HR: 16:45h
AN: G22E-04 INVITED     [PDF]
TI: Relative land/sea-level movements and great Holocene earthquakes, Alaska
AU: * Shennan, I
EM: ian.shennan@durham.ac.uk
AF: Department of Geography, University of Durham, South Road, Durham, DH1 3LE United Kingdom
AU: Hamilton, S
EM: s.l.hamilton@durham.ac.uk
AF: Department of Geography, University of Durham, South Road, Durham, DH1 3LE United Kingdom
AB: The "earthquake deformation cycle" (EDC) model describes 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. By applying quantitative transfer functions to microfossil data collected from contemporary tidal flats, marshes and wetlands together with observational data from 1964 we can calibrate models of Holocene environmental change represented by fossil assemblages in sediment sequences. We use the transfer function models to quantify relative sea-level changes through multiple Holocene EDCs in Alaska and separate co-seismic from non-seismic events. Geological evidence from four sites around the Cook Inlet - Girdwood, Ocean View (Anchorage), Kenai and Kasilof, support a four phase EDC model that in the zone of co-seismic submergence comprises: (1) Relatively rapid land uplift and sedimentation immediately after an earthquake, i.e. the post-seismic period; (2) Centuries-long inter-seismic period of slower relative land uplift caused by strain accumulation along the locked portion of the Alaska-Aleutian subduction zone; (3) A pre-seismic period of relative sea-level rise (relative land subsidence) lasting approximately a decade; (4) Rapid co-seismic land subsidence during a great (Mw $>8$) earthquake. The four sites record evidence of Holocene co-seismic submergence in the form of multiple peat-mud couplets. Not all events are recorded at each site, the interval between events is variable and the magnitude of submergence varies from less than 0.5 m to greater than 2.0 m. A unique finding is the identification and quantification of pre-seismic RSL rise prior to each co-seismic event. Biostratigraphic changes that reveal a pre-seismic RSL rise typically occur over 1 to 5 cm of sediment, are dated at Kenai and Girdwood (for the 1964 earthquake) to start approximately 10 years before the event and indicate pre-seismic RSL rise up to 0.2 m. This is strong evidence to suggest that pre-seismic movements may represent a precursor to a great earthquake. Independent work based on twentieth century observations and limited to only parts of an EDC describe possible mechanisms for this phenomenon. This has potential implications for other subduction zone locations including Chile, Japan and the Pacific Northwest of the USA and Canada.
DE: 1242 Seismic deformations (7205)
DE: 4235 Estuarine processes
DE: 4556 Sea level variations
DE: 7221 Paleoseismology
SC: Geodesy [G]
MN: 2003 Fall Meeting