Shear-wave attenuation along the San Andreas fault zone in central California

1975 ◽  
Vol 65 (2) ◽  
pp. 439-459 ◽  
Author(s):  
William H. Bakun ◽  
Charles G. Bufe
Keyword(s):  
Shear Wave ◽  
Wave Attenuation ◽  
San Andreas Fault ◽  
Source Parameters ◽  
Earthquake Source ◽  
Propagation Path ◽  
Corner Frequency ◽  
Source Control ◽  
Central California ◽  
San Andreas

abstract SH ground-displacement spectra (1 to 12 Hz) for 16 local earthquakes (Δ ≦ 18 km, 1.1 ≦ M ≦ 4.6) recorded at a common site situated atop the active trace of the San Andreas fault are used to estimate attenuation characteristics for propagation paths along the fault trace. t* = 0.10−0.13 (corresponding to an equivalent total path Qβ = 75−100) is appropriate for events with focal depths of ∼ 10 km. Propagation-path effects, and not processes at the earthquake source, control corner frequencies for small (M ≲ 3) earthquakes for these highly attenuating paths. The results obtained here suggest that as a rule of thumb, if the true equivalent total path Q is as low as 4·ƒc·t, where ƒc is the estimated corner frequency and t the travel time, the corner frequency estimate is determined by propagation-path effects, not by processes at the earthquake source. In these cases, reliable estimates of source parameters can only be obtained if the appropriate propagation-path corrections are known. Using Brune's model of shear-wave spectra, source dimensions L = 2r of less than 250 meters and stress drops greater than about 110 bar are estimated for the smaller events (1.1 ≦ M ≦ 2.2), using the equivalent total path Qβ obtained here. The seismic moments obtained in this study, together with data for larger central California events (2.4 ≦ ML ≦ 5.1) obtained by Johnson and McEvilly (1974), imply a linear log seismic moment-magnitude relation for 1 < ML < 5 log ⁡ 10 ( M 0 ) = ( 16.2 ± 0.1 ) + ( 1.52 ± 0.05 ) M L .

Near-field observations and source parameters of central California earthquakes

1974 ◽  
Vol 64 (6) ◽  
pp. 1855-1886 ◽  
Author(s):  
Lane R. Johnson ◽  
Thomas V. McEvilly
Keyword(s):  
San Andreas Fault ◽  
Strong Dependence ◽  
Source Parameters ◽  
Broad Band ◽  
Low Frequency ◽  
Corner Frequency ◽  
Fault Plane Solutions ◽  
Low Frequencies ◽  
Central California ◽  
San Andreas

abstract This is a study of source characteristics of 13 earthquakes with magnitudes between 2.4 and 5.1 located near the San Andreas fault in central California. On the basis of hypocentral locations and fault-plane solutions the earthquakes separate into two source groups, one group clearly related to the throughgoing northwest-trending San Andreas fault zone and the other apparently associated with generally north-trending bifurcations such as the Calaveras fault. The basic data consist of broad-band recordings (0.03 to 10 Hz) of these earthquakes at two sites of the San Andreas Geophysical Observatory (SAGO). Epicentral distances range between 2 and 40 km, and maximum ground displacements from 4 to 4000 microns were recorded. The whole-record spectra computed from the seismograms lend themselves to source parameter studies in that they can be interpreted in terms of low-frequency level, corner frequency, and high-frequency slope. Synthetic seismograms have also been used to estimate source parameters in both the time domain and frequency domain, and the results compare favorably with those estimated directly from the spectra. The influences of tilts and nonlinear response of the seismometer were considered in the interpretation of the low frequencies. Seismic source moments estimated from the low-frequency levels of the spectra show a linear dependence on magnitude with a slope slightly greater than 1. The geology at the recording site can contribute an uncertainty factor of at least 3 to the estimated moments. Observed corner frequencies are only weakly dependent on magnitude. Interpreted in terms of source dimension, these corner frequencies imply values of 1 to 2 km for the earthquakes of this study. The corner frequencies may also be interpreted in terms of the rise time source function, yielding values in the range 0.5 to 1.0 sec. The data indicate that the earthquakes of this study are all surprisingly similar in their fundamental source parameters, with only the seismic moment showing a strong dependence on magnitude.

