John Beavan, Geological and Nuclear Sciences, New Zealand
Jeff Freymueller, Geophysical Institute, University of Alaska

Synopsis

Many years of crustal deformation measurements, made by the USGS and the Lamont-Doherty Earth Observatory, in the Shumagin Islands south of the Alaska Peninsula showed a relative lack of deformation in the overriding plate in the eastern part of the Shumagin seismic gap. This project was begun in 1992 by John Beavan to make GPS measurements in both the western portion of the Shumagin seismic gap and the western portion of the Kodiak segment to spatial variations in the style of subduction along this plate boundary. Three GPS profiles were observed in 1993 and 1995 (from northeast to southwest): the Semidi profile, the Shumagin profile and the Sanak profile.

Field Measurements

• 1993 Semidi, Shumagin and Sanak profiles
• 1994 Port Heiden (in Semidi profile)
• 1995 Semidi, Shumagin and Sanak profiles
• 1996 Sanak profile
• 1995 Port Heiden (in Semidi profile), four sites in Sanak profile

The 1993 and 1995 measurements were carried out by Lamont-Doherty Earth Observatory (LDEO). The 1994 measurement of Port Heiden was made by the Cascades Volcano Observatory. The 1996 measurements were carried out by the Geophysical Institute, University of Alaska, under a subcontract from LDEO. The 1997 measurements were made by the Alaska Volcano Observatory.

Major Findings

• Semidi profile: Fully-coupled subduction zone. Displacement rates can be predicted using a very simple model of a fully locked subduction zone.
• Shumagin profile: Partially-coupled subduction zone. Evidence for accelerated deformation following the 1993 Shumagins earthquake.
• Sanak profile: Partially-coupled subduction zone with extension in the forearc(!).

Dr. Jeffrey T. Freymueller
Assistant Research Professor of Geophysics
Geophysical Institute
University of Alaska, Fairbanks

jeff@giseis.alaska.edu
Phone 907-474-7286
Fax 907-474-5618
Office 307C Elvey