Late Holocene relative sea-level changes and earthquakes around the upper Cook Inlet, Alaska, USA

Abstract

The earthquake deformation cycle (EDC) produces a sequence of specific relative land- and sea-level movements associated with large (magnitude>8) plate boundary earthquakes forming pre-seismic, co-seismic, post-seismic and inter-seismic periods. Measurements available for the 1964 Alaskan earthquake allow a comparison between late Holocene earthquakes and an event of known magnitude. This thesis analyses multiple late Holocene co-seismic events at three sites around the upper Cook Inlet, Alaska, including at least one complete EDC at each site, quantified using transfer function techniques. Results describe five definite co-seismic events at Girdwood and one each at Kenai and Kasilof, with further possible events at all three sites. These fall within six periods of co-seismic submergence, including 1964, spaced at irregular intervals over the last 4000 years. Quantitative reconstructions suggest that submergence resulting from most co-seismic events studied are of similar magnitude to the 1964 earthquake but it is not possible to identify whether pre-1964 events affected the same area. A key finding is the identification and quantification of pre-seismic relative sea-level (RSL) rise before all those events attributed to co-seismic submergence. The maximum magnitude of pre- seismic RSL rise is +0.21 ± 0.10 m and for the 1964 event starts approximately 10 years before the earthquake. This is strong evidence to suggest it represents a precursor to a large event and independent work elsewhere describes possible mechanisms for this phenomenon. Long-term patterns of RSL change show significant differences between sites around Turnagain Arm compared to Kenai and Kasilof. The latter show negligible permanent deformation over multiple EDC's but this is not the case at sites around Turnagain Arm. Before attributing this to only partial recovery between two earthquakes, other factors deserve further attention, especially sediment consolidation, tidal range change through time and improvements in GIA modelling taking into account local ice sheet reconstructions.