Earthquake Of February Research Articles

Particle-velocity attenuation relations: San Fernando earthquake of February 9, 1971

Accelerograms recorded at 61 stations during the San Fernando earthquake were integrated and used to determine the resultant horizontal and the vertical particle velocity as a function of distance. The attenuation of the average value of particle velocity, in the period range from 0.28 to 1.4 sec, is described by the relationship: log10 v = 3.19 − 1.35 log10Δ. The attenuation of Modified Mercalli intensity with distance and the horizontal particle velocity are: log10Δ = 3.05 − 0.25 l and log10 v = −0.93 + 0.29 l. Attenuation relations obtained in the frequency domain for the horizontal and vertical pseudo-spectral velocities of 3-sec-period waves are log10 vH = 3.68 − 1.25 log10Δ and log10 vV = 2.91 − 1.03 log10Δ, respectively. Other relations for different spectral components are also presented. The ratios of horizontal to vertical pseudo-spectral velocities ( vH / vV ) are 3.5 for a 3-sec-period wave at Δ = 10 km, and 3.2 for a 1-sec-period wave in the distance range from 10 to 100 km. Comparison between the particle-velocity attenuation curve found in this study and that determined for a design earthquake for the central United States having mb = 7.2 shows a difference of one order of magnitude in the level of motion at T = 0.3 sec, and the attenuation for mb = 6.2 and T = 3.0 sec agrees fairly well with the observed particle-velocity mean attenuation curve derived from the integrated accelerogram of the San Fernando earthquake, mb = 6.2 (NEIS). The absorption coefficient for a T = 0.1-sec wave is σ = 0.0225/km. The horizontal pseudo-spectral velocity mean curves show a similar rate of attenuation for periods between 0.1 to 3.0 sec, suggesting a decay rate for the displacement-source spectrum of ω −1 in this frequency range.

Closure to “Dynamic Analysis of the Slide in the Lower San Fernando Dam During the Earthquake of February 9, 1971”

Closure to “Dynamic Analysis of the Slide in the Lower San Fernando Dam During the Earthquake of February 9, 1971”

Magnetic Change Accompanying the Haicheng Earthquake of February 4, 1975

Magnetic Change Accompanying the Haicheng Earthquake of February 4, 1975

Aftershocks of the February 21, 1973 Point Mugu, California earthquake

abstract The ML 6.0 Point Mugu, California earthquake of February 21, 1973 and its aftershocks occurred within the complex fault system that bounds the southern front of the Transverse Ranges province of southern California. P-wave fault plane solutions for 51 events include reverse, strike slip and normal faulting mechanisms, indicating complex deformation within the 10-km broad fault zone. Hypocenters of 141 aftershocks fail to delineate any single fault plane clearly associated with the main shock rupture. Most aftershocks cluster in a region 5 km in diameter centered 5 km from the main shock hypocenter and well beyond the extent of fault rupture estimated from analysis of body-wave radiation. Strain release within the imbricate fault zone was controlled by slip on preexisting planes of weakness under the influence of a NE-SW compressive stress.

Aftershocks of the Guatemalan Earthquake of February 4, 1976

Several thousand earthquakes were recorded by portable seismic stations operated in Guatemala within the first week following the destructive earthquake of February 4, 1976. The seismically active zone followed the Motagua fault through eastern and central Guatemala, and then branched sharply southward about 35 km west of Guatemala City. These data support the conclusion from field evidence that the main earthquake was caused by movement within the Motagua fault zone. A total rupture length of at least 250 km, defining a segment of the active boundary between the North American and Caribbean plates, is inferred. The southward trending zone of aftershock activity may be a newly defined section of the boundary, or a secondary fault.

