Tsunami Deposit Research: Fidelity of the Tsunami Record, Ephemeral Nature, Tsunami Deposits Characteristics, Remobilization of Sediment by Later Waves, and Boulder Movements

Abstract

Historically, much that has been reported about tsunamis has been based upon mathematical modeling. However, modeling tells little about the impact of tsunami on land and people, inundation and run-up, drain back, movement of sediments and boulders, the natures of deposition and erosion, the carrying capacity of waves, etc. In order to study these topics, the question can be asked, what do tsunami deposits tell us about tsunamis? Since the Hawaiian Islands have experienced numerous tsunamis it is an ideal place for field studies. Observations and analysis have been carried out on the North Shore of Oahu. The objectives were: to observe tsunami vs. storm activity, document the impact of the 1946 Aleutian and 2006 Kuril Tsunami, observe the nature and distribution of sediments associated with these events, and quantify the nature of transport of mega-boulders, and calculate the transport factor. The results indicate: (1) There was little difference in the sedimentary record and inundation record between the November 2006 Kuril Islands Tsunami and the December 2006 Winter Storm. (2) The 2006 tsunami and storm deposits were ephemeral, i.e., the tsunami record was removed by the winter storm. (3) The sedimentary record is of low fidelity in discriminating and preserving tsunami and storm events. From the analysis of the 1946 Aleutian Tsunami, the real-time aerial photography proved invaluable, since the patterns of inundation and drawback become obvious. A sand sheet, with only isolated and rare boulders, covered the inundation zone. The drain back utilized pre-existing drainages in a limited way. A stratigraphy is found in the archaeological sampling showing that cultural items were relocated by the tsunami and some vegetation was buried in place. The study of mega-boulders suggests that giant waves are capable of moving rock boulders around on the reef platform, but did not significantly displace them. The waves surging over the platform plucked rocks from the back or leeward side of the mega-boulders. The fragments of boulders struck the karstic platform and broke off additional rock. The