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.

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.

Near-field observations and source parameters of central California earthquakes

1974 ◽  
Vol 64 (6) ◽  
pp. 1855-1886 ◽  
Author(s):  
Lane R. Johnson ◽  
Thomas V. McEvilly
Keyword(s):  
San Andreas Fault ◽  
Strong Dependence ◽  
Source Parameters ◽  
Broad Band ◽  
Low Frequency ◽  
Corner Frequency ◽  
Fault Plane Solutions ◽  
Low Frequencies ◽  
Central California ◽  
San Andreas

abstract This is a study of source characteristics of 13 earthquakes with magnitudes between 2.4 and 5.1 located near the San Andreas fault in central California. On the basis of hypocentral locations and fault-plane solutions the earthquakes separate into two source groups, one group clearly related to the throughgoing northwest-trending San Andreas fault zone and the other apparently associated with generally north-trending bifurcations such as the Calaveras fault. The basic data consist of broad-band recordings (0.03 to 10 Hz) of these earthquakes at two sites of the San Andreas Geophysical Observatory (SAGO). Epicentral distances range between 2 and 40 km, and maximum ground displacements from 4 to 4000 microns were recorded. The whole-record spectra computed from the seismograms lend themselves to source parameter studies in that they can be interpreted in terms of low-frequency level, corner frequency, and high-frequency slope. Synthetic seismograms have also been used to estimate source parameters in both the time domain and frequency domain, and the results compare favorably with those estimated directly from the spectra. The influences of tilts and nonlinear response of the seismometer were considered in the interpretation of the low frequencies. Seismic source moments estimated from the low-frequency levels of the spectra show a linear dependence on magnitude with a slope slightly greater than 1. The geology at the recording site can contribute an uncertainty factor of at least 3 to the estimated moments. Observed corner frequencies are only weakly dependent on magnitude. Interpreted in terms of source dimension, these corner frequencies imply values of 1 to 2 km for the earthquakes of this study. The corner frequencies may also be interpreted in terms of the rise time source function, yielding values in the range 0.5 to 1.0 sec. The data indicate that the earthquakes of this study are all surprisingly similar in their fundamental source parameters, with only the seismic moment showing a strong dependence on magnitude.

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.

Directivity in the high-frequency radiation of small earthquakes

1978 ◽  
Vol 68 (5) ◽  
pp. 1253-1263
Author(s):  
W. H. Bakun ◽  
R. M. Stewart ◽  
C. G. Bufe
Keyword(s):  
High Frequency ◽  
Fault Plane ◽  
San Andreas Fault ◽  
Body Waves ◽  
Rupture Propagation ◽  
Central California ◽  
Hypocenter Location ◽  
San Andreas ◽  
Road Section ◽  
Frequency Radiation

abstract On December 12, 1972 at 0351 and 0355 GMT, two earthquakes with magnitudes equal to 3.0 and 2.8, respectively, occurred on the Cienega Road section of the San Andreas fault in central California. The two events have the same hypocenter location and fault-plane soultion. Observed seismograms for these two events at 28 stations within about 65 km of and surrounding the epicenters are systematically different in a pattern that is consistent with different directions of rupture expansion for the two events. The 0351 GMT event preferentially radiated high-frequency (f ⪚ 10 Hz) body waves to the southeast consistent with unilateral rupture propagation toward the southeast while the 0355 GMT event rupture expanded more toward the northwest.

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.

A detailed seismicity study of the Middle Mountain zone at Parkfield, California

1990 ◽  
Vol 80 (3) ◽  
pp. 577-588
Author(s):  
Gail K. Nishioka ◽  
Andrew J. Michael
Keyword(s):  
San Andreas Fault ◽  
Velocity Model ◽  
Strike Slip ◽  
Seismogenic Zone ◽  
Fault Plane Solutions ◽  
Motion Data ◽  
Station Corrections ◽  
The North ◽  
San Andreas ◽  
First Motion

Abstract In order to better understand the preparation zone of the predicted Parkfield earthquake, a detailed study of the seismicity at middle Mountain in the Parkfield, California, area was made using 71 digitally recorded earthquakes that located within, or close to, the Middle Mountain alert box. These earthquakes were retimed on an interactive graphics system. Based on these new arrival times, new station corrections were developed; however the data did not support changing the velocity model developed from refraction and 1966 aftershock data. The process of retiming the earthquakes and using the new station corrections reduced the rms travel-time residuals by 70 per cent to 0.025 sec, halved the location errors, and clustered the earthquakes closer to the surface trace of the San Andreas fault. The seismicity can be approximated by a plane on the scale of several kilometers, but at finer scales two clusters were discovered that show demonstrable width to the seismogenic zone. Previous workers had proposed a 5° bend in the fault at the hypocenter of the 1966 main shock on the basis of patterns in the first motion data in the 1966 aftershocks. We find that this pattern also exists in the first-motion data from 1969 to 1987, but the 5° bend was not evident in the hypocentral distribution. This suggests that a more complicated explanation is needed to explain the first-motion data. Fault plane solutions were determined for the 71 events and 69 of these were compatible with strike-slip motion on a vertical San Andreas fault. An event located in the north end of the study area co-locates with the strike-slip solutions and may be a thrust or oblique solution. The other earthquake, located 2½ kilometers northeast of the fault, has a thrust or NNE-SSW striking right lateral solution but can not be explained by a San Andreas style mechanism. Both possible solutions can be explained by structures observed in the geology.

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