Geology topics

G.A. Marshall

Publications and source records attributed to G.A. Marshall.

3 recordsLinked to original sources

USGS2012

Effects of geothermal development on deformation in the Long Valley Caldera, eastern California, 1985-1994

Long Valley caldera in east central California has been the site of crustal unrest in the form of seismicity and ground deformation in response to magmatic inflation since 1980. Uplift of the resurgent dome has totaled ∼0.6 m over the 1975–1992 period. Within this region of uplift, and near the southwestern edge of the resurgent dome, is the Casa Diablo area, which experienced relative subsidence of 0.17 m between 1985 and 1992. Geothermal fluid production began at Casa Diablo in 1985 to supply a binary electric power plant; currently, three such plants generate a total of about 40 MW of electricity. The plants are supplied by 170°C water pumped from depths near 150m; all the produced water is reinjected at cooler temperatures at depths near 600 m. Analyses of data from (1) regional leveling lines and a network of bench marks in the geothermal well field area, (2) tilt observations from an L‐shaped array of bench marks, and (3) calculations relating pressure and temperature changes to subsidence indicate that relative subsidence at Casa Diablo results mainly from reductions in pressure in the shallow production reservoir and overlying formations and reductions in temperature in the underlying injection reservoir. Secondary effects on deformation are caused by offsets along the Taylor‐Bryant fault bounding the west side of the well field.

California↗

USGS1997

A coccidioidomycosis outbreak following the Northridge, Calif, earthquake

Objective. —To describe a coccidioidomycosis outbreak in Ventura County following the January 1994 earthquake, centered in Northridge, Calif, and to identify factors that increased the risk for acquiring acute coccidioidomycosis infection. Design. —Epidemic investigation, population-based skin test survey, and casecontrol study. Setting. —Ventura County, California. Results. —In Ventura County, between January 24 and March 15, 1994, 203 outbreak-associated coccidioidomycosis cases, including 3 fatalities, were identified (attack rate [AR], 30 cases per 100 000 population). The majority of cases (56%) and the highest AR (114 per 100 000 population) occurred in the town of Simi Valley, a community located at the base of a mountain range that experienced numarous landslides associated with the earthquake. Disease onset for cases peaked 2 weeks after the earthquake. The AR was 2.8 times greater for persons 40 years of age and older than for younger persons (relative risk, 2.8; 95% confidence interval [CI], 2.1-3.7; P <.001). Environmental data indicated that large dust clouds, generated by landslides following the earthquake and strong aftershocks in the Santa Susana Mountains north of Simi Valley, were dispersed into nearby valleys by northeast winds. Simi Valley case-control study data indicated that physically being in a dust cloud (odds ratio, 3.0; 95% CI, 1.6-5.4; P<.001) and time spent in a dust cloud ( P <.001) significantly increased the risk for being diagnosed with acute coccidioidomycosis. Conclusions. —Both the location and timing of cases strongly suggest that the coccidioidomycosis outbreak in Ventura County was caused when arthrospores were spread in dust clouds generated by the earthquake. This is the first report of a coccidioidomycosis outbreak following an earthquake. Public and physician awareness, especially in endemic areas following similar dust cloud-generating events, may result in prevention and early recognition of acute coccidioidomycosis.

Journal of the American Medical Association↗

USGS1996

The 1992 M=7 Cape Mendocino, California, earthquake: Coseismic deformation at the south end of the Cascadia megathrust

We invert geodetic measurements of coseismic surface displacements to determine a dislocation model for the April 25, 1992, M = 7 Cape Mendocino, California, earthquake. The orientation of the model slip vector, which nearly parallels North America-Juan de Fuca relative plate convergence, and the location and orientation of the model fault relative to the offshore Cascadia megathrust, suggest that the 1992 Cape Mendocino earthquake is the first well-recorded event to relieve strain associated with the Cascadia subduction zone. We use data from three geodetic techniques: (1) the horizontal and vertical displacements of 13 monuments surveyed with the Global Positioning System, corrected for observed horizontal interseismic strain accumulation, (2) 88 section-elevation differences between leveling monuments, and (3) the uplift of 12 coastal sites observed from the die-off of intertidal marine organisms. Maximum observed displacements are 0.4 m of horizontal movement and 1.5 m of uplift along the coast. We use Monte Carlo techniques to estimate an optimal uniform slip rectangular fault geometry and its uncertainties. The optimal model using all the data resolves 4.9 m of slip on a 14 by 15 km fault that dips 28° SE. The fault extends from 1.5 to 8.7 km in depth and the main-shock hypocenter is close to the downdip projection of the fault. The shallowly dipping fault plane is consistent with the observed aftershock locations, and the estimated geodetic moment is 3.1 × 10 19 N m, 70% of the seismic moment. Other models that exclude leveling data collected in 1935 and 1942 are more consistent with seismological estimates of the fault geometry. If the earthquake is characteristic for this segment, the estimated horizontal slip vector compared with plate convergence rates suggests a recurrence interval of 140 years, with a 95% confidence range of 100–670 years. The coseismic uplift occurred in a region that also has high Quaternary uplift rates determined from marine terrace studies. If repeated ruptures of this southernmost segment of the Cascadia megathrust are responsible for the Quaternary uplift, a comparison of the coseismic uplift with coastal uplift rates suggests a recurrence interval of 200–400 years. Thus comparing horizontal and vertical coseismic to long-term deformation suggests a recurrence interval of about 100–300 years for M = 7 events at the south end of the Cascadia megathrust.

Journal of Geophysical Research B: Solid Earth↗