In June 1993, June 1995 and Aug. 1997 geodetic measurements were made at approximately 25 sites in south central Alaska (Figure 1, Sauber et al., 1997, 2000) and across Kodiak Island using the Global Positioning System (GPS). The Yakataga region of south central Alaska has not had a large earthquake since 1899. The Kodiak Island segment of the plate boundary ruptured in the great 1964 Prince William Sound earthquake (M = 9.2). This study builds upon the earlier geodetic VLBI (very-long-baseline-interferometry) measurements made between 1984 and 1990 at 5 sites as part of the NASA's Crustal Dynamics Project (Ma et al., 1990). The horizontal rates of deformation (1984-1993) have been estimated from a combination of VLBI and GPS data in southern Alaska (Sauber et al., 1994). The results for 1993 and 1995 (GPS only) have been used to estimate the horizontal site velocities and to test alternative tectonic models (Sauber et al., 1997). Although we have estimated the vertical rate of deformation at many of sites given in Figure 1, and we have related our preliminary results to the mountain building process in southern Alaska (Sauber et al., 1998), we need additional measurements before we are confident of our vertical rates of deformation (Sauber et al., 2000). In addition to determining the spatial and temporal variations in rates of deformation across the plate boundary zone, geodetic measurements have been used to estimate coseismic strain release in large earthquakes. Earlier geodetic, geologic, and tsunami data have been used to invert for a detailed fault slip model for the 1964 earthquake (Holdahl and Sauber, 1993; Johnson et al., 1996). Also, the offset of the VLBI Cape Yakataga site has been used to estimate the average slip during the 1987-1988 Gulf of Alaska earthquakes (M=6.9, 7.6, 7.6) (Sauber et al., 1993).
FIGURE 1. Location of geodetic sites occupied in 1993, 1995, and 1997 in which horizontal velocities have been estimated. The vector velocities of sites with 1-sigma errors in the components of less than 2 mm/yr, in a North American fixed reference frame, are given with error ellipses representing region of 95% confidence. The shaded regions (with year) indicate the rupture zones of major earthquakes in southern Alaska this century. The Nuvel-1A predicted rate of motion of the Pacific plate relative to a fixed North American plate (52 mm/yr at N14W, DeMets et al. 1994) is given by the vector in the lower right hand corner. Faults (solid lines): BRF = Border Ranges, CF = Contact, CSEF = Chugach-St. Elias, DF = Denali, FF = Fairweather, PFZ = Pamplona, TFZ = Transition. IB = Icy Bay, MW = Mount Wrangell. (References: Sauber et al., 1997, 2000)
It is hard to make measurements anywhere in southern Alaska without being close to a glacier. During this century there has been a regional pattern of glacier retreat in southern Alaska that may be related to climatic warming. While deploying the GPS instruments in the Cape Yakataga region we made glaciological measurements that have been used with earlier glaciological measurements made by G. Plafker and B. Molnia (of the U.S.Geological Survey) to constrain rates of glacier thinning and recession (Sauber et al., 2000). During the same time period that the overall ice volume of southern Alaska glaciers has decreased, the Bering Glacier has undergone at least six surges. This surging is a periodic, very rapid movement of large quantities of ice within the glacier, alternating with a much longer period of near stagnation or retreat. The most recent surge began in the late spring of 1993 and ended in the late summer of 1995. A rapid transfer of ice from the Bagley Icefield and upper reaches of the Bering Glacier to the glacier's piedmont lobe advanced the terminus (horizontally) an average of 5 km. GPS measurements made in June 1993, combined with measurements in 1995, yielded discrete sampling of the elastic displacement field of the solid Earth associated with the change in surface load. We used these results for understanding ice redistribution during the surge [Sauber et al., 1995a,b, 2000]. The four stations near the source region, where the drawdown of ice occurred, showed uplift values which ranged from 2 to 4 cm, similar to preliminary predicted values (Sauber and Molnia, 1997). The one station located south of the advancing Bering Glacier showed approximately 4 cm of subsidence.
For the inhabitants of Alaska, understanding the hazards and underlying physical processes associated with earthquakes has immediate personal relevance. Large earthquakes occur yearly, great earthquakes have occurred during the lifetime of many residents, and the weekly testing of the tsunami warning system in coastal towns is a reminder of the devastation caused by earlier tsunamis generated near Alaska and serve as a cautionary note about future inundation.
