Volcanic, tectonic, and tsunamigenic events recorded in peats near Millers Landing, Homer, Alaska

Abstract

The Millers Landing peat deposit is located on the western side of Kachemak Bay, near Homer, Alaska. Distal tephra deposits from past eruptions of Augustine Volcano, Redoubt Volcano, Spurr Volcano, and Katmai Volcano are preserved within the peat. Evidence of active tectonism is found where a meter of marine silt overlies the peat deposits at Millers Landing. The marine mud deposits record co-seismic subsidence and post-seismic uplift as a result of a prehistoric great earthquake, ca. 1000 yr. B.P. along the northern Pacific plate boundary along the subduction interface. The uplift rate of Millers Landing over the past 1000 years has a minimum uplift rate of 3.0 mm/yr. Since 1995 Millers Landing has been an experiencing a post-seismic uplift rate of 5.4 +/- 0.6 mm/yr from the 1964 Prince William Sound earthquake. The Millers Landing peat deposits also contained nine layers of sand and beach gravel. The sedimentology is identical to classic tsunami depositional facies that have been identified in other tectonically active areas and we interpret these deposits as evidence of prehistoric tsunami events. The upper layer of two thick sand units, dated at ca. 3600 yr. B.P. by radiocarbon dating, is directly overlain by a 1.2 cm thick grayish white tephra. The tephra is from Redoubt Volcano and records a tsunami triggered by Mt. Redoubt's debris avalanche and lahar which are also dated at 3600 yrs B.P. The other sand deposits present within the peat are evidence of tectonically-triggered tsunamis. The recognition of tephras, tsunami deposits, and evidence of prehistoric co-seismic subsidence indicates the potential for geohazard assessment of Millers Landing and the entire Homer, Alaska region.