Directivity in the high-frequency radiation of small earthquakes

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.

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Microearthquake seismicity and tectonics of Coastal Northern California

Bulletin of the Seismological Society of America ◽

10.1785/bssa0600051669 ◽

1970 ◽

Vol 60 (5) ◽

pp. 1669-1699 ◽

Cited By ~ 7

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.

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The earthquake sequence of November 1964 near Corralitos, California

Bulletin of the Seismological Society of America ◽

10.1785/bssa0560030755 ◽

1966 ◽

Vol 56 (3) ◽

pp. 755-773 ◽

Cited By ~ 3

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.

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

Bulletin of the Seismological Society of America ◽

10.1785/bssa0670061565 ◽

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.

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