Sources and depositional processes of tsunami deposits: Analysis using foraminiferal tests and hydrodynamic verification

Abstract

Foraminiferal tests are commonly found in tsunami deposits and provide evidence of transport of sea floor sediments, sometimes from source areas more than 100 m deep and several kilometers away. These data contribute to estimates of the physical properties of tsunami waves, such as their amplitude and period. The tractive force of tsunami waves is inversely proportional to the water depth at sediment source areas, whereas the horizontal sediment transport distance by tsunami waves is proportional to the wave period and amplitude. We derived formulas for the amplitudes and periods of tsunami waves as functions of water depth at the sediment source area and sediment transport distance based on foraminiferal assemblages in tsunami deposits. We applied these formulas to derive wave amplitudes and periods from data on tsunami deposits in previous studies. For some examples, estimated wave parameters were reasonable matches for the actual tsunamis, although other cases had improbably large values. Such inconsistencies probably reflect: (i) local amplification of tsunami waves by submarine topography, such as submarine canyons; and (ii) errors in estimated water depth at the sediment source area and sediment transport distance, which mainly derive from insufficient identification of foraminiferal tests.

Internal structure of event layers preserved on the Andaman Sea continental shelf, Thailand: tsunami vs. storm and flash-flood deposits

“…A depth of ca. 20 m is substantially lower compared to the depths found during previous studies of tsunami impact based on wave theory and the analysis of microfossils (e.g., Nanayama and Shigeno, 2006;Weiss and Bahlburg, 2006;Weiss, 2008;Uchida et al, 2010). It is unknown whether the local shelf morphology offshore Khao Lak prohibited the substantial erosion of material from deeper waters or whether the sediments transported onshore from deeper waters were not preserved.…”

A systematic review of geological evidence for Holocene earthquakes and tsunamis along the Nankai-Suruga Trough, Japan

“…The identification of landward thinning, fining upward sequences, marine macro-and microfossils, erosional lower contacts and rip-up clasts in geoslicer samples of up to 6 m in length supports the hypothesis that Ōsatsu records a sequence of tsunamis (Okahashi et al, 2005b). The presence of the sublittoral zone foraminifera species Heterolepa haidingeri and Rectobolivina raphana in the sand layers suggests reworking of sediments from water depths deeper than would be expected during typhoons Uchida et al 2010). The corresponding transport distance, derived from the offshore bathymetry, is 8 -14 km and transport from such a depth and distance would require a tsunami with an amplitude of 6 m and a period of 60 minutes .…”

Sedimentary deposits left by the 2004 Indian Ocean tsunami on the inner continental shelf offshore of Khao Lak, Andaman Sea (Thailand)

“…The sea bottom is mainly composed of sand, sandy mud, muddy sand and bedrock outcrops (Usiriprisan et al, 1987;Di Geronimo et al, 2009;Feldens et al, 2009Feldens et al, , 2012. In some regions, the seabed was subjected to offshore tin mining (Usiriprisan et al, 1987). The climate of this region is dominated by monsoons, with the southwest monsoon remaining active from May to September and generating heavy rainfall during this period.…”

“…The shelf is relatively flat and sloping offshore, reaching water depths of 50 m at a distance of 25 km from the coastline. The sea bottom is mainly composed of sand, sandy mud, muddy sand and bedrock outcrops (Usiriprisan et al, 1987;Di Geronimo et al, 2009;Feldens et al, 2009Feldens et al, , 2012. In some regions, the seabed was subjected to offshore tin mining (Usiriprisan et al, 1987).…”

“…Tsunami impacts (erosion) on the seafloor have primarily been examined indirectly through studies of onshore deposits containing marine microfossils such as foraminifera (e.g., Hawkes et al, 2007;Uchida et al, 2010) or diatoms (e.g., Dawson, 2007;Sawai et al, 2009). The studies of shallow marine tsunami deposits that have been conducted to date have indicated that there is a large variability in the characteristics of these deposits.…”