HR: 0800h
AN: T11A-0362    [Abstracts]
TI: A comparison of grading in deposits from five tsunamis: Does tsunami wave duration affect grading patterns?
AU: * Higman, B
EM: hig314@u.washington.edu
AF: University of Washington, 310 Condon Hall Box 351310, Seattle, WA 98195
AU: Jaffe, B
EM: bjaffe@usgs.gov
AF: USGS Pacific Science Center, 400 Natural Bridges Dr., Santa Cruz, CA 95060
AB: The December 26, 2004 tsunami left behind sandy deposits that had more complex vertical grading than those seen in several other recent tsunamis. We propose that this is because this great subduction zone tsunami was composed of waves with longer inundation duration, providing time to record complex fluctuations in the structure of the wave. Great subduction zone earthquakes such as the Dec. 26 2004 Sumatra earthquake produce some of the longest duration tsunami waves. This reflects their wide source, about 250 km in the case of the 2004 Sumatra earthquake. The long period waves in these tsunamis inundate far inland and only decrease tens of centimeters per 100 meters inland. They wax and then wane, but spend a significant period of time as "river-like" flows moving across the land. Generally they are more energetic near the beach where they erode sediment, and lose energy as they travel inland. Tsunamis generated by smaller earthquakes and landslides generally lead to a shorter inundation duration, reflecting their smaller sources. They do not inundate for long enough to reach their maximum elevation at the limit of inundation; flow elevation can drop a meter or two per 100 meters inland from the shore. The steep front of these shorter waves means that little time is spent in the waxing of the flow; the flow suddenly appears and then wanes. They are also most energetic near the beach. Deposition by a tsunami occurs when water in the flow loses its sediment transport capacity. Sediment capacity may decrease temporally as the flow wanes, or it may decrease spatially as the flow moves inland. Shorter period tsunamis decrease in sediment capacity rapidly through time and space, and result in normally graded deposits. Conversely, long period tsunamis have times of temporal increase in flow coupled with spatial decrease. These may be during the initial inundation, or just minor increases on top of the larger wave structure. The complicated flow history coupled with longer duration spatial decrease in transport capacity results in a grading that is complex, including normal, massive, and inverse grading. We measured vertical grading in the 1992 Nicaragua, 1998 Papua New Guinea, 2001 Peru, 2004 Sumatra, and 2005 Sumatra tsunami deposits. The events with narrower source regions are more likely to be simply normally graded, and the events with the widest source have the most complex deposits. This effect of inundation duration could provide a basis for distinguishing the deposits of great subduction zone earthquakes from more local tsunamis.
DE: 3022 Marine sediments: processes and transport
DE: 3040 Plate tectonics (8150, 8155, 8157, 8158)
DE: 3060 Subduction zone processes (1031, 3613, 8170, 8413)
DE: 7215 Earthquake source observations (1240)
DE: 8100 TECTONOPHYSICS
SC: Tectonophysics [T]
MN: Fall Meeting 2005