;DR: In this paper, the relation between remanent magnetization and volumetric strain for gabbro samples stressed in uniaxial compression inside a near zero-field μ-metal shield has been examined.
Abstract: The relation between remanent magnetization and volumetric strain for gabbro samples stressed in uniaxial compression inside a near zero-field μ-metal shield has been examined. For samples with an induced IRM parallel to the axis of compression, remanent magnetization decreased linearly up to the onset of dilatancy. As increased stress produced additional dilatancy, the variation of remanent magnetization became nonlinear, and the stress dependence continually decreased until the rock failed. Stress cycling with the peak stress augmented for each cycle produced a continuous decrease in the zero stress value of the IRM although an appreciable amount of recovery was observed during unloading. When the sample was loaded in constant stress increments after the onset of dilatancy and held for several minutes at each level, time-dependent variations in remanent magnetization coincided with time-dependent increases in inelastic volumetric strain. In general as the inelastic creep rate increases, the rate of change in remanent magnetization increases. These results suggest that dilatancy related effects of the intensity of rock magnetization should be observed in magnetic rocks in epicentral regions prior to earthquakes and may serve as both long- and short-term precursors.
TL;DR: In this paper, a physical interpretation of empirical formulas for earthquake magnitude and spatial extents of land deformation and aftershock area is attempted on the basis of a dilatancy model.
Abstract: A physical interpretation of empirical formulas for earthquake magnitude (M) and spatial extents of land deformation and aftershock area is attempted on the basis of a dilatancy model. It is demonstrated that the precursor time interval (τ) vs.M relation approximately fits in that for the τ vs. typical length of earthquake zone provided the above formulas hold good. A diffusion process in a dilatant region seems also to account for these approximate formulas.
TL;DR: The annual mean sea levels of all permanently recording tide gauge stations along the North American west coast were inspected for evidence of elevation changes before and after large strike-slip earthquakes as mentioned in this paper.
Abstract: The annual mean sea levels of all permanently recording tide gauge stations along the North American west coast were inspected for evidence of elevation changes before and after large strike-slip earthquakes. The Yakutat earthquake, 1958 with M = 7.9 was preceded by 3 ± 3 cm of uplift 50 km west of the central part of the 400 km rupture. The Sitka earthquake, 1972 with M = 7.1, was preceded by 3 ± 3 cm of uplift at a station 40 km east of the center of the 170 km rupture. The Queen Charlotte earthquake, 1949 with M= 8.1, was followed by 5 ± 1 cm of subsidence 200 km east of the center of the 300 km rupture. This subsidence is interpreted as dilatancy recovery. In this case the uplifted area had a radius of approximately 200 km, and the precursor time may have been about 30 years. The San Francisco earthquake, 1906 with M = 8.3, was preceded by no uplift during seven years before it and was followed by no elevation change during 20 years after the event. The measurements were taken at the Presidio 6 km from the fault trace. The Long Beach earthquake, 1933 with M= 6.3, was preceded by 1 ± 1 cm of uplift at a point 7.4 km from the 50 km long aftershock zone.
TL;DR: In this paper, a conceptual three-dimensional model of midlatitude organized convection along with its implications for parameterization, particularly in mesoscale modeling, is presented, and the effects of moist downdrafts are also considered.
Abstract: Observations of air flow in and around convective clouds are summarized and discussed in light of the requirements for parameterization of midlatitude convection. Both theory and observation indicate that a substantial portion of the compensating subsidence occurs as dry downdrafts in the immediate vicinity of convective clouds, which suggests that an additional physical mechanism is necessary in linking convection to the large scale. A conceptual three-dimensional model of midlatitude organized convection along with its implications for parameterization, particularly in mesoscale modeling, is presented. The effects of moist downdrafts are also considered and some differences between midlatitude and tropical convection are discussed.
TL;DR: In this paper, acoustic emissions during high temperature frictional sliding in faulted granite were detected with a piezoelectric transducer placed outside the furnace and pressure vessel.
