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.

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.

P-wave spectra for ML 5 foreshocks, aftershocks, and isolated earthquakes near Parkfield, California

1981 ◽  
Vol 71 (2) ◽  
pp. 423-436
Author(s):  
Willian H. Bakun ◽  
Thomas V. McEvilly
Keyword(s):  
Stress Drop ◽  
High Stress ◽  
P Wave ◽  
Wave Radiation ◽  
Focal Region ◽  
Wave Spectra ◽  
Rupture Length ◽  
Main Shocks ◽  
Relative Stress ◽  
Fault Trace

abstract Wood-Anderson seismograms recorded at Mount Hamilton (MHC, 185 km, 327°), Santa Barbara (SBC, 180 km, 158°), and Tinemaha (TIN, 240 km, 56°) provide data for comparing P-wave spectra for two immediate (17-min) foreshocks, one early (55-hr) foreshock, two aftershocks, and two “isolated” Parkfield earthquakes. All are ML 5.0 shocks with epicenters within 7 km of the common epicenter of the 1934 and 1966 Parkfield main shocks. The set of events is well suited for testing the hypothesis that foreshocks are high-stress-drop sources. Calculated stress drops are controlled by source directivity at azimuths aligned with the fault break (at MHC and SBC). P-wave radiation from the three foreshocks is focused along one fault trace azimuth, suggesting that foreshock sources are characterized by pronounced unilateral rupture expansion. At TIN, broadside to the fault where directivity has minimum effect on calculated relative stress drop, the two immediate foreshocks are higher stress-drop sources. The early foreshock is a low-to-average stress-drop source, indicating the possibility that stress concentration is a rapidly occurring phenomenon in rupture nucleation. Alternatively, the stress field is highly variable on the scale of 2 to 3 km in the focal region of an impending earthquake with a rupture length of 20 to 30 km.

Active displacement on the Calaveras fault zone at Hollister, California

1971 ◽  
Vol 61 (2) ◽  
pp. 399-416
Author(s):  
Thomas H. Rogers ◽  
Robert D. Nason
Keyword(s):  
San Francisco ◽  
Fault Zone ◽  
Active Fault ◽  
San Andreas Fault ◽  
Fault System ◽  
Fault Creep ◽  
San Andreas ◽  
San Andreas Fault System ◽  
Fault Trace ◽  
Fault Displacement

abstract The Calaveras fault zone, which is a major branch of the San Andreas fault system in northern California, passes through the City of Hollister 160 km (100 miles) southeast of San Francisco. Active fault displacement (fault creep slippage) has occurred in and near Hollister along a fault trace within the Calaveras fault zone. Various man-made structures crossing the fault trace have been deformed and gradually offset in a right-lateral sense. The amount of offset varies directly with age of the structure. The maximum offset is 33 cm (13 in) of a sidewalk constructed in the period 1909 to 1914. Offsets on dated structures indicate displacement rates of approximately 2 mm/yr (0.08 in/yr) from 1909 to 1925 and 6 mm/yr (0.24 in/yr) from 1925 to 1967. Data obtained from periodic measurement of specially designed survey lines and instruments have indicated a displacement rate of 9 mm/yr (0.4 in/yr) since May 1967. Displacements of the survey lines are not associated with local earthquake events. Rates of active fault displacement vary with time and position along the Calaveras and San Andreas fault zones in the Hollister area. The pattern of this variation suggests that active displacement on the San Andreas fault zone may be transferring northeastward to the Calaveras fault zone.

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.

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.

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.

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.

Observations of creep-related tilt, strain, and water-level changes on the central San Andreas fault

1977 ◽  
Vol 67 (3) ◽  
pp. 641-649 ◽  
Author(s):  
C. E. Mortensen ◽  
R. C. Lee ◽  
R. O. Burford
Keyword(s):  
Water Level ◽  
San Andreas Fault ◽  
Slip Surface ◽  
Lower Boundary ◽  
Water Level Fluctuations ◽  
Fault Creep ◽  
San Andreas ◽  
Strain Data ◽  
Data Source ◽  
Surface Creep

abstract Several simultaneous observations of surface fault creep, tilt, strain, and water-level fluctuations have been recorded along the San Andreas fault in the vicinity of the Almaden-Cienega Winery south of Hollister, California. Creep events recorded on the winery creepmeters on February 16, 1975, and by the winery and Harris Ranch creepmeters on September 17, 1975, were modeled as migrating dislocations with geometries chosen to give results that match the observed tilt and strain data. Source depths for the February 16th and September 17th creep events were found to be relatively shallow, the depth to the lower boundary of the slip surface being 0.4 and 2.0 km, respectively. In both cases slip was found to propagate from the northwest toward the southeast, which is consistent with changes in water level observed in a well near the winery. Since the installation of the tiltmeter and strainmeter 0.8 km northwest of the Cienega Winery, six tilt and strain signals with durations typical of creep events have been related to observed surface creep, while 11 such signals appear unrelated to recorded surface creep. The latter may result from surface creep of limited extent or creep at depth.

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