 Current Research Efforts
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| Thermotectonics of the California Coast Ranges - The California Coast Ranges south of about 40°N latitude coincide with a region of high heat flow (>75 mW/m2). Two possible source mechanisms have been identified for this high heat flow: asthenospheric upwelling into a "slab window" formed south of the Mendocino Triple Junction (MTJ) or mechanical heat generation in a broad shear zone between the North American and Pacific plates. In the past two years we have incorporated 57 new USGS heat-flow measurements with the older USGS data and geothermal industry data to provide additional constraints on the nature, extent, and origin of the Coast Ranges high. The work focuses on revised models for the slab window forming in the wake of the MTJ as well as the nature of the Coast Range high itself, including the transition between the Coast Range high and the adjoining region of low heat flow in the Great Valley (<50 mW/m2). We have developed thermal models of the magnitude and spatial extent of this transition, consistent with the heat flow as well as the varying depth extent of seismicity. We continue to identify heat flow through the northern and central Coast Ranges with the asthenospheric upwelling model but note that the persistence of high heat flow through the southern Coast Ranges, where the decaying thermal effects of the upwelling should result in decreasing heat flow, allows for the existence of a secondary mechanical heat source with a long thermal time delay. Another important aspect of our work is the clear confirmation of the absence of any significant frictional heat source centered on the San Andreas fault itself. |
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| Thermal Controls on the Nucleation Depth of the 1994 Northridge Earthquake - In recent years we have measured heat flow in a number of oil wells located in the eastern Ventura basin to the north of the Northridge epicenter. When coupled with existing measurements from the central Ventura basin, these data provided valuable insights into thermal conditions at depth for the 1994 Northridge Earthquake, 1971 San Fernando Earthquake, and a number of deep seismic events in the central Ventura basin. Although the complicated structure and rapid vertical tectonic movements of the Transverse Ranges complicate the interpretation, we can make a tie between temperatures and hypocentral depths for the three areas. The shallowing of seismicity from the central Ventura Basin (>20 km) to the Northridge event (17 km) to the San Fernando event (about 14 km) follows an isotherm between 325 and 375°C. We can extend these results for the heat flow data spanning the entire length of the Santa Clara River valley above the Oak Ridge fault system to estimate likely rupture depths and identify natural segment boundaries. |
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| Heat Flow and the Seismotectonics of the Los Angeles Basin - Starting in 1996 and continuing through 1998, we acquired thermal data from 17 selected oil wells in the Los Angeles basin. We are now working through the final interpretive steps in preparing our work for publication. These new data fill an enormous gap in heat-flow coverage (one previously published measurement for the entire area) and allow us to address a number of important tectonic issues. For example, we can now say with some certainty that the shallowing of seismicity west of the Newport-Inglewood Fault Zone (NIFZ) is due to increasing heat flow rather than a change in basement composition. We are also able to characterize thermal conditions on thrust ramps and detachments that appear on balanced cross sections of the basin. In many cases these supposed structures attain temperatures which should render them aseismic. Our results also have significance for the tectonic history of the basin as well as revising previous estimates of thermal conditions at depth. |
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| Studies of the Deep Seismic-Aseismic Transition in the San Francisco Bay Area - Our most recent work on heat flow in California and thermal controls on the base of seismicity along active faults (the seismic-aseismic transition), focused on two portions of the San Francisco Bay Area with anomalously deep or shallow seismicity. We are measuring heat flow in the Suisun Bay region to investigate possible thermal controls on deep seismicity along the Greenville, Concord-Green Valley and northern Calaveras faults. We are also measuring heat flow along the Rodgers Creek fault near Petaluma and plan to combine the new measurements with existing data related to northward shallowing of seismicity from the northern end of the Hayward fault. Both contain major seismogenic structures for which the catalogue seismicity suggests substantial deviations from the maximum depth of rupture characterizing other major faults in the Bay Area. Seismicity is relatively shallow along the Rodgers Creek fault, particularly its northern end, but the number and spatial distribution of events leave open the possibility that the seismicity catalogue is simply incomplete. If heat flow is high at our Petaluma sites, we will have strong evidence for reducing the predicted moment release rate for the Rodgers Creek fault in the Working Group 99 seismic hazard report. In contrast, seismicity is anomalously deep near Suisun Bay. Relatively low temperature gradients in this area (due to low heat flow) would confirm the tie between seismicity and temperature and raise the estimated moment magnitude for the faults in this area. |
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| Contributing to the FOQUS-LA Investigations of Quaternary Stratigraphy, Geohydrology and Earthquake Hazards in the Los Angeles Basin - Our goals in this new project are to (1) establish the value of thermal studies in understanding the geohydrology of basins, (2) investigate the interrelationships among ground-water flow, heat flow, and the physical properties (especially permeability) of faults within the Los Angeles basin, and (3) utilize the water well heat flow measurements to augment the oil well heat flow study described above. This involves measuring thermal and physical properties of core samples recovered from dedicated coreholes, measuring subsurface temperatures in coreholes and USGS-Water Resources Division (WRD) monitoring wells, and relating thermal measurements to hydrologic data through a mathematical model for the thermal effects of ground-water transport through the basin. Preliminary results from the coupled thermal-hydrologic model were presented at the Fall 1999 American Geophysical Union meeting. |
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| For more information on FOQUS-LA
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| Investigations of Surprise Spring Fault Near the Hector Mine Rupture and Facilitation of GD-WRD Contacts - The Hector Mine earthquake resulted in a dramatic coseismic water level decline in water wells supplying the Marine Corps base at 29 Palms. Additional declines could adversely affect the water supply for the base, and the Marines are contracting with USGS-WRD to investigate the cause. The wells pump from an aquifer bounded by the Surprise Spring fault, and the decline is almost certainly due to a change in fault permeability, possibly as a result of sympathetic coseismic slip. The USGS is planning a integrated program of investigations including drilling, heat flow, borehole geophysics, reflection seismics, and surface gravity. |
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| For more information on the Hector Mine Earthquake |
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