Prehistoric human settlement patterns in a tectonically unstable environment: Outer shumagin islands, southwestern alaska

Abstract

Human population movements into and around the outer Shumagin Islands of southwestern Alaska during the last 5000 years, and temporal gaps in Shumagin habitation correlate inversely with geologically inferred prehistoric earthquakes. Clusters of inferred seismic activity correlate with temporal gaps or with small, sparsely distributed archaeological sites; periods of relative seismic quiescence coincide with settlement florescence. Variations in terrace and archaeological site heights across the study area indicate differential uplift of the outer Shumagin Islands when corrected for the Holocene sea level rise and post-glacial isostatic adjustments. INTRODUCTIONCoastal people in tectonically active areas can be dramatically affected by co-seismic events that may include permanent uplift or subsidence, severe shaking, tilting, and inundation by large tsunamis. Major food resources such as shellfish beds can be destroyed by increased or decreased shelf depths, landslides can bury settlements, collapsing seacliffs can take settlements and boats with them, and changes in harbor geometries by creation or removal of natural barriers can make a locality more or less favorable for future habitation. Therefore, following a great earthquake, major disruption of the human record, either a hiatus or marked reduction in size of sites, might be expected (Black, 1981).The interplay between tectonic uplift or subsidence and the early Holocene sea level rise not only predicts the location of desirable coastal habitats and the maximum age of archaeological sites, but also affects preservation of former settlements (Black, 1977). Therefore, recognition of temporal and spatial patterns of crustal movements and the timing and approximate size of geologically inferred prehistoric earthquakes and tsunamis is of critical importance to the evaluation of the prehistoric archaeological record. Conversely, interpretations of long term crustal behavior must not violate human

History of earthquakes and tsunamis along the eastern Aleutian-Alaska megathrust, with implications for tsunami hazards in the California Continental Borderland

“…Based on this record, estimates of the recurrence interval for this part of the Aleutian-Alaska megathrust vary from 333 to 875 yrs. In the Shumagin Islands, Winslow and Johnson (1989) show evidence for uplifted Holocene terraces that yield a clustering of radiocarbon ages that correlate inversely with native habitation in the Shumagin Islands. This suggests that following times of terrace uplift (in other words, earthquakes), the natives abandoned the islands.…”

“…On Unga Island, water levels rose as much as 90 m (Lander and Lockridge, 1989). The occupation of Unga Island by native Shumagin people ended in 1788 as a result of the destruction and abandonment of the last native village on the island, most likely from tsunami inundation (Winslow and Johnson, 1989).…”

Coastal paleogeography and human occupation of the Western Alaska Peninsula

“…However, paleoshoreline features and chronostratigraphic data indicate that both isostatic and tectonic factors have contributed to the postglacial trend of sea level, which has also been influenced by eustatic changes in global ocean volume. Uplifted shorelines of Holocene age are common throughout the Aleutian arc (Powers, 1961;Plafker et al, 1969;Morris and Bucknam, 1972;Black, 1976Black, , 1980Black, , 1981Winslow and Johnson, 1989;Winslow, 1991;Plafker et al, 1992), but no broad pattern or correlation of sea level changes is apparent because of the lack of chronological control and the disparity between terrace elevations among islands, island groups, and the peninsula.…”

Tsunami inundation maps for the city of Sand Point, Alaska

“…According to Winslow and Johnson (1989), the occupation of Unga Island by Native Shumagin people ended in 1788 as a result of the destruction and abandonment of the last Native village on the island, most likely from tsunami inundation. We emphasize that in addition to the major tectonic tsunami in 1788, a dormant Augustine Volcano began to erupt (Lander and Lockridge, 1989).…”

“…The goal was to investigate effects of the 1788 tsunami and to collect evidence of tectonic deformation at the outer Shumagin Islands. Simeonof Island, the closest island to the trench, was chosen as a good candidate, based on archeological study by Winslow and Johnson (1989). The latter interpreted coastal landforms on Simeonof Island as tectonically uplifted marine terraces.…”