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.

Implication of seismicity for failure of a section of the San Andreas Fault

1980 ◽  
Vol 70 (1) ◽  
pp. 185-201
Author(s):  
W. H. Bakun ◽  
R. M. Stewart ◽  
C. G. Bufe ◽  
S. M. Marks
Keyword(s):  
San Andreas Fault ◽  
P Wave ◽  
Seismogenic Zone ◽  
Wave Radiation ◽  
Focal Region ◽  
Fault Creep ◽  
Aftershock Activity ◽  
Discontinuous Surface ◽  
San Andreas ◽  
Fault Trace

abstract On January 15, 1973, a magnitude ML 4.1 earthquake occurred near Cienega Road on the San Andreas Fault about 20 km south of Hollister, California. A 3-km-long segment of the fault southeast of the earthquake was aseismic for the 7 weeks preceding the event, although microearthquakes occurred at both its ends. The first day's aftershocks occurred at the northwest end of the aseismic segment; later aftershock activity migrated to the southeast, filling the remainder of the segment. If the discontinuous surface trace of the fault can be extrapolated to the focal region of the earthquakes to define fault geometry at depth, then aftershocks occurred primarily on one continuous segment of the fault and epicenter locations and direction of rupture propagation (inferred from the azimuthal pattern of P-wave radiation) of the precursory shocks correlate with the discontinuities in the trace that terminate the segment. The 1970 to 1976 deficit in seismic slip within the segment suggests that fault creep accounts for a significant part of cumulative slip within the segment. The pattern of seismicity is consistent with the hypothesis that creep on the segment before the main shock caused a buildup of stress at the ends of the segment or at the ends of adjacent offset segments. Correlation of seismicity and discontinuities or bends in the mapped fault trace are the basis for an extension and refinement of the “stuck” and “creeping” patch model of the San Andreas Fault in central California. Patch boundaries extend from the free surface down through the seismogenic zone. Creeping patches lie beneath smooth continuous segments of the fault trace. Stuck patches lie beneath discontinuities or bends in the fault trace.

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.

scholarly journals Velocity contrast across the San Andreas fault in central California: Small-scale variations from P-wave nodal plane distortion

1977 ◽  
Vol 67 (6) ◽  
pp. 1565-1576
Author(s):  
Karen C. McNally ◽  
Thomas V. McEvilly
Keyword(s):  
Fault Zone ◽  
Fault Plane ◽  
San Andreas Fault ◽  
P Wave ◽  
Wave Radiation ◽  
Small Scale ◽  
Depth Range ◽  
Radiation Patterns ◽  
Central California ◽  
San Andreas

abstract Systematic variations in P-wave radiation patterns, evident in a data set of 400 central California earthquakes, have been analyzed for variations in velocity contrast across the San Andreas fault zone. Vertical strike-slip faulting characterizes the region, with radiation patterns well constrained by the dense local seismographic station network. A discontinuity in crustal velocity occurs across the San Andreas fault. The distribution of systematically inconsistent first motions indicates that first arrivals observed along the fault plane within the northeastern block have followed refracted paths through the higher velocity crustal rocks to the southwest, retaining P-wave polarities characteristic of the quadrant of origin, and thus appearing reversed. A simple geometrical interpretation, with P waves refracted at the fault plane near the focus, yields the velocity contrast across the fault zone; the distribution of hypocenters allows its mapping in time and space. The velocity contrast so determined ranges up to 15 per cent, for a depth range of 1 to 10 km. The observed pattern of contrast values is coherent, with the greatest contrast related apparently in space, and possibly in time, to the larger earthquakes occurring on the fault. We suggest the phenomenon reflects changes in stress state at the fault and, by virtue of its ease of measurement, offers a new and valuable technique in earthquake studies.

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.

scholarly journals Parkfield earthquakes of June 27-29, 1966, Monterey and San Luis Obispo Counties, California—Preliminary report

1966 ◽  
Vol 56 (4) ◽  
pp. 961-971
Author(s):  
Gordon B. Oakeshott ◽  
Clarence R. Allen ◽  
Stewart W. Smith ◽  
L. C. Pakiser ◽  
T. V. McEvilly ◽  
...  
Keyword(s):  
Fault Zone ◽  
Fault Plane ◽  
San Andreas Fault ◽  
Strike Slip ◽  
P Wave ◽  
Surface Zone ◽  
Horizontal Acceleration ◽  
San Luis ◽  
San Andreas ◽  
Fault Trace

abstract Two earthquakes, M = 5.3 and 5.5, shook the Parkfield area in southern Monterey County, California, at 0409:56.5 and 0426:13.8 GMT, 28 June 1966. They were preceded by foreshocks on the same day at 0100 and 0115. A third shock, M = 5.0, occurred in the same area at 1953:26.2 on 29 June. The earthquakes were followed by a heavy sequence of aftershocks with epicenters along the San Andreas fault zone extending for about 15 miles southward beyond Cholame in San Luis Obispo County. A P-wave first-motion fault plane solution shows strike of vertical fault plane is N 33°W, coinciding with a surface zone of en echelon fault fractures in the pattern characteristic of right-lateral, strike-slip movement. The motion appears to have an upward component on the west side, at about 20° from pure strike slip. Extensive instrumentation within a few miles of the epicentral district gave unusually complete records from foreshock to aftershock sequence. A strong-motion instrument in the fault zone near Cholame recorded the unusually high horizontal acceleration of 0.5 g. The epicentral region of the earthquakes is on a known active segment of the San Andreas fault. Earthquakes in 1901, 1922, and 1934 in this region were also accompanied by surface faulting. On the published State geologic map, scale 1:250,000, the San Andreas fault zone shows a braided pattern of several branching en echelon major faults. Topographic forms, typical of the features of rift valleys, testify to the recency of fault movements. Small right-lateral surficial displacements had been recognized prior to the late June earthquakes in at least three places on the Parkfield-Cholame trace of the fault. Similar creep, or slippage, has continued since the earthquakes. Extensive nets of survey markers installed by 30 June across the active fault trace had recorded slippage as great as 0.1 inch per day by 12 July. The fault trace associated with the earthquakes is principally in alluvium of unknown depth in Cholame Valley, apparently a faulted graben within the San Andreas fault zone. Under a blanket of Tertiary and Quaternary sedimentary rocks in this part of the southern Coast Ranges, the great fault separates Jurassic-Cretaceous granitic and metamorphic rocks in the western block from Late Jurassic eugeosynclinal sedimentary and volcanic rocks of the Franciscan Formation in the eastern block. In spite of the large horizontal acceleration recorded near the fault, very little building damage occurred in this sparsely populated region. Small concrete and steel bridges in, and adjacent to the fault trace, did not have their structural strength impaired.

Sign in / Sign up

Export Citation Format

Share Document