A study of the aftershocks and focal mechanism of the Salinas-Watsonville earthquakes of August 31 and September 14, 1963

1965 ◽  
Vol 55 (1) ◽  
pp. 85-106 ◽  
Author(s):  
Agustin Udias
Keyword(s):  
Focal Mechanism ◽  
Seismic Activity ◽  
Main Shock ◽  
San Andreas Fault ◽  
San Andreas ◽  
Principal Axes ◽  
Mobile Stations ◽  
Earthquake Sequences ◽  
Background Seismic Activity

Abstract The earthquake sequences connected with the earthquakes of August 31 and September 14, 1963 in the Salinas-Watsonville region of California are here studied with reference to the background seismic activity. A very favorable distribution of permanent and mobile stations in this area permits the analysis to include earthquakes of small magnitudes. The mechanism of the larger aftershocks of both sequences is found to be similar to the mechanism of the main shock of September 14, 1963. The orientation of the principal axes of stress derived from the focal mechanism of the September 14 earthquake, is related to the strike of the San Andreas fault.

Seismic activity on the southern Calaveras Fault in central California

1980 ◽  
Vol 70 (4) ◽  
pp. 1181-1197
Author(s):  
William H. Bakun
Keyword(s):  
Seismic Moment ◽  
Seismic Activity ◽  
Main Shock ◽  
San Andreas Fault ◽  
San Jose ◽  
Central California ◽  
Seismicity Pattern ◽  
San Andreas ◽  
Fault Trace ◽  
The Right

abstract Cumulative seismic moment ΣM0 for earthquake on 50-km-long creeping section of the Calaveras Fault from near Mount Hamilton southeast to San Felipe Lake correlates with mapped fault-trace characteristics. In general, ΣM0 is lower at the left-stepping offset in the trace at the south end of Anderson Lake and along linear segments of the fault than near right-stepping offsets and bends in the trace and intersections of the Calaveras with other faults. Rupture expansion for the August 6, 1979 Coyote Lake sequence main shock, 10 km NNE of Gilroy, California, was unilateral to the southeast (Archuleta, 1979) away from the right-stepping offset in the fault trace near its epicenter. Rupture expansion for the two felt shocks (ML 4.2 and ML 3.9) on August 29, 1978 located 112 km apart near Halls Valley east of San Jose was unilateral for each away from the other, suggesting the existence of a rupture-expansion blocking discontinuity between them. The correlations of seismic activity and fault-trace characteristics are similar to those for shocks along the creeping section of the San Andreas Fault in central California and suggest that the specific “stuck” and “creeping” patch model of Bakun et al. (1980) developed for the San Andreas is applicable to the creeping Calaveras Fault as well. Cumulative seismic moment (January 1, 1969 to August 6, 1979) within the 16-km-long 1979 Coyote Lake sequence aftershock zone was less than that near the fault-trace discontinuities at its ends. Microearthquakes along the Calaveras Fault near the Coyote Lake aftershock zone increased before the sequence beginning with a cluster on June 22, 1978 near the southeast end of the aftershock zone. A similar seismicity pattern preceded the August 29, 1978 shocks and the ML 4.5 May 8, 1979 shock near Halls Valley.

Control of rupture by fault geometry during the 1966 parkfield earthquake

1981 ◽  
Vol 71 (1) ◽  
pp. 95-116 ◽  
Author(s):  
Allan G. Lindh ◽  
David M. Boore
Keyword(s):  
Main Shock ◽  
Fault Plane ◽  
San Andreas Fault ◽  
Strong Motion ◽  
P Wave ◽  
Fault Geometry ◽  
Dynamic Rupture ◽  
San Andreas ◽  
Hill Station ◽  
Parkfield Earthquake

abstract A reanalysis of the available data for the 1966 Parkfield, California, earthquake (ML=512) suggests that although the ground breakage and aftershocks extended about 40 km along the San Andreas Fault, the initial dynamic rupture was only 20 to 25 km in length. The foreshocks and the point of initiation of the main event locate at a small bend in the mapped trace of the fault. Detailed analysis of the P-wave first motions from these events at the Gold Hill station, 20 km southeast, indicates that the bend in the fault extends to depth and apparently represents a physical discontinuity on the fault plane. Other evidence suggests that this discontinuity plays an important part in the recurrence of similar magnitude 5 to 6 earthquakes at Parkfield. Analysis of the strong-motion records suggests that the rupture stopped at another discontinuity in the fault plane, an en-echelon offset near Gold Hill that lies at the boundary on the San Andreas Fault between the zone of aseismic slip and the locked zone on which the great 1857 earthquake occurred. Foreshocks to the 1857 earthquake occurred in this area (Sieh, 1978), and the epicenter of the main shock may have coincided with the offset zone. If it did, a detailed study of the geological and geophysical character of the region might be rewarding in terms of understanding how and why great earthquakes initiate where they do.

Seismoscope results from three earthquakes in the Hollister, California, Area

1967 ◽  
Vol 57 (6) ◽  
pp. 1445-1448 ◽  
Author(s):  
William K. Cloud
Keyword(s):  
Main Shock ◽  
San Andreas Fault ◽  
Large Angle ◽  
Maximum Amplitude ◽  
Transient Motion ◽  
San Andreas ◽  
Close Proximity

Abstract Seismoscope records from a magnitude 5.0 earthquake on January 19, 1960, and from magnitudes 5.6 and 5.5 earthquakes on April 8, 1961, in the Hollister, California, area, like those from the Parkfield, California, main shock of June 27, 1966, indicate that the maximum amplitude of transient motion in close proximity to the San Andreas Fault was oriented at a large angle to the strike of the fault.

