Natasha L.M. Barlow

Great tsunamigenic earthquakes during the past 1000 yr on the Alaska megathrust

Large to great earthquakes and related tsunamis generated on the Alaska megathrust produce major hazards for both the area of rupture and heavily populated coastlines around much of the Pacific Ocean. Recent modeling studies suggest that single-segment ruptures, as well as multi-segment, 1964-type ruptures, can produce great earthquakes, >M8, and significant hazards both in the near field and to distant locations through the generation of tsunamis. We present new paleoseismological data from Kodiak Island and a new analysis of radiocarbon data based on Bayesian age modeling to combine our observations with previous geological, historical, and archaeological investigations. We suggest that, in addition to multi-segment ruptures in A.D. 1964 and 1020–1150 (95% age estimate), a single-segment rupture occurred in 1788, with coseismic land-surface deformation across Kodiak Island and a tsunami that is recorded in historical documents and in sediment sequences, and another, similar rupture of the same Kodiak segment at A.D. 1440–1620. These indicate shorter intervals between ruptures of the Kodiak segment than previously assumed, and more frequent ruptures than for the Prince William Sound segment.

Paleoseismological Records of Multiple Great Earthquakes in Southcentral Alaska: A 4000‐Year Record at Girdwood

Analysis of sediment sequences beneath a tidal marsh at Girdwood, Alaska, record seven great earthquakes in the past 4000 years, including the M = 9.2 earthquake of March 27, 1964. The key theme that arises from studies of crustal deformation for the southcentral Alaska earthquake zone over timescales of the last few millennia is one of temporal and spatial variability. We have quantitative data to show both temporal and spatial similarities and differences for different earthquake cycles. There is not a fixed recurrence interval. The shortest interval is between ∼180 and 720 years. The longest interval is 790-920 years, which is between the penultimate and the 1964 earthquakes. Estimates of subsidence at Girdwood for each earthquake show values similar to or less than that recorded in 1964. Similarities between each earthquake cycle leads to a model for the Girdwood area with coseismic subsidence, followed by rapid postseismic uplift in the decades after the earthquake. This merges into centuries of slower interseismic uplift before a period of preseismic subsidence. Correlations with sites beyond Girdwood reveal regional-scale temporal variability and spatial heterogeneity in the crustal deformation processes.

Lack of evidence for a substantial sea-level fluctuation within the Last Interglacial

During the Last Interglacial, global mean sea level reached approximately 6 to 9 m above the present level. This period of high sea level may have been punctuated by a fall of more than 4 m, but a cause for such a widespread sea-level fall has been elusive. Reconstructions of global mean sea level account for solid Earth processes and so the rapid growth and decay of ice sheets is the most obvious explanation for the sea-level fluctuation. Here, we synthesize published geomorphological and stratigraphic indicators from the Last Interglacial, and find no evidence for ice-sheet regrowth within the warm interglacial climate. We also identify uncertainties in the interpretation of local relative sea-level data that underpin the reconstructions of global mean sea level. Given this uncertainty, and taking into account our inability to identify any plausible processes that would cause global sea level to fall by 4 m during warm climate conditions, we question the occurrence of a rapid sea-level fluctuation within the Last Interglacial. We therefore recommend caution in interpreting the high rates of global mean sea-level rise in excess of 3 to 7 m per 1,000 years that have been proposed for the period following the Last Interglacial sea-level lowstand.Robust evidence for a previously proposed sea-level fall and rise during the Last Interglacial is lacking, according to a synthesis. This calls estimates of high rates of sea-level rise at the end of the Last Interglacial into question.