Discussion of “Dynamic Analysis of the Slide in the Lower San Fernando Dam During the Earthquake of February 9, 1971”

Discussion of “Dynamic Analysis of the Slide in the Lower San Fernando Dam During the Earthquake of February 9, 1971”

Inference of Granitic Batholiths by Gravity Studies in South India

Abstract Three well-defined gravity 'lows' in Kodaikanal, Gudalur and Erode regions are explained as due to granitic batholiths at depth. The scattered outcrops of granitic bodies exposed in these regions are suggested to be interconnected at depth, and, therefore, are merely the surface expressions of the three concealed batholiths. Although the exposures of the individual granitic bodies in the Kodaikanal-Coimbatore region are elongated in an east-west direction, it is surmised, from structural evidences, that the main batholith is elongated in a roughly NNW-SSE direction. A striking correspondence between the isoseismals for the Coimbatore earthquake of February 8, 1900 and the gravity contours is noticed. It is suggested that the Coimbatore earthquake (and the reported seismicity near Kottayam and 'Palghat Gap') could be the result of some minor readjustments of the boundaries of the batholith along fault planes.

Dynamic analysis of the slide in the lower San Fernando dam during the earthquake of February 9, 1971 13F, 16R : Seed, HB Univ. Calif. Berkeley, USA J. Geotech. Engng Div. ASCE, V101, GT9, Sept. 1975, P889–911

Dynamic analysis of the slide in the lower San Fernando dam during the earthquake of February 9, 1971 13F, 16R : Seed, HB Univ. Calif. Berkeley, USA J. Geotech. Engng Div. ASCE, V101, GT9, Sept. 1975, P889–911

Time variations in teleseismic reisduals prior to the magnitude 5.1 Bear Valley earthquake of February 24, 1972

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.

Vertical crustal movements preceding and accompanying the San Fernando earthquake of February 9, 1971: A summary

Comparative elevations referred chiefly to a tidal bench mark with a history of relatively positive movement show that much of the Transverse Ranges of southern California sustained major changes in elevation both before and in association with the M L 6.4 San Fernando earthquake of February 9, 1971. Preseismic changes in elevation in the southern Transverse Ranges were almost uniformly positive and generally episodic. Maximum uplift measured between 1960/61 and 1968/69 was 0.205 m (observed) or 0.200 m (adjusted); it occurred about 30 km northeast of the 1971 epicenter. As much as 0.129 m (observed) of preseismic uplift was also measured about 30 km northwest of the epicenter between 1964 and 1968. A comparison between 1968 and 1969 elevation data revealed 0.078 m (observed) of differential uplift centered about 10 km west of the epicenter; earlier leveling indicates that this uplift began between February 1967 and May 1968. Changes in elevation measured during the interval 1968/69–71 (postearthquake) are interpreted as chiefly coseismic and were concentrated along a 15-km ruptured segment of the east-trending, north-dipping San Fernando fault. These changes were characterized by subsidence of as much as 0.111 m (observed) or 0.092 m (adjusted) south of the fault and a ridge of uplift of up to at least 2.195 m (observed) or 2.196 m (adjusted) immediately north of the fault. The more episodic preseismic movements are interpretable as deep-seated creep events on the San Fernando fault. Alternatively, these episodic movements may be due partly to the operation of dilatancy; the onset of the 1968–1969 epicentral uplift accords closely with the initiation of a V p V s anomaly recognized in this same area. The 1968/69–71 elevation changes can be attributed almost entirely to slip on the San Fernando fault.

The slides in the San Fernando dams during the earthquake of February 9, 1971. 23F, 2T, 15R : Seed, HB et al J. Geotech. Engng Div., ASCE. V101, GT7, July, 1975, P651–688

The slides in the San Fernando dams during the earthquake of February 9, 1971. 23F, 2T, 15R : Seed, HB et al J. Geotech. Engng Div., ASCE. V101, GT7, July, 1975, P651–688

Dynamic Analysis of the Slide in the Lower San Fernando Dam during the Earthquake of February 9, 1971

