PRE-SEISMIC RELATIVE SEA-LEVEL MOVEMENTS AND GREAT HOLOCENE EARTHQUAKES, ALASKA

The “earthquake deformation cycle” (EDC) accounts for relative sea-level (RSL) movements associated with large plate boundary earthquakes. On March 27, 1964, an earthquake of magnitude 9.2 occurred along the south coast of Alaska, USA. With observational data from 1964 together with microfossil data collected from contemporary tidal flats, marshes and wetlands, we can calibrate models of environmental change represented by fossil assemblages. We use these models to analyse late Holocene earthquakes in Alaska.

Initial analyses of the 1964 event suggests a four stage EDC model (Shennan et al., 1999; Zong et al., 2003):

1. Co-seismic submergence 2. Post-seismic rapid land uplift / RSL fall 3. Centuries-long more gradual inter-seismic strain accumulation / RSL fall 4. Pre-seismic RSL rise

We evaluate the stratigraphic evidence for the 1964 event, and preceding events, at four sites around the Cook Inlet: Kenai, Girdwood, Kasilof and Ocean View and focus on the evidence for pre-seismic RSL rise as a possible precursor to a large earthquake.

At Kenai, Girdwood and Ocean View there is evidence for a rise in RSL before the 1964 earthquake, dated by 137Cs. At Girdwood, Karlstrom (1964) observed tidal inundation of the marsh commencing in 1953. Comparable changes occur before other late Holocene events at all sites. There are differences in the detail of each pre-seismic RSL change, but mixing of microfossil assemblages from the overlying silt into the peat cannot explain the distinct diatom assemblage found within this phase. Quantitative diatom transfer functions provide estimates of the magnitude of the RSL changes for all events at each site. Estimates of late Holocene co-seismic submergence range from 0.28±0.22 m to 1.97±0.42 m and pre-seismic RSL rise from 0.13±0.13 m to 0.21±0.10 m.

The hypothesis that a pre-seismic RSL rise represents a precursor to a large earthquake remains valid and supports a similar hypothesis proposed by recent work on the Cascadia margin of the Pacific Northwest, USA (Shennan et al., 1998; Long & Shennan 1998).

Long AJ & Shennan I 1998 The Holocene 8, 129-142 Karlstrom TNV 1964 US Geological Survey Professional Paper 443, 1-69 Shennan I et al. 1998 Quaternary Science Reviews 17, 365-393 Shennan I et al. 1999 Quaternary International 60, 55-73 Zong Y et al. 2003 The Holocene, 13, 7-20