Abstract: Acoustic emissions during high temperature frictional sliding in faulted granite were detected with a piezoelectric transducer placed outside the furnace and pressure vessel. Suitable choice of materials allowed the transmission of signals sufficiently large for measurement and analysis. Acoustic emissions were also monitored during the fracture runs at room temperature and could be compared with earlier studies. As observed previously, the emission rate increased dramatically prior to failure and the Ishimoto-Iida relationship between emission amplitude, A, and number of emissions, N(A), of amplitude, A, i. e., log N(A) = K′ − (b + 1) log A, was followed. The constant, b, was typically about 1.4, higher than the value of 1 or less indicated by previous experimental and theoretical studies.
TL;DR: In this article, the residence time for SO2 in the Mediterranean marine atmosphere is approximately one-half day, valid for the calm weather conditions which prevailed during the observations, and this figure is an upper limit.
Abstract: New measurements of SO2 and sulfate aerosol concentrations were obtained during April, 1974, in a series of observations made along the French Mediterranean coast, inland and at sea. By comparing the variations in the concentrations of these substances with variations in the concentrations of Radon 222 and Lead 212, which were measured concurrently, we have found that the residence time for SO2 in the Mediterranean marine atmosphere is approximately one-half day. This figure is an upper limit, valid for the calm weather conditions which prevailed during the observations.
TL;DR: In this article, it was shown that the gravity change depends on the uplift immediately beneath the station and the deformation of cavities at depth, and the approximation was made that density is uniform except for cavities, but no assumptions about the constitutive behavior of the medium are necessary.
Abstract: Deformation causes changes in gravity even though the measuring station is fixed relative to the center of the earth and the total mass in the vicinity of the station does not change. Deformation has two effects: the density at a point changes in response to changes in local stresses and the point itself moves relative to the station. The general expression derived here shows that the gravity change depends on the uplift immediately beneath the station and the deformation of cavities at depth — cracks, pores, magma reservoirs, etc. The approximation is made that density is uniform except for cavities, but no assumptions about the constitutive behavior of the medium are necessary.
TL;DR: In this paper, a more refined model was built where, although the shear and the Reynolds stresses are approximated by dispersion terms, the tidal stress was explicitly calculated using a preliminary tidal model.
Abstract: The determination of the residual circulation is essential to the interpretation of the sedimentation pattern. The classical picture of the residual circulation in the Southern Bight could not explain the observation of mud deposition in front of the Belgian coast. Hence, a more refined model was built where, although the shear and the Reynolds stresses are approximated by dispersion terms, the tidal stress was explicitly calculated using a preliminary tidal model. It is shown that the new model predicts the existence of a residual gyre off the Belgian coast which, increasing the residence time of the water masses and in particular the entrained water from the Scheldt estuary, fully explains the observed sedimentation pattern.
TL;DR: In this paper, a one-dimensional, time-dependent numerical cloud model is used to analyze the factors in the dynamic and thermodynamic equations which lead to a steady-state or nonsteady-state solution for the cloud vertical motion, buoyancy, precipitation, and cloud water fields.
Abstract: A one-dimensional, time-dependent numerical cloud model is used to analyze the factors in the dynamic and thermodynamic equations which lead to a steady-state or nonsteady-state solution for the cloud vertical motion, buoyancy, precipitation, and cloud water fields. ‘Bulk water’ microphysical techniques are used for the cloud, rain, and hail variables. An atmospheric sounding from a severe storm situation is used as initial and environmental conditions, yielding model updrafts of 40 m sec−1 maximum and more than 10 m sec−1 over the entire cloud region.
TL;DR: In this paper, the first place selected was in the city of Iwatsuki, about 30 km from the centre of Tokyo, and the main construction was completed at the end of 1973.
