Since the early 1990's I have been studying crustal deformation in southcentral Alaska (figure 1), a region that was severely affected by the 1964 Prince William Sound earthquake. This earthquake had an estimated moment magnitude of 9.2 and is the largest instrumentally recorded earthquake to occur in North America.  Horizontal displacements of up to 25 m or more and vertical displacements of several meters occurred as a portion of the Pacific Plate slid in a north-northwesterly direction under the North America Plate. The region of significant coseismic crustal deformation extended northeast from Kodiak Island to well beyond Prince William Sound, a region about 500-600 km in length and 200-300 km in width. Our investigations of the crustal deformation in this region have several components. The field investigations consist of Global Positioning System (GPS) observations of the ongoing three dimensional crustal motion (figure 2, figure 3). We also analyze apparent sea level height observations from tide gauge stations (figure 4) and have reanalyzed historic leveling data . We have developed numerical models that provide an explanation of our observations and insight into the subsurface processes responsible for them. This research began as a collaborative effort with Sanford Holdahl, now retired from the National Geodetic Survey. More recently, Jeffrey Freymueller and his colleagues at the Geophysical Institute, University of Alaska Fairbanks have taken the lead role in producing and analyzing the GPS results.
 

Figure1: Map of southcentral Alaska

Figure 2: Cumulative crustal uplift between 1964 and 1995 relative to a site in Seward. Notice the nearly 1 m uplift in the center of the Kenai Peninsula and the general SE-NW trend of the uplift pattern.. The uplift is computed by comparing relative height profiles from a 1964 leveling survey and contemporary GPS observations using a high precision local relative geoid height model to transform between the orthometric (leveling) and geometric (GPS) coordinate systems. From Cohen and Freymueller, 1997.

Figure 3: Contemporary horizontal velocities from GPS observations. The North-northwest motion of sites on the eastern half of the figure is attributable to elastic strain accumulation along the Pacific-North America tectonic plate interface. The southeasterly directed motions on the west side of the figure are more enigmatic but may be due to long term (or delayed) deep postseismic fault slip. The shallow portion of the plate boundary up-dip of the southeasterly moving sites probably exhibits weak coupling, hence no shallow elastic strain accumulation.  From Freymueller, Cohen, and Fletcher, 2000.

Figure 4 Uplift rates versus time. The results are normalized by the amount of coseismic slip (in meters). The data types include: tide gauge data - crosses, maxima from leveling surveys and GPS-leveling comparisons - circles, Very Long Baseline Interferometery -asterisk, Specific locales are indicated as follows: AB - Anchorage, KI - Kodiak Island, NI - Nikiski, SL-Seldovia, and SW - Seward, General area are TN - Turnagain Arm and KP - Kenai Peninsula. The dashed curved follows Omori's Law, B/(1 + t/T) with t equal to the time since the earthquake and B=7 cm/yr, T=3 yrs. The dotted curve is an exponential, Aexp(-t/r) with A = 8 cm/yr and r = 6 years. Recent evidence suggests that there may be additional long term component to the deformation not modeled by the simple behavior shown here. From Cohen, 1998.

References:

Cohen, S.C., S. Holdahl, D. Caprette, S. Hilla, R. Safford, and D. Schultz, "Uplift of the Kenai Peninsula Alaska since the 1964 Prince William Sound Earthquake," Journal of Geophysical Research, 100, 2031-2038, 1995.

Cohen, S.C., "Time Dependence of the Uplift of the Kenai Peninsula and Adjacent Areas of Southcentral Alaska since the 1964 Prince William Sound Earthquake," Journal of Geophysical Research, 101, 8595-8604, 1996.

Cohen, S.C., and J.T. Freymueller, "Deformation on the Kenai Peninsula, Alaska,," Journal of Geophysical Research, 102, 20,479-20,487, 1997.

Cohen, S.C., "On the Rapid Postseismic Uplift Along Turnagain Arm, Alaska following the 1964 Prince William Sound Earthquake, Geophysical Reseach Letters, 25, 1213-1215, 1998.

Freymueller, J.T., S.C. Cohen, and H. Fletcher, Variations in Present-Day deformation, Kenai Peninsula, Alaska, and Their Implications, Journal of Geophysical Research, 105, 8079-8101, 2000.

Cohen, S.C. and J. T. Freymueller, Crustal Uplift in the Southcentral Alaska Subduction zone: A New Analysis and Interpretation of Tide Gauge Observations, accepted by Journal of Geophysical Research, November, 2000.

Zweck, C., J. T. Freymueller, and S .C. Cohen, Elastic Dislocation Modeling of the Postseismic Response to the Alaska Earthquake, submitted to Journal of Geophysical Research, May, 2000.