In 1995 a new strategy for making geodetic measurements was implemented that included local residents in the measurement program. In June, 1995 high school students and their Earth science teacher, Eric Linscheid, from Kodiak, Alaska joined our earthquake hazard research project and made GPS measurements at six stations on Kodiak Island. This led to a paper by Earth Science educator, S. Stockman (Stockman et al., 1997; Web site: http://lptwww.gsfc.nasa.gov/education/alaska.html) and an article in the local newspaper ("Students help plot islands movement", Kodiak Daily Mirror, 1995). Based on the success of this pilot effort we expanded the program to include 5 Alaskan schools in our 1997 observation program. Our approach was published (Sauber et al., 1998) and broadcasted extensively (National Public Radio, Voice of America, local Alaska newspapers, AGU lecture at the National Science Teachers Boston Meeting in 1999).
Now we are expanding the scope of the projects in which students participate. In addition to making GPS observations at geodetic sites in 1999 (Sauber et al., 1999) two teachers and three students from Kodiak Island assisted in making kinematic GPS profiles across marine terraces as part of a sea-level study led by G. Carver. A new joint venture between Goddard scientists (J. Sauber, T. Clark, and S. Stockman), Ashtech (M. Bryant and B. LeMoine), Kodiak Island High School (E. Linscheid), and Scripps Orbit and Permanent Array Center (P. Jamason and Y. Bock) has led to the construction of a new permanent GPS station on Kodiak Island that is part of two international monitoring networks. Kodiak students will use the GPS data from this new station, along with data from a U.S. Coast Guard GPS station located about 20 km away, to monitor crustal deformation and assess earthquake hazard on Kodiak Island.
The author would like to acknowledge the substantial contributions of Mark Bryant(AshTech), Lynda Bell (NVI, Inc.), Stephanie Stockman (Univ. of Maryland), Jim Ryan (GSFC), Douglas Caprette (Hughes STX), Troy Carpenter (ATSC), Bjorn Johns (UNAVCO), Garth Franklin (JPL), and Steve Fisher (Sterling Scientific), B. LeMoine (AshTech), and E. Linscheid (Kodiak Island High School) in the planning and implementation of the field studies.
References
Holdahl, S. and J. Sauber, Coseismic slip in the 1965 Prince William Sound earthquake: a new geodetic inversion, Pageoph., 14(4), 55-82, 1993.
Johnson, J., K. Satake, S. Holdahl, and J. Sauber, The 1964 Prince William Sound earthquake: joint inversion of tsunami and geodetic data, J. Geophys. Res., 101(B1), 523-532, 1996.
Ma, C., J. M. Sauber, L. J. Bell, T. A. Clark, D. Gordon, W. E. Himwich, and J. Ryan, Measurement of horizontal motions in Alaska using very long baseline interferometry, J. Geophys. Res., 95, 21,991-22,011, 1990.
Meigs, A. and J. Sauber, Southern Alaska as an example of the long-term consequences of mountain building under the influence of glaciers, Quaternary Science Reviews, I. Stewart, J. Sauber, and J. Rose (eds), 13/14, 2000. Sauber, J. and B. Molnia, Ice mass "moves" the Earth, Physics News in 1996, edited by P. F. Schewe and B. P. Stein, New York: American Institute of Physics, 1997.
Sauber, J., S. Cohen, S. Stockman, and E. Linscheid, The seismic cycle and topography at Kodiak Island, Alaska, Supplement to Eos, Trans. AGU, 80(46), F226, 1999.
Sauber, J., T. Herring, and A. Meigs, Short-term uplift rates and the mountain building process in southern Alaska, Supplement to Eos, Trans. AGU, 79(45), F205, 1998.
Sauber, J., S. McClusky, and R. King, Relation of ongoing deformation rates to the subduction zone process in Southern Alaska, Geophys. Res. Letter, 24(22) 2853-2856, 1997.
Sauber, J. , G. Plafker, and J. Gipson, Geodetic measurements used to estimate ice transfer during Bering Glacier surge, Eos, Trans., Am. Geophys. Un., 76(29), 289-290, 1995a.
Sauber, J., G. Plafker, and J. Gipson, Measuring a moving glacier, Earth in Space, 4-5, Nov. 1995b.
Sauber, J., G. Plafker, B.F. Molnia, and M. A. Bryant, Crustal deformation associated with glacial fluctuations in the eastern Chugach Mountains, Alaska, J. Geophys. Res., 105(B4), 8055-8077, 2000.
Sauber, J., T. vanDam, J. Gipson, T. Herring, W.E. Himwich, and T. Clark, Rates of deformation in southern and central Alaska from a combination of VLBI and GPS data: 1984-1993, Supplement to Eos Trans., AGU, 112, 1994.
Stockman, S., J. Sauber, and E. Linscheid, A high school and NASA join forces to investigate the Alaska-Aleutian subduction zone near Kodiak Island, J. of Geoscience Education, 45, 440-446, 1997.
Contact: Jeanne Sauber
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