Research Projects |
Postseismic Deformation In AlaskaClick on each image for a larger version in a new window 
Figure 1a. Contour plots of gravitational perturbation rates (in μGal/year) associated with the 1964 Alaska earthquake. Model a shows present day rates computed for a model with upper mantle viscosity of 1019 Pa.s and a lower mantle viscosity of 1021 Pa.s. Reproduced with permission after Soldati et al. [1998]; please note the authors presented four scenarios for comparison. The locations of Fairbanks and Palmer are also included. 
Figure 1b. Contour plots of gravitational perturbation rates (in μGal/year) associated with the 1964 Alaska earthquake. Model d computes rates for a mantle with a uniform ν = 1021 Pa.s. 
Figure 2. The FG5/111 absolute gravimeter(Fig. 2a.), set in the Gilmore Creek VLBI station. This station is colocated with a VLBI antenna (Fig. 2b.), and was originally designed for a superconducting gravimeter. 
Figure 3. The FG5/111 Gravimeter at Petersville, Alaska (Fig. 3a.). This station is outdoors, using a small tent for the instrument, and is set on rock outcrop. It takes a fair amount of hard work to establish such stations! Pictured below is Max Kaufmann, UAF (Fig. 3b). Tests of Postseismic Deformation Models with Continued Absolute Gravity and GPS Measurements in Alaska, Phase IThe transient deformation after great earthquakes has been explained with hypotheses involving viscoelastic response, aseismic creep and combinations of the two. Different models of postseismic viscoelastic relaxation predict observable gravity changes over 1000 km length scales, with a complex spatial pattern. To date, the viscosity profile is best determined by postglacial rebound studies. Gravity measurements offer the opportunity to obtain viscosity profile estimates for the Alaskan plate boundary. The area is especially interesting, as the viscosity of subduction zone materials may be very different from that beneath the centers of continental and oceanic plates. Aseismic creep is predicted to produce deformation of a more limited and markedly different spatial character than that of viscoelastic relaxation [Cohen et al, 1995, Cohen and Freymueller, 1997]. Determining the contribution of aseismic creep in postseismic deformation is important in estimating recurrence intervals for great earthquakes. Soldati et al. [1998] predicted viscoelastic relaxation specifically for the 1964 Prince William Sound earthquake. Assuming an upper mantle viscosity ν = 1019 Pa.s, their model predicts gravity rates of change as high as 10 μGal/year in the vicinity of the fault rupture. This ν value is similar to estimates suggested by recent postseismic deformation studies in California. Models with uniform ν = 1021 Pa.s predict maximum rates of change of 0.5 μGal/year. Thus, to the extent that viscoelastic processes can explain the observed postseismic deformation, gravity change observations can place important constraints on the viscosity structure. If only small gravity changes are observed, a suite of low viscosity models would be ruled out. Gravity measurements are uniquely sensitive to both surface vertical displacements and subsurface density variations. The FG5 absolute gravimeter has a ±2 μGal field accuracy [Niebauer et al., 1995, Sasagawa et al., 1995]. Combining such observations with continuous and campaign GPS measurements would provide the data for testing the various models of postseismic deformation. We have completed a feasibility study, which made new absolute gravity observations at sites measured 10-15 years ago. Two sites in Fairbanks, Alaska show near zero gravity rates of change and are consistent with model predictions. A site in Palmer, Alaska showed a near zero rate as well. The Palmer site is located near a nodal line predicted by viscoelastic deformation models. The new observations demonstrate the resolution and stability of absolute gravity measurements. As of September 2003, the project established a baseline gravity network of fifteen stations in Alaska. The bulk of the network formed a transect perpendicular to the predicted contours of maximum gravity change in the far field. Additional points are located within the deformation near field. Gravity measurements were collocated with existing GPS measurements. This project also repeated a portion of a 1964-1965 relative gravity survey [Rice, 1969], which will determine the total gravity change over the Kenai Peninsula since the earthquake. These data will be combined with other measures of cumulative postseismic deformation to constrain postseismic deformation models. We are now post-processing the gravity and GPS data, and combining the results with other supporting data sets, such as water table records. These initial measurements will be the basis for testing different postseismic deformation models. Future measurements will further improve mantle viscosity estimates. References
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