Intensity map and structural damage, Parkfield, California earthquake of June 27, 1966

1967 ◽  
Vol 57 (6) ◽  
pp. 1161-1178 ◽  
Author(s):  
William K. Cloud
Keyword(s):  
Fault Zone ◽  
Structural Damage ◽  
Small Area ◽  
Main Shock ◽  
San Andreas Fault ◽  
Acceleration Pulse ◽  
Isoseismal Map ◽  
San Andreas ◽  
Earthquake Series ◽  
Intensity Map

Abstract The isoseismal map indicates that the main shock of the Parkfield, California earthquake series on June 27, 1966 was felt over an area of approximately 20,000 square miles, and that the maximum Modified Mercalli scale intensity of VII was limited to a small area near the San Andreas fault zone in the vicinity of the Parkfield and Cholame. In this area an acceleration pulse of 0.5g was recorded. However, damage to homes, pipe lines, bridges and other structures was not severe.

Accelerograms—Parkfield earthquake

1967 ◽  
Vol 57 (6) ◽  
pp. 1179-1192 ◽  
Author(s):  
William K. Cloud ◽  
Virgilio Perez
Keyword(s):  
Short Duration ◽  
Main Shock ◽  
San Andreas Fault ◽  
Response Spectra ◽  
Acceleration Pulse ◽  
San Andreas ◽  
High Acceleration ◽  
Earthquake Series ◽  
Parkfield Earthquake ◽  
Electric Analog

Abstract Five accelerograms obtained during the main shock of the Parkfield earthquake series at locations adjacent to and within 10 miles of the trace of the San Andreas fault are presented together with limited analysis by electric analog methods. With one exception records from horizontal accelerometers are characterized by a short duration section of high acceleration. Adjacent to faulting an acceleration pulse of 0.5 gravity was recorded. Judged by a record from a station east of the fault on rock and a record from a station approximately the same distance west of the fault on alluvium, magnification of response spectra by alluvium was less than a factor of 3.

Seismicity of northern California and western Nevada

1964 ◽  
Vol 54 (2) ◽  
pp. 845-850
Author(s):  
Mansour Niazi
Keyword(s):  
Fault Zone ◽  
Frequency Distribution ◽  
Seismic Activity ◽  
San Andreas Fault ◽  
Seismic Energy ◽  
Northern California ◽  
Time Intervals ◽  
San Andreas ◽  
Strain Release ◽  
Surface Creep

abstract Nearly 1500 recent northern California and western Nevada earthquakes of magnitude 2 and above have been used to study aspects of the seismicity of the region. In an area covering 512 degrees of latitude by 8 degrees of longitude, each square degree was subdivided into 16 approximately equal quadrilateral blocks. The seismic energy and associated strain release in each block were computed for time intervals up to five years. The computed representations of strain release for each year are contoured to define the distribution of seismic zones in the area and to give a dynamic measure of thier variation. The observed surface creep along the San Andreas fault at the Vineyard seismographic station is compared with the intensity of the seismic activity in the immediate neighborhood of the fault zone. The frequency distribution of earthquakes of different magnitude is discussed.

The Parkfield, California, earthquakes of 1966

1967 ◽  
Vol 57 (6) ◽  
pp. 1221-1244 ◽  
Author(s):  
T. V. McEvilly ◽  
W. H. Bakun ◽  
K. B. Casaday
Keyword(s):  
Main Shock ◽  
San Andreas Fault ◽  
P Wave ◽  
Wave Radiation ◽  
Aftershock Activity ◽  
Central California ◽  
The North ◽  
San Andreas ◽  
High Incidence ◽  
Fault Trace

Abstract The characteristics of the Parkfield, California earthquake sequence of 1966 are presented. Historically, the epicentral region is one of the three most seismic areas along the San Andreas fault in central California. It is characterized, however, by a relatively high incidence of large earthquakes in proportion to smaller shocks, compared to other active zones. The 1966 sequence occurred in an area where measured deformation across the fault for 1959-1965 shows a decrease from about 2 cm/year to the north to zero to the south of the area. Neither micro-earthquake nor normal seismic activity prior to the sequence gave indication of its coming. Seismicity before the sequence was confined to the north of the active zone, with some indication of convergence of foci toward the location of the initial shocks. The early aftershock distribution extended 20 km south of the main shock; cracking occurred to 33 km south of the main shock; and intense aftershock activity for the entire sequence extended 27 km south of the main shock. At least 95 per cent of the earthquakes, including the three largest, have P-wave radiation patterns consistent with right lateral transcurrent motion on the San Andreas fault. Earthquakes of the sequence fall very closely along the fault trace. About 75 per cent of the total strain release for the sequence can be accounted for by earthquakes in the main shock region, the principal shock (M = 5.5) contributing only 25 per cent of the total. The sequence is characterized by a high incidence of large aftershocks, an extensive area of aftershock activity, and average focal depths near 5 km-three properties apparently related, and distinguishing two types of sequence traits in central California.

The San Andreas Fault

1993 ◽  
Author(s):  
Sandra S. Schulz ◽  
Robert E. Wallace
Keyword(s):  
San Andreas Fault ◽  
San Andreas
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