Glacier ice mass fluctuations and fault instability in tectonically active Southern Alaska

Abstract

Across southern Alaska the northwest directed subduction of the Pacific plate is accompanied by accretion of the Yakutat terrane to continental Alaska. This has led to high tectonic strain rates and dramatic topographic relief of more than 5000 meters within 15 km of the Gulf of Alaska coast. The glaciers of this area are extensive and include large glaciers undergoing wastage (glacier retreat and thinning) and surges. The large glacier ice mass changes perturb the tectonic rate of deformation at a variety of temporal and spatial scales. We estimated surface displacements and stresses associated with ice mass fluctuations and tectonic loading by examining GPS geodetic observations and numerical model predictions. Although the glacial fluctuations perturb the tectonic stress field, especially at shallow depths, the largest contribution to ongoing crustal deformation is horizontal tectonic strain due to plate convergence. Tectonic forces are thus the primary force responslble for major eartnquakes. Xowever, for geodefic sites located < 10-20 km from major ice mass fluctuations, the changes of the solid Earth due to ice loading and unloading are an important aspect of interpreting geodetic results.The ice changes associated with Bering Glacier's most recent surge cycle are large enough to cause discernible surface displacements. Additionally, ice mass fluctuations associated with the surge cycle can modify the shod-term seismicity rates in a local region. For the thrust faulting environment of the study region a large decrease in ice load may cause an increase in seismic rate in a region close to failure whereas ice loading may inhibit thrust faulting.Prior to the 1979 St. Elias earthquake (M=7.2), the main thrust zone below the study region had been locked since the 1899 earthquakes and strain had been accumulating. During this same time period ongoing wastage of southern Alaska glaciers was 100's of meters up to almost a 1 km. We used estimates of ice thickness decrease to calculate the changes in the fault stability margin around the region of the 1979 St. Elias earthquake and aftershocks. Our results suggest that the cumulative decrease in the fault stability margin due to ice wastage between 1899 and 1979 was large and would promote thrust faulting. Since earthquake hazard evaluations are based on the paleoseismic history of the region, for glaciated areas the concurrent glacial history should be considered in this evaluation. Glacier terminus. In addition to modifying the surface displacements rates, we evaluated the influence ice changes during the Bering glacier surge cycle had on the background seismic rate.We found an increase in the number of earthquakes (ML 2 2.5) and seismic rate associated with ice thinning and a decrease in the number of earthquakes and seismic rate associated with ice thickening. These results support the hypothesis that ice mass changes can modulate the background seismic rate.

Quantitative Analysis of Glaciers Changes From Passage Canal Based on Gis and Satellite Images, South Alaska

“…We did not expect that in 1985-1999 the melting area of ice mass to be almost equal compared with ice mass melting during 1999-2013. The retreat of all three GLs in the same period, without earthquakes and landslides (Sauber and Molnia, 2003), help us understand that the main factor contributing to the melting is climate change (Oerlemans, 1994;Haeberli et al, 1998).…”

“…Surface displacement and ice mass fluctuation were demonstrated by GLs from South Alaska related to earthquakes and active tectonics (Sauber and Molnia, 2003). The landslides and thick debris cover triggered by earthquakes can significantly change the ablation rate of GLs (Post, 1967;Krimmel and Meier, 1989).…”

Potential for a hazardous geospheric response to projected future climate changes

“…Questions remain, however, most particularly in relation to the time scales over which a geospheric response may be detectable. Although increases in the incidence of climate-change-driven, large-volume rock and ice avalanches (Huggel et al 2008(Huggel et al , 2010Huggel 2009), and the suggested modulation of seismicity in areas of large-scale ice wastage (Sauber & Molnia 2004;Sauber & Ruppert 2008), lead to speculation that climate change is already drawing out a crustal response, no increase in the global incidence of either volcanic activity or seismicity has been identified to date, nor has any change in the stability of submarine slopes been detected. It may be the case that modulation of potentially hazardous geological and geomorphological processes due to anthropogenic climate change proves to be too small a signal to extract from the background noise of 'normal' geophysical activity, at least in the short to medium term.…”

“…Glacier mass fluctuations in south central Alaska have been charged with modulating the recent seismic record, and even implicated in the triggering of the 1979 magnitude 7.2 St Elias earthquake Sauber & Molnia 2004;Sauber & Ruppert 2008). Rapid ice-mass loss at the many glaciated volcanoes in Alaska and Kamchatka, driven by surface temperature rises that could exceed 15…”

Revision of Time-Independent Probabilistic Seismic Hazard Maps for Alaska

“…Although some studies have been carried out in this region since 1999 (Pavlis and others, 2004), it is not possible at this time to construct a better model of the faulting, and the model assumed is that used in the 1999 map. As a proxy for a more detailed understanding, a flat fault surface (that is, with a 0° dip) at a depth of 15 km was assumed, extending from 59.1° to 61.0° N. and from 139.5° to 145.4° W. This is nearly identical to the dislocation model proposed by others (1997, 1998; see also Sauber and Molnia, 2004) to explain GPS observations in this region. Sauber and others (1997) estimate that sufficient strain has accumulated in this region since 1899 to generate an earthquake of magnitude Mw 8.1.…”