Little late Holocene strain accumulation and release on the Aleutian megathrust below the Shumagin Islands, Alaska

Abstract

Can a predominantly creeping segment of a subduction zone generate a great (M > 8) earthquake?Despite Russian accounts of strong shaking and high tsunamis in 1788, geodetic observations above the Aleutian megathrust indicate creeping subduction across the Shumagin Islands segment, a well-known seismic gap. Seeking evidence for prehistoric great earthquakes, we investigated Simeonof Island, the archipelago's easternmost island, and found no evidence for uplifted marine terraces or subsided shorelines. Instead, we found freshwater peat blanketing lowlands, and organic-rich silt and tephra draping higher glacially smoothed bedrock. Basal peat ages place glacier retreat prior to 10.4 ka and imply slowly rising (<0.2 m/ka) relative sea level since~3.4 ka. Storms rather than tsunamis probably deposited thin, discontinuous deposits in coastal sites. If rupture of the megathrust beneath Simeonof Island produced great earthquakes in the late Holocene, then coseismic uplift or subsidence was too small (≤0.3 m) to perturb the onshore geologic record.

Tsunami recurrence in the eastern Alaska-Aleutian arc: A Holocene stratigraphic record from Chirikof Island, Alaska

“…Hutchinson and Crowell (2007) suggested that six episodes of village abandonment, inferred from archaeological stratigraphy throughout the region, may have been caused by subsidence or tsunamis accompanying great earthquakes. Approximately 250 km west of Chirikof Island, Witter et al (2014) could not find clear evidence of any high tsunamis in the past 3000 yr, despite Russian accounts of severe flooding during the 1788 tsunami.…”

“…Although it is unclear if reports of severe flooding in the western part of the ruptured zone occurred following the 21 July earthquake or the later 6 August earthquake, 1 or 2 high tsunamis (reported run-up heights range from 3 to >30 m) caused much damage and many deaths from Kodiak Island to Sanak Island ( Fig. 1; Lander, 1996;Ryan et al, 2012b;Witter et al, 2014). The highest tsunami inundation at Southwest Anchorage was probably on 21 July, when the tsunami severely flooded Three Saints Harbor on Kodiak Island, 215 km to the northeast.…”

“…The variable source characteristics and tsunami-generating potential of megathrust ruptures in the region (Ryan et al, 2012b;Briggs et al, 2014aBriggs et al, , 2014bShennan et al, 2014aShennan et al, , 2014bWitter et al, 2014) suggest that Southwest Anchorage has been exposed to a large range in inundation heights that may only loosely correlate with tsunami volume, rupture extent, and earthquake magnitude. For this reason, forecasting future great earthquakes in the eastern Aleutian arc accompanied by southwarddirected high tsunamis will require carefully reconstructed histories of rupture lengths and inundation extents for the past 3-5 k.y.…”

Detection limits of tidal-wetland sequences to identify variable rupture modes of megathrust earthquakes

“…Atwater and Hemphill-Haley, 1997;Hayward et al, 2006;Kelsey, 2015;Nelson, 2015;Nelson et al, 1996;Witter, 2015) debate moved on to questions critical for hazard assessment, emergency planning and international building code design (Mueller et al, 2015;Wesson et al, 2007). Key questions include the extent of past great earthquake ruptures (a proxy for magnitude), the identification of the boundaries between rupture segments, the persistence of these boundaries over multiple earthquake cycles, recurrence intervals of great earthquakes in each segment, the role of aseismic slip, and whether segments of plate boundaries that are currently creeping can generate great earthquakes Goldfinger et al, 2012;Hayward et al, 2015;Kelsey et al, 2015;Mueller et al, 2015;Scholz, 2014;Witter et al, 2014).In order to address these questions, coastal paleoseismology continues to seek evidence to discriminate between alternative hypotheses. This inevitably results in returning to the field evidence and the quantifiable resolution of the age, the extent, and the pattern of vertical surface displacement of each rupture.…”

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

“…Kodiak model estimates the age of the event to AD 1440-1620 (Figure 2c were two earthquakes in 1788 (Briggs et al, 2014). Opinions range from one great earthquake on 22 131…”