During the San Fernando, California earthquake of February 9, l97l, a major slide occurred in the upstream slope of the Lower San Fernando Dam. Based on data obtained from tests on undisturbed samples, a detailed dynamic analysis of the seismic stability of the embankment is presented. It is concluded that an analysis procedure incorporating: (1) Consideration of the initial static stresses in the embankment: (2) the use of a dynamic finite element analysis procedure to determine the dynamic stresses induced in individual elements of the embankment by the earthquake; (3) the use of cyclic loading triaxial compression test data to determine the response of the soil elements to the induced stresses; and (4) consideration of progressive failure effects provides a satisfactory basis for assessing the stability of the embankment. This type of analysis indicates the development of a zone of liquefaction along the base of the upstream shell that would be sufficiently extensive near the end of the earthquake shaking to lead to a condition of instability.

The Slides in the San Fernando Dams During the Earthquake of February 9, 1971

The February 9, 1971 earthquake in San Fernando Valley, California, caused slides in the embankments of two old adjacent hydraulic fill dams. At the 140-ft high Lower dam, the entire upstream face slid into the reservoir leaving only 5 ft of freeboard. At the 65-ft high Upper dam, the entire embankment from the waterline to the downstream toe moved downstream 6 ft, leaving a scarp just below the reservoir level on the upstream face. It is concluded that the slide in the Lower dam resulted from the development of an extensive zone of liquefaction near the base of the embankment. Some liquefaction is also believed to have occurred in the Upper dam; however, since a significant body of the sand in the upstream and downstream shells of this dam retained considerable strength, complete failure could not occur and the movements were limited in extent. Conclusions are presented concerning the applicability of analytical procedures for predicting the failure and movements that occurred.

President's page: Concerning the new AGU committees

Earthquakes are among mankind's most feared scourges. In a sense they represent the small price earthlings must pay to enjoy their beneficent environment, for it is the same internal heat energy that has provided us with atmosphere, oceans, and mineral wealth that is the source of the stresses that periodically fracture the crust.Earthquake prediction is a subject as old as soothsaying and astrology; men who could pretend to foretell catastrophes exercised considerable sway in their time. In recent years, research in earthquake prediction has become scientifically respectable. Hundreds of geophysicists and geologists principally in the United States, the USSR, Japan, and the People's Republic of China are engaged in research with earthquake prediction as the direct goal. Most of these investigators believe that the goal is attainable. The Chinese claim that they have saved lives by predicting ten earthquakes in the past four years. News dispatches from China indicate that the destructive earthquake (M = 7.2) of February 4, 1975, in northeast China was predicted by Chinese scientists.

Bear Valley, California, earthquake sequence of February-March 1972

abstract The earthquake sequence of late February and March 1972 involved movement along the San Andreas fault and within the crustal wedge enclosed by the branching San Andreas and San Benito faults near Bear Valley, San Benito County, California. Activity was mainly confined to three distinct zones of strike-slip faulting: the short north-trending aftershock zone of the M 3.5 earthquake of February 22, 1972, the aftershock zone of the M 5.0 Bear Valley earthquake of February 24, 1972 located along the San Andreas fault, and the west-trending aftershock zone of the M 4.6 earthquake of February 27, 1972. The north-trending and west-trending zones lie between the two major splays of the branching fault system. Focal mechanism solutions from events in these zones are consistent with the transfer of horizontal, dextral displacement from the San Andreas fault to the San Benito, Paicines and Calaveras faults within the Bear Valley region. During the 18 months preceding the February 1972 sequence, the hypocentral regions of both the M 5.0 and M 4.6 shocks were characterized by concentrations of small earthquakes. Aftershock source areas of these two events progressively expanded during the course of the aftershock sequence. Estimates of the mainshock rupture surface for these events based on the distribution of aftershocks range over a factor of 4 owing to the irregular distribution of aftershocks and the rapid growth of the aftershock zone.

