Comprehensive study of the seismotectonics of the easter Aleutian arc and associated volcanic systems. Annual progress report, March 1, 1978--February 28, 1979

• Main Authors: Jacob, K.H., Davies, J.N., Beavan, J., Johnson, D., House, L., Krause, J., Hickman, S., Winslow, M., Hauptman, J., Mori, J., Sykes, L.R.
• Language: unknown
• Published: 2014
• Subjects: 58 GEOSCIENCES; ALEUTIAN ISLANDS; SEISMICITY; TECTONICS; BERING SEA; ACCELERATION; EARTH CRUST; EARTHQUAKES; GEODETIC SURVEYS; GEOLOGIC FAULTS; GROUND MOTION; RISK ASSESSMENT; SEISMIC P WAVES; STRESS ANALYSIS; VOLCANOES; GEOLOGIC STRUCTURES; GEOPHYSICAL SURVEYS; ISLANDS; MOTION; PACIFIC OCEAN; SEAS; SEISMIC EVENTS; SEISMIC WAVES; SURFACE WATERS; Bering Sea; Pacific; Tilting; Alaska; Aleutian Islands
• Online Access: http://www.osti.gov/servlets/purl/6389953; https://www.osti.gov/biblio/6389953; https://doi.org/10.2172/6389953

Based on the historic seismic record and accurate hypocenter data obtained from the eastern Aleutian seismic network, a relationship between the subduction-zone seismicity, volcano-trench separation, and the occurrence of great thrust earthquakes has been established for the Aleutian arc. On the basis of strong-motion accelerometer data it was found that high stress drops (540 t 650 bars) were associated with two moderate-size earthquakes (m/sub b/ = 6.0 and 5.8) within the Shumagin Islands seismic gap. This indicates that near the down-dip end of the major thrust zone, at depths of about 40 km, high tectonic stresses have accumulated within the gap segment of the arc. That such accumulation of stress is presently an ongoing process is corroborated by results from geodetic precision leveling on Unga Island. The leveling data indicate tilt rates of about 1 microradian/year. The tilting is directed down towards the trench and up towards the volcanic arc. Whether the recent activity of Pavlof, Shishaldin and Westdahl volcanoes indicates transmission of high tectonic stresses from the major thrust zone to the volcanic arc is unresolved. The search for a shallow magma chamber beneath the seismically monitored Pavlof volcano is still inconclusive although large amounts of recently acquired data remain to be analyzed. A geologic reconnaissance of the Shumagin Islands and the Adjacent Alaska Peninsula revealed Quaternary uplifted marine terraces and evidence for Holocene faulting. Both findings have severe implications for long-term tectonic activity and seismic hazards in the region of this seismic gap, portions of which are presently considered for off-shore hydrocarbon exploration and development. A critical analysis of th presently operating seimic data acquisition system reveals that a major change in remote sensing and central recording equipment is urgently needed for the Pavlof, Cold Bay, and Shumagin sections of the seismic array.