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.

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.

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.

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.

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.

Microearthquake seismicity and tectonics of Coastal Northern California

1970 ◽  
Vol 60 (5) ◽  
pp. 1669-1699 ◽  
Author(s):  
Leonardo Seeber ◽  
Muawia Barazangi ◽  
Ali Nowroozi
Keyword(s):  
High Frequency ◽  
Fault Plane ◽  
San Andreas Fault ◽  
High Gain ◽  
Strike Slip ◽  
Fault System ◽  
Northern California ◽  
Fault Plane Solutions ◽  
Sharp Bend ◽  
San Andreas

Abstract This paper demonstrates that high-gain, high-frequency portable seismographs operated for short intervals can provide unique data on the details of the current tectonic activity in a very small area. Five high-frequency, high-gain seismographs were operated at 25 sites along the coast of northern California during the summer of 1968. Eighty per cent of 160 microearthquakes located in the Cape Mendocino area occurred at depths between 15 and 35 km in a well-defined, horizontal seismic layer. These depths are significantly greater than those reported for other areas along the San Andreas fault system in California. Many of the earthquakes of the Cape Mendocino area occurred in sequences that have approximately the same magnitude versus length of faulting characteristics as other California earthquakes. Consistent first-motion directions are recorded from microearthquakes located within suitably chosen subdivisions of the active area. Composite fault plane solutions indicate that right-lateral movement prevails on strike-slip faults that radiate from Cape Mendocino northwest toward the Gorda basin. This is evidence that the Gorda basin is undergoing internal deformation. Inland, east of Cape Mendocino, a significant component of thrust faulting prevails for all the composite fault plane solutions. Thrusting is predominant in the fault plane solution of the June 26 1968 earthquake located along the Gorda escarpement. In general, the pattern of slip is consistent with a north-south crustal shortening. The Gorda escarpment, the Mattole River Valley, and the 1906 fault break northwest of Shelter Cove define a sharp bend that forms a possible connection between the Mendocino escarpment and the San Andreas fault. The distribution of hypocenters, relative travel times of P waves, and focal mechanisms strongly indicate that the above three features are surface expressions of an important structural boundary. The sharp bend in this boundary, which is concave toward the southwest, would tend to lock the dextral slip along the San Andreas fault and thus cause the regional north-south compression observed at Cape Mendocino. The above conclusions support the hypothesis that dextral strike-slip motion along the San Andreas fault is currently being taken up by slip along the Mendocino escarpment as well as by slip along northwest trending faults in the Gorda basin.

Elastic displacements in the near-field of a propagating fault

1969 ◽  
Vol 59 (2) ◽  
pp. 865-908
Author(s):  
N. A. Haskell
Keyword(s):  
Fault Plane ◽  
Near Field ◽  
Strong Motion ◽  
Dislocation Motion ◽  
Displacement Discontinuity ◽  
Wave Form ◽  
Strong Motion Station ◽  
Ramp Function ◽  
Wave Forms ◽  
Parkfield Earthquake

abstract Displacement, particle velocity, and acceleration wave forms in the near field of a propagating fault have been computed by numerical integration of the Green's function integrals for an infinite medium. The displacement discontinuity (dislocation) on the fault plane is assumed to have the form of a unilaterally propagating finite ramp function in time. The calculated wave forms in the vicinity of the fault plane are quite similar to those observed at the strong motion station nearest the fault plane at the Parkfield earthquake. The comparison suggests that the propagating ramp time function is roughly representative of the main features of the dislocation motion on the fault plane, but that the actual motion has somewhat more high frequency complexity. Calculated amplitudes indicate that the average final dislocation on the fault at the Parkfield earthquake was more than an order of magnitude greater than the offsets observed on the visible surface trace. Computer generated wave form plots are presented for a variety of locations with respect to the fault plane and for two different assumptions on the relation between fault length and ramp function duration.

Aftershocks of the February 21, 1973 Point Mugu, California earthquake

1976 ◽  
Vol 66 (6) ◽  
pp. 1931-1952
Author(s):  
Donald J. Stierman ◽  
William L. Ellsworth
Keyword(s):  
Fault Zone ◽  
Main Shock ◽  
Fault Plane ◽  
Body Wave ◽  
P Wave ◽  
Fault System ◽  
Wave Radiation ◽  
Fault Plane Solutions ◽  
Complex Deformation ◽  
Transverse Ranges

abstract The ML 6.0 Point Mugu, California earthquake of February 21, 1973 and its aftershocks occurred within the complex fault system that bounds the southern front of the Transverse Ranges province of southern California. P-wave fault plane solutions for 51 events include reverse, strike slip and normal faulting mechanisms, indicating complex deformation within the 10-km broad fault zone. Hypocenters of 141 aftershocks fail to delineate any single fault plane clearly associated with the main shock rupture. Most aftershocks cluster in a region 5 km in diameter centered 5 km from the main shock hypocenter and well beyond the extent of fault rupture estimated from analysis of body-wave radiation. Strain release within the imbricate fault zone was controlled by slip on preexisting planes of weakness under the influence of a NE-SW compressive stress.

Crustal structure southwest of the San Andreas fault from quarry blasts

1964 ◽  
Vol 54 (1) ◽  
pp. 67-77
Author(s):  
Robert M. Hamilton ◽  
Alan Ryall ◽  
Eduard Berg
Keyword(s):  
San Francisco ◽  
Crustal Structure ◽  
San Andreas Fault ◽  
P Wave ◽  
Geological Survey ◽  
San Andreas ◽  
Crustal Model ◽  
Crystalline Crust ◽  
The U.S

abstract To determine a crustal model for the southwest side of the San Andreas fault, six large quarry blasts near Salinas, California, were recorded at 27 seismographic stations in the region around Salinas, and along a line northwest of the quarry toward San Francisco. Data from these explosions are compared with results of explosion-seismic studies carried out by the U.S. Geological Survey on a profile along the coast of California from San Francisco to Camp Roberts. The velocity of Pg, the P wave refracted through the crystalline crust, in the Salinas region is 6.2 km/sec and the velocity of Pn is about 8.0 km/sec. Velocities of the direct P wave in near-sur-face rocks vary from one place to another, and appear to correlate well with gross geologic features. The thickness of the crust in the region southwest of the San Andreas fault from Salinas to San Francisco is about 22 kilometers.

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.

scholarly journals Displacement on the San Andreas fault subsequent to the 1966 Parkfield earthquake

1968 ◽  
Vol 58 (6) ◽  
pp. 1955-1973
Author(s):  
Stewart W. Smith ◽  
Max Wyss
Keyword(s):  
Shear Stress ◽  
Ground Motion ◽  
San Andreas Fault ◽  
High Rate ◽  
Aftershock Activity ◽  
Fault Surface ◽  
San Andreas ◽  
Parkfield Earthquake ◽  
Fault Displacement ◽  
Displacement Measurements

ABSTRACT Immediately following the 1966 Parkfield earthquake a continuing program of fault displacement measurements was undertaken, and several types of instruments were installed in the fault zone to monitor ground motion. In the year subsequent to the earthquake a maximum of at least 20 cm of displacement occurred on a 30 km section of the San Andreas fault, which far exceeded the surficial displacement at the time of the earthquake. The rate of displacement decreased logarithmically during this period in a manner similar to that of the decrease in aftershock activity. After the initial high rate of activity it could be seen that most of the displacement was occurring in 4–6 day epochs of rapid creep following local aftershocks. The variation of fault displacement along the surface trace was measured and shown to be consistent with a vertidal fault surface 44 km long and 14 km deep, along which a shear stress of 2.4 bars was relieved.

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