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C. G. Bufe

Publications and source records attributed to C. G. Bufe.

31 records · Page 2Linked to original sources
USGS1979

Fault-crossing P delays, epicentral biasing, and fault behavior in central California

The P delays across the San Andreas fault zone in central California have been determined from travel-time differences at station pairs spanning the fault, using off-fault local earthquake or quarry blast sources. Systematic delays as large as 0.4 sec have been observed for paths crossing the fault at depths of 5-10 km. These delays can account for the apparent deviation of epicenters from the mapped fault trace. The largest delays occur along the San Andreas fault between San Juan Bautista and Bear Valley and Between Bitterwater Valley and Parkfield. Spatial variations in fault behavior correlate with the magnitude of the fault-crossing P delay. The delay decreases to the northwest of San Juan Bautista across the "locked" section of the San Andreas fault and also decreases to the southeast approaching Parkfield. Where the delay is large, seismicity is relatively high and the fault is creeping.

California↗
USGS1978

Seismic monitoring at the Geysers geothermal field, California

Two distinct clusters of microearthquakes have been identified at The Geysers, possibly relating to two independent pressure sinks resulting from steam production described by Lipman, and others (1977). Unlike earthquakes in the Maacama-Rodgers Creek fault zone to the south and west, earthquakes at The Geysers are confined to depths of less than 5 km. The present level of seismicity at The Geysers appears to be higher than the preproduction level and is higher and more continuous than the seismicity in the surrounding region. Earthquakes in the steam production zone at The Geysers resemble earthquakes in the surrounding region with regard to focal plane solutions, source characteristics and magnitude distribution (b slope). Subtle differences in earthquake characteristics may be resolved by analysis of more extensive data now being gathered in the region.

Open-File Report↗
USGS1977

Search for seismic forerunners to earthquakes in central California

The relatively high seismicity of the San Andreas fault zone in central California provides an excellent opportunity to search for seismic forerunners to moderate earthquakes. Analysis of seismic traveltime and earthquake location data has resulted in the identification of two possible seismic forerunners. The first is a period of apparently late (0.3 sec) P-wave arrival times lasting several weeks preceding one earthquake of magnitude 5.0. The rays for these travel paths passed through — or very close to — the aftershock volume of the subsequent earthquake. The sources for these P-arrival time data were earthquakes in the distance range 20–70 km. Uncertainties in the influence of small changes in the hypocenters of the source earthquakes and in the identification of small P-arrivals raise the possibility that the apparantly delayed arrivals are not the result of a decrease in P-velocity. The second possible precursor is an apparent increase in the average depth of earthquakes preceding two moderate earthquakes. This change might be only apparent, caused by a location bias introduced by a decrease in P-wave velocity, but numerical modeling for realistic possible changes in velocity suggests that the observed effect is more likely a true migration of earthquakes. To carry out this work — involving the manipulation of several thousand earthquake hypocenters and several hundred thousand readings of arrival time — a system of data storage was designed and manipulation programs for a large digital computer have been executed. This system allows, for example, the automatic selection of earthquakes from a specific region, the extraction of all the observed arrival times for these events, and their relocation under a chosen set of assumptions.

California↗
USGS1976

The Anderson Reservoir seismic gap - Induced aseismicity?

A persistent 10-km seismicity gap along the Calaveras fault appears to be related to the presence of the Leroy Anderson Reservoir in the Calaveras-Silver Creek fault zones southeast of San Jose, California. A magnitude-4.7 earthquake occurred at a depth of 5 km in the centre of the gap on October 3, 1973. The sequence of immediate aftershocks usually accompanying shallow earthquakes of this magnitude in central California did not occur. A bridge crossing the reservoir near its southeast end has been severely deformed, apparently the result of tectonic creep on the Calaveras fault. The occurrence of creep and absence of small earthquakes along the Calaveras in the vicinity of the reservoir suggest a transition from stick slip to stable sliding, possibly brought about by increased pore pressure.

Engineering Geology↗
USGS1976

Oroville earthquakes: Normal faulting in the Sierra Nevada foothills

Aftershocks of the Oroville, California, earthquake of 1 August 1975 define a 16- by 12-kilometer fault plane striking north-south and dipping 60 degrees to the west to a depth of 10 kilometers. Focal mechanisms from P-wave first motions indicate normal faulting with the western, Great Valley side downdropped relative to the Sierra Nevada block. The northward projection of the fault plane passes beneath Oroville Dam and crops out under the reservoir.

California↗