Time functions appropriate for some aftershocks of the point Mugu, California earthquake of February 21, 1973

abstract Broad-band recordings of aftershocks of the Pt. Mugu earthquake at small epicentral distances provided an excellent opportunity to test source models for small earthquakes. Simple events recorded at nearly vertical incidence produced a single P-wave pulse of a duration of about 0.07 sec and a somewhat more complicated S wave with a slightly longer duration. Such events are consistent with a point dislocation source for which Qβ = 100 or for which there is directivity with the fault breaking downward. We attribute the more usual complexities of small earthquake records to multiple events, some of which we observed, layering effects combined with greater epicentral distances, and scattering.

The Delaware-New Jersey earthquake of February 28, 1973

abstract The largest earthquake to affect northern Delaware in 100 years occurred at 08:21 GMT on February 28, 1973. The event was perceptible over 15,000 km2 in New Jersey, Delaware, Pennsylvania, and Maryland with a maximum Modified Mercalli intensity of V-VI near the Fall Zone between Wilmington and Claymont, Delaware. The isoseismals are elongate in a northeast-southwest direction. The main shock had a magnitude of 3.8 and was located at 39°43.1′N, 75°26.4′W with a depth of 14.1 km as calculated by NOAA. The five aftershocks located in this study were centered within a few kilometers of the region of greatest intensity at about 39°47′N and 75°25′W with depths ranging from 5 to 8.4 km. The fault plane solution determined from the mainshock and aftershocks indicates dip-slip motion on a nearly vertical plane striking N28°E. The southeast side is down in agreement with geological observations of subsidence of the Coastal Plain and uplift of the Piedmont. The strike of this fault is similar to that of the border faults of a graben in the basement rocks about 30 km southwest along the strike from the epicenter suggesting that the seismic activity may be associated with such faults.

A three dimensional dislocation model for the San Fernando, California, earthquake of February 9, 1971 : 10F, 3T, 42R Bull. Seism. Soc. Am. V64, N1, Feb. 1974, P149–172

A three dimensional dislocation model for the San Fernando, California, earthquake of February 9, 1971 : 10F, 3T, 42R Bull. Seism. Soc. Am. V64, N1, Feb. 1974, P149–172

Prediction of peak ground motion from earthquakes

Abstract A statistical analysis shows that the peak horizontal accelerations recorded from the San Fernando earthquake of February 1971 attenuate with focal distance as R−1.39. This attenuation rate is nearly identical to that reported for peak accelerations from underground nuclear explosions. Assuming that the derived attenuation is independent of source parameters and using data from a number of other California earthquakes, the scaling of peak horizontal acceleration with magnitude was determined statistically. Assuming that the attenuation of peak velocity and displacement with distance is identical for earthquakes and underground nuclear explosions, the scaling of earthquake peak velocity and displacement with magnitude was also determined. The equations resulting from these analyses are: a = 6.6 × 10−2 100.40MR−1.39, v = 7.26 × 10−1 100.52M R−1.34 and d = 4.71 × 10−2 100.57MR−1.18, where a, v, and d are maximum acceleration (g), velocity (in centimeters per second) and displacement (in centimeters), respectively, M is the local magnitude, and R is the focal distance (in kilometers). In this analysis, no attempt was made to account for effects of recording site geology.

Premonitory vertical migration of microearthquakes in central California—Evidence of dilatancy biasing?

Mean apparent focal depths of microearthquakes occurring along a 20‐km stretch of the San Andreas fault southeast of Hollister, California, increased by 25 percent some 60 days before the magnitude 4.6 Stone Canyon earthquake of September 4, 1972. The shape of the time‐depth anomaly is virtually identical to the time plots of Vp/Vs preceding moderate earthquakes at Garm, U.S.S.R. A less well‐defined depth anomaly occurred in October‐December 1971, preceding the Limekiln Road swarm in December 1971 and the magnitude 5.0 Melendy Ranch earthquake in February 1972. The observed depth anomalies can be attributed to dilatancy biasing of hypocenters, although true vertical migration of seismicity cannot be ruled out.