Adak Island, Alaska, Microearthquake survey: Preliminary Hypocenter Determinations
Microearthquakes, defined as shocks having magnitudes less than 4, are commonly recorded in the vicinity of geothermal manifestations and volcanism. They have been mapped from producing geothermal fields as well as those not yet developed, in such places as Iceland, El Salvador, Japan, Kenya and the US. Microearthquakes have been recorded at several geothermal sites in the Imperial Valley and Coso Hot Springs, California; Kilbourne Hole, New Mexico; Yellowstone National Park, Wyoming; and The Geysers, California, where there is debate over whether or not the seismicity is induced by steam production. Seismicity occurs around active volcanoes, but appears reduced directly over zones of high temperature or magma, where the depth of the brittle fracture zone is shallow, as over Yellowstone caldera. In areas of active hydrothermalism, regional stress is likely to be relieved by low-level seismicity rather than occasional large ruptures, owing to the high temperatures, presence of fluids, and crustal weakening due to alteration and fracturing. Active faulting maintains the permeability of the system, which in its absence, might otherwise seal. on the microscopic scale, pore-fluid pressures rise as a result of heating, resulting in the decrease of effective pressure at the pore-mineral boundary. When this effective pressure becomes less than the rock's tensile strength, the pore ruptures; and if it intersects a through-going fracture under hydrostatic pressure can result in a shock detectable on seismographs at the surface. Such a mechanism might also account for the swarms of very small events seen in a number of geothermal areas. A microearthquake survey was conducted on Adak Island, Alaska for the purpose of identifying seismicity associated with a possible geothermal reservoir. During 30 days of recording in September and October 1982, 190 seismic events were recorded on two or more stations of a nine-station network. Of the total, 33 were of local origin, and of these 24 were locatable. Utilizing a 5 km/sec constant velocity earth model, the hypocenters define a structure dipping north-northwestward toward the Bering Sea, beneath Mt. Adagdak. many of the events took place beneath the Adagdak peninsula in an area in which hot springs discharge and where other geophysical evidences suggest a geothermal reservoir. A similar NNW-dipping fault plane was deduced from a 9-day microearthquake survey conducted in 1974. At that time all of the activity occurred beneath the sea. the projected surface trace lies NNW of that deduced form the present survey. It is quite likely that the mapped structure and attendant fractures control a hydrothermal system by providing the necessary permeability for maintaining circulation of hot waters within the upper several kilometers of the surface. Only preliminary analysis of the records fell within the scope of the present project. The work should be supplemented with the application of a locally appropriate earth model, 3D fault-mapping, first-motion studies leading to fault-plane solutions, and computations of event magnitudes.
- Research Organization:
- Mincomp Exploration Resources, Wheat Ridge, CO
- Sponsoring Organization:
- USDOE
- DOE Contract Number:
- AC07-80ID12079
- OSTI ID:
- 890973
- Report Number(s):
- UURI-82-011; TRN: US200622%%171
- Country of Publication:
- United States
- Language:
- English
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