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.

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.

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 moments, local magnitudes, and coda-duration magnitudes for earthquakes in central California

1984 ◽  
Vol 74 (2) ◽  
pp. 439-458 ◽  
Author(s):  
William H. Bakun
Keyword(s):  
Seismic Moment ◽  
San Andreas Fault ◽  
Seismic Network ◽  
San Juan ◽  
Local Magnitude ◽  
Central California ◽  
San Andreas ◽  
Duration Magnitude ◽  
San Juan Bautista ◽  
The U.S

Abstract Onscale seismograms recorded at stations in the U.S. Geological Survey's (USGS) central California seismic network (CALNET) have been used to estimate the seismic moment M0 and local magnitude ML for earthquakes of 1 ≦ ML ≦ 4 located on the San Juan Bautista and Parkfield sections of the San Andreas fault, the Coyote Lake section of the Calaveras fault, the Sargent fault, and near Livermore. These data, together with M0 and ML estimates for 4 ≦ ML ≦ 6 earthquakes in these areas, cannot be fit with a single linear log M0-versus-ML relation. Rather, the data are consistent with log M0 = 1.5 ML + 16 for 3 ≲ ML ≲ 6, with log M0 = 1.2 ML + 17 for 1 1/2 ≲ ML ≲ 3 1/2 and with a slope of ⅔ to 1 fro 1/2 ≲ ML ≲ 1 1/2. Whereas USGS coda duration magnitude MD is consistent with ML for 1 1/2 ≲ ML ≲ 3¼, MD is larger than ML at ML ≲ 1 1/2 and smaller than ML at ML ≳ 3¼. Log M0 can be estimated to a precision of 0.2 for 1 ≦ MD ≦ 3 1/2 earthquakes in central California by applying log M0 = 1.2 MD + 17 to the MD that have been routinely published by the USGS.

Central California earthquakes of the 1830's

1947 ◽  
Vol 37 (1) ◽  
pp. 33-74
Author(s):  
George D. Louderback
Keyword(s):  
San Francisco ◽  
Strong Shock ◽  
Fault Rupture ◽  
San Jose ◽  
Central California ◽  
San Andreas ◽  
The People ◽  
Great Fear ◽  
Fault Trace ◽  
North And South

Summary The information and inferences therefrom concerning central California earthquakes reported as occurring in the 1830's, according to the conclusions reached in the foregoing discussion, may be summarized as follows. 1836—April 25, 5 a.m. A “strong” shock. Monterey. This may possibly be a foreshock of the June 10 earthquake. If so, it must have been felt at points around the Bay of San Francisco, but no reports are available. 1836—June 9, about 4 p.m. Monterey. Described as strong and short by Gomez; strong, and of about a half minute's duration, by Vallejo. It is probably a foreshock of the earthquake of the next morning, but no reports are available from other localities. 1836—June 10, 7:30 a.m. X. East of San Francisco Bay. An earthquake of great intensity comparable with and possibly stronger than that of October 21, 1868. Large fissures were formed and “innumerable” aftershocks occurred, decreasing in violence, but “continuous” for a month. The account stresses the effects along the foothill belt from San Pablo to Mission San Jose, which indicates an origin in the Haywards fault. The fissures were probably, at least in part, fault-trace phenomena. Reported by Gomez in Monterey as of more violence and duration than the shock of June 9. Described by Vallejo as strong, lasting close to a minute in Monterey, causing havoc in Monterey and Santa Clara, and arousing great fear among the people. Intensity apparently at least VII at Monterey and Mission Carmel. 1838—Late in June, just after noon. X. Comparable with the earthquake of April 18, 1906. Originated in the San Andreas fault, and violent fault-trace phenomena described by Charles Brown as observed in the hills behind Palo Alto near the present Searsville Lake: a great fissure which he describes as ten to twelve feet wide and running from near San Francisco to the latitude of of Santa Clara; the ground cracked in all directions; thousands of trees broken off; water thrown from creek bed; adobe walls cracked. Violent at Yerba Buena (San Francisco), walls cracked at Presidio, a sand hill bodily shifted (according to Spear); house shaken down at San Jose (town), and walls badly injured at the San Francisco, Santa Clara, and San Jose missions (Captain Paty); crockery and glassware broken, walls of adobe buildings cracked, inhabitants “frightened out of their wits” in Monterey (Major Warren). Captain Paty reported aftershocks as “frequent since” (his ship left the coast September 19, 1838). The fault rupture may have occurred throughout all or most of the line active in 1906, but north and south beyond the limits indicated by Brown it lay under water or in wild country uninhabited by whites (except at Fort Ross, from which we have no report). The evidence of greater intensity at Monterey than in 1906 may mean that the fault rupture extended farther south in 1838 than in 1906.

The earthquake sequence of November 1964 near Corralitos, California

1966 ◽  
Vol 56 (3) ◽  
pp. 755-773 ◽  
Author(s):  
Thomas V. McEvilly
Keyword(s):  
Fault Plane ◽  
San Andreas Fault ◽  
Focal Region ◽  
Fault Plane Solutions ◽  
Central California ◽  
San Andreas ◽  
Motion Characteristic ◽  
First Motion ◽  
The Times ◽  
The Right

abstract A sequence of more than 100 aftershocks with magnitudes as low as −0.1 was recorded following a magnitude 5.0 earthquake on November 16, 1964, in the San Andreas fault zone of central California. The sequence was monitored in detail by three temporary seismographic stations at distances less than 15 km and the surrounding telemetry array. Nearly all of the 35 earthquakes which could be located clustered in a focal region about 4 km in diameter at a depth near 12 km and exhibited uniform first motion radiation patterns. First motion fault plane solutions are consistent with the right lateral transcurrent motion characteristic of the San Andreas fault. Exceptions to this uniform radiation pattern in the concentrated focal region occurred near the times of two large aftershocks apparently on another fault about 5 km away.

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