Coulomb stress, surface displacement, and seismic rate changes associated with ice mass fluctuations on seasonal, annual, and decadal time scales
Abstract
Between Yakutat and Cordova in southern Alaska, large ice mass fluctuations occur in a region of upper crustal faulting and folding associated with collision and accretion of the Yakutat terrane. Following the Little Ice Age, glacial retreat in this area commenced in the late 19th century and the last great earthquakes occurred in 1899. Annual ablation rates of meters/year have been estimated over the last several decades from aircraft and satellite lidar and radar measurements. For the Chugach-St. Elias region, GRACE 1 deg x 1 deg mascon estimates indicate a general trend of annual ice mass loss (2003-2010) with large, variable seasonal mass fluctuations. The station position of a continuous GPS site near Cape Yakataga (Alaska EarthScope PBO site, AB35, pbo.unavco.org) shows large seasonal excursions in the detrended vertical and horizontal components consistent with snow loading in the late fall and winter. Surface displacements of the solid Earth associated with these annual and seasonal ice and snow changes were calculated using a layered elastic half-space model. For instance, during a warm period in 2002-2006 the predicted elastic displacement rates of the solid Earth due to average annual ice changes were up to 20 mm/yr for the vertical and 3 mm/yr for the horizontal. To empirically evaluate the influence of short-term ice fluctuations on fault stability, we compared the seismic rate (M>1.8) from a reference background time period against other time periods with variable ice or tectonic change characteristics. After probable icequakes were removed from the AEIC catalog, the seismic rate change for the time periods of 1993-1996 and 1997-2001 relative to 1988-1992 was not statistically significant and the tectonic events did not show a strong seasonal dependence. However, we found that the frequency of small earthquakes increased significantly in the 2002-2006 time interval relative to the reference time period of 1988-1992 and there was a significant increase in these small events in the late summer and fall. For most months the temperature at the Yakutat NWS in 2002-2006 was several degrees warmer than the long-term (1917-2010) average. For 2007-2009 average monthly temperatures were cooler than the long-term average and there was a lower rate of ice mass loss as measured by GRACE. To calculate the longer-term influence of Little Ice Age glacial build-up and subsequent retreat on Coulomb stress changes, we are using PYLITH finite element model (FEM) calculations assuming a linear Maxwell viscoelastic Earth model. Our earlier 2-D TECTON FEM calculations indicated cumulative changes in the fault stability margin over 80 years of 0.2-1.2 MPa between the Gulf of Alaska coast and epicenter of the 1979 St. Elias earthquake.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2011
- Bibcode:
- 2011AGUFM.U53E..04S
- Keywords:
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- 1223 GEODESY AND GRAVITY / Ocean/Earth/atmosphere/hydrosphere/cryosphere interactions;
- 8104 TECTONOPHYSICS / Continental margins: convergent;
- 8118 TECTONOPHYSICS / Dynamics and mechanics of faulting;
- 4321 NATURAL HAZARDS / Climate impact