LITTLE STRAIN ACCUMULATION AND RELEASE ON THE ALASKA-ALEUTIAN MEGATHRUST BENEATH THE SHUMAGIN ISLANDS IN THE LAST 3400 YEARS?

Russian accounts of a strong earthquake followed by a tsunami in 1788 imply rupture of the Alaska-Aleutian megathrust beneath the Shumagin Islands, Alaska. Geodetic observations in the Shumagin archipelago are consistent with a partially locked plate interface near the trench. Motivated by reports of uplifted marine terraces and features resembling raised shorelines on satellite imagery, we investigated Simeonof Island (120 km northwest of the Aleutian trench) for evidence of tsunami deposits and relative sea-level (RSL) change related to the earthquake deformation cycle.

Field work at 8 sites included descriptions of soil pits, outcrops, and 75 sediment cores along the island’s northwest coast. Freshwater peat, 0.5–2-m thick, occurs in thermokarst lake basins and stream valleys in lower elevations. Organic silt and tephra, <2-m thick, uniformly drapes stepped bedrock topography. Topographic profiles, soil pits, and ¹⁴C ages from basal silts suggest that the stepped landscape probably has a glacial, rather than marine, origin. A ¹⁴C age on basal peat limits glacial retreat to before 9.9 ka. Subsequently, late Holocene RSL rise and/or coastal erosion breached lakes near the coast forming tidal lagoons. At two sites, freshwater peat overlies sand inferred to be marine based on diatoms and δ¹³C values of detrital algae. Interbedded peat-sand contacts suggest lagoons gradually reverted to fresh bogs behind barriers; their elevations and ¹⁴C ages imply stable or slowly rising (<0.2 m/ka) RSL since ~3.4 ka. We infer storms rather than tsunamis deposited thin sand layers of limited extent that interrupt peat at elevations below high tide.

The absence of raised marine terraces on Simeonof and drowned coasts of neighboring islands suggest that sea-level rise since the mid Holocene has outpaced any net tectonic and/or isostatic uplift. We found no late Holocene evidence on Simeonof for sudden tectonic uplift, subsidence, or tsunami deposits. Therefore, if the megathrust below Simeonof has ruptured since 3.4 ka then vertical displacements were too small or too infrequent to perturb a record of stable or slowly-rising RSL. Our explanation accounts for the apparent lack of geological evidence for the 1788 earthquake and tsunami and implies little cyclic strain accumulation and release on the megathrust below Simeonof Island.