Strong ground motion of the San Fernando, California, earthquake: Ground displacements

Two hundred and thirty-four components of ground displacement are the basis of an investigation of long-period strong ground motion in southern California arising from the San Fernando, California, earthquake. The displacement data are obtained from the double integration of strong-motion accelerograms via the base-line adjustment and filtering operations routinely performed in the series “Strong Motion Earthquake Accelerograms”. These procedures can recover long-period data from strong-motion accelerograms with considerable accuracy. Many-station comparisons of displacement data for which the station spacing is small compared to the wavelengths of interest reveal that uncertainties in displacement are less than 1 cm in the period range 5 to 8 sec, 1 to 2 cm at periods near 10 sec, and 2 to 4 cm in the period range 10 to 15 sec, for a data sensitivity of approximately 7.6 cm/g. For limited variations in epicentral distance (R) and source-station azimuth (ϕ), ground displacements show a strong coherence; for wider variations in R and ϕ, many of the observed variations in the displacement wave forms are easily attributable to well-understood seismological phenomena. Seismic moment, source dimension, radiation pattern, rupture propagation, the development of surface waves and their subsequent dispersion, and azimuthal variations in the gross geological structure all appear to have first-order significance in fashioning the gross amplitude and frequency content of the displacement wave forms and in explaining observed variations with R and ϕ. The essential simplicity of these displacement wave forms offers considerable optimism that long-period strong ground motion can be realistically synthesized with advance knowledge of the earthquake source parameters and gross geological structure

Source Parameters of the Borrego Mountain Earthquake

Spectral analysis of teleseismic body phases at several azimuths was used to determine the moment, fault length, dislocation, stress-drop, and radiated energy of the Borrego Mountain earthquake. The results agree well with the same parameters obtained from the surface fracture and aftershock distribution and local observations of radiated energy

The use of body-wave spectra in the determination of seismic-source parameters

Teleseismic determinations of body-wave (P, S) spectra, interpreted in terms of the Brune (1970) seismic-source model, are used to estimate the parameters seismic moment (M_o) and source dimension (r) for three large, shallow, strike-slip earthquakes occurring on nearly vertical fault planes and for which the same parameters can be determined from field (F) data. These earthquakes are (1) the Borrego Mountain, California, earthquake (April 9, 1968) for which [M̅_o(P) = 10, M̅_o(S) = 6.6, and M_o(F) = 3.6] × 10^(25) dyne-cm and [r̅(p) = 14, r̅(S) = 23, and L/2(F) = 17] km; (2) the Mudurnu Valley, Turkey, earthquake (July 22, 1967) for which [M̅_o(P) = 9.1, M̅_o(S) = 8.5, and M_o(F) = 7.4] × 10^(26) dyne-cm, and [r̅(P) = 39, r̅(S) = 48, and L/2(F) = 40] km; and (3) the Dasht-e-Bayāz, Iran, earthquake (August 31, 1968) for which [M̅_o(P) = 4.8, M̅_o(S) = 8.6, and M_o(F) = 18] × 10^(26) dyne-cm, and [r̅(P) = S1, r̅(S) = 48, and L/2(F) = 40] km. The Brune (1970) model is well-calibrated with respect to the determination of these parameters for the earthquakes considered. A minimum estimate for the radiated energy can be expressed in terms of M_o and r; this estimate is low by a factor of 10 with respect to the estimate obtained from energy-magnitude relations for these three earthquakes. The stress drops of these events are of the order of 10 bars

The source parameters of the San Fernando earthquake inferred from teleseismic body waves

The accuracy of teleseismic estimates of moment, fault area, dislocation and stress drop was tested for the case of a thrust fault: the San Fernando, California, earthquake of February 9, 1971. On the basis of P-wave spectra of 25 stations and S-wave spectra of 9 stations, the respective values were found to be 0.7 · 10^(26) dyne-cm, 570 km^2, 45 cm, and 14 bars. They agree well with the same parameters obtained from field observations. It is concluded that Brune's (1970) seismic source model is valid for the area determination of thrust earthquakes. The energy radiated in the form of S wave is estimated to be 5 · 10^(21) ergs

A Moment Magnitude Scale

The nearly coincident forms of the relations between seismic moment M_0 and the magnitudes M_L, M_S, and M_w imply a moment magnitude scale M = ⅔ log M_0 - 10.7 which is uniformly valid for 3 ≲ M_L ≲ 7, 5 ≲ M_s ≲ 7½, and M_w ≳ 7½

The Elmore Ranch and Superstition Hills earthquakes of 24 November 1987: Introduction to the special issue

On 24 November 1987, two significant earthquakes occurred along the southern San Jacinto fault zone and related structural elements in southern California, not far from the International Border. These two events, the Elmore Ranch earthquake (M = 6.2 at 0154 GMT) and the Superstition Hills earthquake (M = 6.6 at 1315 GMT, both moment magnitudes from Sipkin, 1989), and their aftershocks have yielded a rich harvest of geological, seismological, and engineering data pertinent to the cause and effect of earthquakes in this region, where the southern San Jacinto fault zone enters the Salton Depression from the Peninsula Ranges bordering it on the southwest (Fig. 1). This special issue of the Bulletin presents 18 geologic and seismologic investigations of these earthquakes, a collection of papers born in El Centro, California, on 8 and 9 February 1988 at a meeting attended by approximately 60 scientists interested in these earthquakes for one reason or another

Seismological Studies of the San Fernando Earthquake and Their Tectonic Implications

Improved hypocentral locations have been obtained for the San Fernando earthquake and its larger aftershocks through the use of data from portable stations installed in and around the aftershock area subsequent to the main shock. The main shock, at 14 00 41.8 GMT on 9 February 1971, is now assigned a magnitude (M_L) of 6.4 and a location at 34° 24.7' N, 118° 24.0' W, h = 8.4 km. Fifty-five aftershocks of magnitude 4.0 and greater had occurred through 31 December 1971. The lunate-shaped epicentral distribution of aftershocks is consistent with the idea of southward thrusting along a disc-shaped fault surface, and aftershock depths as well as aftershock focal mechanisms suggest that the thrust surface dips about 35° toward N 20° E. However, a distinct linear alignment of left-lateral strike-slip aftershocks parallel to the motion direction near the west boundary of activity suggests that the fault surface has a steep flexure along this line, down-stepped to the west, and both the planar distribution of aftershocks and the local geology support this concept

An array of strong-motion accelerographs in Bear Valley, California

Fifteen strong-motion accelerographs, each with the capability of writing the WWVB absolute time code on the recorded accelerogram, have been deployed in an elliptical array, at a station spacing of several kilometers, along the San Andreas Fault in the Bear Valley region of central California. Ten accelerograms were obtained for the June 22, 1973, earthquake (M = 3.9), located near the center of the array. Preliminary analyses of these accelerograms support previous suggestions that the crystalline rocks of the Gabilan Range possess higher material velocities and lower intrinsic absorption than do the Cretaceous and Cenozoic sedimentary rocks northeast of the fault zone. These accelerograms clearly indicate that a strong-motion accelerograph array of this sort can provide the basic data for source mechanism, wave propagation, and local ground-motion studies for earthquakes with magnitudes as small as 3.5-4.0

Fault mechanics

[no abstract

Twelfth award of the Medal of the Seismological Society of America

The medal of the Seismological Society of America was established as Article XII of the Constitution and Bylaws in the 1975 annual election. The Medal recognizes outstanding contributions in Seismology and Earthquake Engineering. The twelfth award, in 1989, was made to Dr. Robert E. Wallace