Source parameters of moderate size earthquakes and the importance of receiver crustal structure in interpreting observations of local earthquakes

1977 ◽  
Vol 67 (2) ◽  
pp. 301-313
Author(s):  
Donald V. Helmberger ◽  
Lane R. Johnson
Keyword(s):  
Crustal Structure ◽  
San Andreas Fault ◽  
Source Parameters ◽  
Accurate Estimate ◽  
Strike Slip ◽  
Wave Shape ◽  
Earthquake Excitation ◽  
Central California ◽  
San Andreas ◽  
Crustal Model

Abstract Broadband observations of three central California earthquakes as recorded on opposite sides of the San Andreas fault zone are studied. The earthquake mechanisms are of the strike-slip type occurring along the fault at epicentral distances between 15 and 30 km. The seismograms obtained at the two sites are distinctly dissimilar in both amplitude and wave shape even though they are at roughly the same azimuth. We suppose that the earthquake excitation is identical for the two sites and that the differences in seismograms are caused by the receiver structure. The problem is idealized by assuming that the first 10 sec of each record can be modeled synthetically with a point shear dislocation embedded in a half-space with a two-layer upper-crustal model appropriate for each site. The results determined by matching the observations indicate that the durations for these events with ML = 4 to 5 are about 0.3 to 0.6 sec. Furthermore, the results demonstrate that accurate estimate of source parameters can only be accomplished after a detailed appreciation of crustal structure.

Creep on the San Andreas fault

1960 ◽  
Vol 50 (3) ◽  
pp. 389-415
Author(s):  
Karl V. Steinbrugge ◽  
Edwin G. Zacher ◽  
Don Tocher ◽  
C. A. Whitten ◽  
C. N. Claire
Keyword(s):  
Creep Rate ◽  
San Andreas Fault ◽  
Total Duration ◽  
Fault Line ◽  
Fault Movement ◽  
Central California ◽  
San Andreas ◽  
Different Types ◽  
Present Rate ◽  
Local Earthquake

ABSTRACT Progressive destruction of buildings and other works of man at the W. A. Taylor Winery near Hollister, California, indicates that one side of a segment of the San Andreas fault is creeping relative to the other. Three different types of measurements all yield an annual rate of creep of approximately one-half inch per year. Steinbrugge and Zacher: Measurements of the separations of pairs of reference marks adjacent to the line of creep have been repeated periodically since 1956, and damage to structures provides a good measure of the total creep since 1948. Reports of damage to older buildings on the same site suggest that the creep may have been going on at about the present rate for fifty or more years. Tocher: Creep recorders designed to measure continuously the differential lateral movement of adjacent sections of the concrete floor have been installed in the main winery building. The creep rate so determined has been about one-half inch per year (with right-lateral sense) for the past two years. Creep accumulates largely in spasms of rather short duration (on the order of a week) separated by intervals of weeks or months during which little or no creep takes place. Ninety-two per cent of the movement in a recent 371-day period accumulated in four spasms of total duration 34 days. Three of these spasms began at times when no local earthquakes were recorded on near-by seismographs; the fourth began with a sudden right-lateral fault movement of 3 mm. at the time of a sharp local earthquake (Richter magnitude 5.0) on January 20, 1960 (GCT). Whitten and Claire: Resurveys over monumented points established near the winery also yield a rate of slippage or creep along the fault line of one-half inch per year. A new method for analyzing the data obtained by retriangulating over monumented points at wide intervals (10 to 20 years) is presented and applied to two triangulation networks which cross the San Andreas fault in central California. Results from a net near Hollister show an average creep rate of about one-half inch per year; results from a net near Cholame (about 75 miles southeast of Hollister) show an average creep rate of about one-tenth inch per year. The results also give an angular value which represents the deformation in the crust adjacent to the fault line.

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