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Recurrent Holocene Paleoseismicity and Associated Land / Sea Level Changes in the Greater Anchorage Area
A research project of the Department of Geography.
This project involves, to date, field and laboratory analyses of tidal marsh sediment sequences around Cook Inlet, at Ocean View (Anchorage), Girdwood and Kenai. They provide evidence of six great earthquakes in the Greater Anchorage area during the last ~3300 years at intervals ranging from ~400 to ~900 years. Calibration of fossil sequences using diatom transfer functions enable elevation reconstructions of land-level changes associated with each earthquake deformation cycle. The following paragraphs summarise our main findings to date and give details of published articles.
The penultimate great earthquake, ~900 BP, has a different spatial pattern of subsidence compared to AD 1964. It is not recorded at Kenai and subsidence is less at Ocean View, ~0.2 m. Co-seismic subsidence at Girdwood is similar to AD 1964 but pre-seismic subsidence is less. The ~1500 BP great earthquake shows yet another spatial pattern of subsidence. It is of similar magnitude at Girdwood and Ocean View but larger than AD 1964 at Kenai. Different spatial patterns of co-seismic subsidence for the AD 1964, ~900 BP and ~1500 BP great earthquakes may indicate variations in the location or depth of the rupture zone.
Current evidence from older peat-silt contacts indicate subsidence only at Girdwood, during great earthquakes ~2100 BP, ~2500 BP and ~3300 BP.
Observations in AD 1964 recorded no tsunamis at Kenai, Ocean View or Girdwood. There is no litho-stratigraphic or bio-stratigraphic evidence at any of the sites described here to indicate a tsunami within upper Cook Inlet caused by any of the great earthquakes during the last 3300 years.
Diatom analysis of the peat-silt couplets that record six great earthquakes in the last 3300 years provide evidence of pre-seismic land subsidence (relative sea-level rise) for each earthquake. Some of the evidence is more conclusive than others, and we can indentify where the signal is possibly enhanced by mixing of sediment and diatoms from the overlying silt into the peat or where the quantitative reconstruction shows a poor modern analogue. Many of the estimates show pre-seismic elevation change smaller than the error term but the fact that it is recorded in the litho- and bio-stratigraphy and the estimates are all negative, indicating relative sea-level rise, rather than a mixture of values either side of zero suggest that this is not a random effect. There is no relationship between the magnitude of pre-seismic and co-seismic subsidence at the same site.
Pre-seismic land subsidence (relative sea-level rise) does appear to be a common feature of late Holocene great earthquakes and seismological models developed on observational data from the last 40 years need to take account of these longer term movements. Quantifiable pre-seismic relative land- and sea-level movements may be a pre-cursor to a great plate-boundary earthquake.
Hamilton, S., and Shennan, I. (2005a). Late Holocene relative sea-level changes and the earthquake deformation cycle around upper Cook Inlet, Alaska. Quaternary Science Reviews 24, 1479.
Hamilton, S., Shennan, I., Combellick, R., Mulholland, J., and Noble, C. (2005). Evidence for two great earthquakes at Anchorage, Alaska and implications for multiple great earthquakes through the Holocene. Quaternary Science Reviews 24, 2050.
Hamilton, S. L., and Shennan, I. (2005b). Late Holocene great earthquakes and relative sea-level change at Kenai, southern Alaska. Journal of Quaternary Science 20, 95-111.
Zong, Y., Shennan, I., Combellick, R. A., Hamilton, S., and Rutherford, M. (2003). Microfossil evidence for land movements associated with the AD 1964 Alaska earthquake. The Holocene 13, 7-20.
Shennan, I., Scott, D. B., Rutherford, M. M., and Zong, Y. (1999). Microfossil analysis of sediments representing the 1964 earthquake, exposed at Girdwood Flats, Alaska, USA. Quaternary International 60, 55-73.