Microearthquakes and the nature of the creeping-to-locked transition of the San Andreas fault zone near San Juan Bautista, California

1984 ◽  
Vol 74 (1) ◽  
pp. 235-254
Author(s):  
William H. Bakun ◽  
Marcia McLaren
Keyword(s):  
Fault Zone ◽  
Stress Drop ◽  
Seismic Moment ◽  
Dynamic Stress ◽  
San Andreas Fault ◽  
Static Stress ◽  
P Wave ◽  
Apparent Stress ◽  
San Andreas ◽  
Static Stress Drop

Abstract Eighteen digital event recorders were deployed during May-June 1981 along the creeping-to-locked transition of the San Andreas fault zone near San Juan Bautista, California, as a supplement to the U.S. Geological Survey's central California seismic network. Analysis of 18 well-recorded microearthquakes (0.7 ≦ ML ≦ 2.8) located along the transition confirms the complexity of the crust and fault-zone structure of the transition region. Seismic-wave site amplification is 2 to 10 times greater at sites between the San Andreas and Sargent fault traces, consistent with other evidence for lower velocities in the upper 3 km of crust there. Routine mislocation of epicenters 2 to 4 km northeast of the Sargent fault are consistent with greater P-wave velocity northeast of the Sargent fault. Microearthquake source parameters are consistent with a more segmented and splayed fault geometry toward the northwest locked end of the transition. P-wave nodal planes for 10 microearthquakes located to the northwest, 9 on the Sargent fault, and 1 near the San Andreas, are oriented more westerly than nodal planes commonly obtained for the frequent moderate-size earthquakes on the creeping section of the San Andreas fault to the southeast. Static stress drop, dynamic stress drop, and apparent stress estimates all increase with seismic moment, with the apparent stress and dynamic stress drops equal to about 3 and 20 per cent, respectively of the static stress drop. Average fracture energies, calculated assuming complete stress drop, generally increase with source size (seismic moment, rupture area, seismic slip, etc.) from 7 to 3000 J/m2; the two events with anomalously low fracture energies occurred on the creeping section of the San Andreas fault.

Source physics interpretation of non-self-similar double-corner frequency source spectral model JA19_2S

2021 ◽  
Author(s):  
Chen Ji ◽  
Ralph Archuleta
Keyword(s):  
Aspect Ratio ◽  
Stress Drop ◽  
Dynamic Stress ◽  
Static Stress ◽  
Corner Frequency ◽  
Rupture Velocity ◽  
Dynamic Stress Drop ◽  
Self Similar ◽  
Static Stress Drop ◽  
Frequency Source

<p>Source spectral models developed for strong ground motion simulations are phenomenological models that represent the average effect that the source processes have on near fault ground motion. Their parameters are directly regressed from the observations and often do not have clear meaning for the physics of the source process. We investigate the relation between the kinematic double-corner frequency (DCF) source spectral model JA19_2S (Ji and Archuleta, BSSA, 2020) and static fault geometry scaling relations proposed by Leonard (2010). We derive scaling relations for the low and high corner frequency in terms of static stress drop, dynamic stress drop, fault rupture velocity, fault aspect ratio, and relative hypocenter location. We find that the non-self-similar low corner frequency  scaling relation of JA19_2S model for 5.3<<strong>M</strong><6.9 earthquakes is well explained using the fault length scaling relation of Leonard’s model combined with a constant rupture velocity. Earthquakes following both models have constant average static stress drop and constant average dynamic stress drop. The high frequency source radiation is controlled by seismic moment, static stress drop and dynamic stress drop but strongly modulated by the fault aspect ratio and the hypocenter’s relative location. The mean, scaled energy  (or apparent stress) decreases with magnitude due to the magnitude dependence of the fault aspect ratio. Based on these two models, the commonly quoted average rupture velocity of 70-80% of shear wave speed implies predominantly unilateral rupture.</p>

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.

Water-level fluctuations and earthquakes on the San Andreas fault zone

Geology ◽  
1975 ◽  
Vol 3 (8) ◽  
pp. 437 ◽  
Author(s):  
Robert L. Kovach ◽  
Amos Nur ◽  
Robert L. Wesson ◽  
Russell Robinson
Keyword(s):  
Water Level ◽  
Fault Zone ◽  
San Andreas Fault ◽  
Water Level Fluctuations ◽  
San Andreas

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.

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