Abstract: As a part of the Japanese earthquake prediction programme, deep borehole observation of the earth’s crustal activities has been planned in and around Tokyo by the National Research Center for Disaster Prevention. The first place selected was in the city of Iwatsuki, about 30 km from the centre of Tokyo. Starting in 1970, the main construction was completed at the end of 1973. The tentative observation of micro-earthquakes and crustal movement by tiltmeters has been carried out since May, 1973. As expected, noise at the bottom of the hole (3500 m depth) is extremely small, about 1/1000–1/300 of that at the surface. We could, possibly, detect earthquakes with magnitudes greater than 2.0 within 50 km radius from the Iwatsuki observatory.
TL;DR: In this paper, the force between a charged conducting sphere and an uncharged dielectric sphere is computed and given in terms of a force coefficient which depends on the relative geometry of the two spheres and the dielectrics constants of the sphere and the medium.
Abstract: The force between a charged conducting sphere and an uncharged dielectric sphere is computed and given in terms of a force coefficient which depends on the relative geometry of the two spheres and the dielectric constants of the spheres and the medium. In addition, approximate solutions for the force between a charged conducting sphere and a charged dielectric sphere are presented. The application of our computed electrostatic forces to the scavenging of atmospheric aerosol particles by water droplets is discussed.
TL;DR: In this paper, a dilatancy model is presented which seems capable of simulating the results of laboratory tests on rock samples, where the fundamental assumption incorporated in the model is that dilatance is caused by the opening of cracks, where cracks open in the least compressive stress direction.
Abstract: A dilatancy model is presented which seems capable of simulating the results of laboratory tests on rock samples. The fundamental assumption incorporated in the model is that dilatancy is caused by the opening of cracks, where the cracks open in the least compressive stress direction (Brace et al., 1966). Its simplicity and compatibility with numerical techniques, involving the simulation of both static and dynamic stress fields, permits prediction of dilatancy induced effects for stress states and loading conditions which are not experimentally attainable.
TL;DR: In this article, hydraulic fracturing stress measurements have been performed in a limestone quarry near the Hohenzollern-Graben, a fault structure in SW Germany, and the values of the two horizontal principal stresses were 24 and 15 bars at a depth of 25 m.
Abstract: Hydraulic fracturing stress measurements have been performed in a limestone quarry near the Hohenzollern-Graben, a fault structure in SW Germany. The values of the two horizontal principal stresses were 24 and 15 bars at a depth of 25 m. The magnitude and the direction of the stresses agree with the results obtained by door-stopper measurements at the same location and the direction of the maximum horizontal principal stress derived from fault plane solutions of shallow earthquakes of this tectonic active area.
TL;DR: In this article, the authors used two-station residuals and corrected for observed azimuthal variations to analyze P-velocity variations prior to the February 24, 1972, Bear Valley earthquake at several stations in the vicinity of Bear Valley, California.
Abstract: Teleseismic arrivals for large earthquakes occurring in the Circum-Pacific seismic source regions have been analysed for P- velocity variations prior to the February 24, 1972, Bear Valley earthquake at several stations in the vicinity of Bear Valley, California. The teleseismic arrivals have been analysed by the method of two-station residuals and corrected for observed azimuthal variations. The data covers the time period of July, 1971, through April, 1972, and suggests that during part of January, 1972, a P-velocity anomaly occurred beneath station BVL 2 km from the epicenter of the magnitude 5.1 Bear Valley event. A maximum vertical travel time delay of 0.15 seconds is observed. No other anomalous behavior associated with this event is suggested by the data for the other stations ranging from 7 to 19 km from the event’s epicenter. These results support an anomalous zone of limited size with a maximum horizontal extent of less than 5 km perpendicular and about 10 km parallel to the San Andreas fault relative to the epicenter and confined within a 5 to 10 km portion of the uppermost crust. Also a P-velocity delay for waves travelling essentially along the intermediate stress axis would imply in this case that the P-velocity anomaly is caused by a bulk modulus mechanism such as that proposed by the dilatancy-fluid flow theory.