HR: 1340h
AN: S53A-1040    [Abstracts]
TI: Role of Splay Faulting and Dispersion in Tsunami Waveforms
AU: * DeDontney, N
EM: ndedontn@fas.harvard.edu
AF: Dept. Earth Planet. Sci., Harvard Univ., 20 Oxford St., Cambridge, MA 02138, United States
AU: Rice, J R
EM: rice@esag.deas.harvard.edu
AF: Dept. Earth Planet. Sci., and Sch. Engin. Appl. Sci., Harvard Univ., 29 Oxford St., Cambridge, MA 02138, United States
AB: Tsunami theory based on standard earthquake source models and shallow-water gravity wave methodology suggests that the 2004 Indian Ocean tsunami should have arrived in Sri Lanka as a large wave followed by a depression. However, eyewitness accounts of the tsunami arrival indicate that there were multiple wave arrivals separated by a recession (Liu et al., 2005). The tsunami waveform was also measured by the Jason-1 satellite two hours after the event, which is approximately the same time as the first arrival in Sri Lanka. The altimetry data agrees with those accounts and shows a double peaked lead wave (Smith et al., 2005). This may be the result of two areas of vertical uplift in the source region caused by slip on both the plate interface and on a steeper splay fault branching from this detachment. Only a small amount of slip is needed on the steep splay to vertically displace the seafloor enough to create a double peaked wave. There have been attempts to distinguish between coseismic slip on the detachment and slip on a splay or a combination of both of these surfaces. GPS inversions by Banerjee et al., attempted to distinguish between these possibilities, but one model of slip distribution was not preferred. The tsunami waveform and arrival time should vary with these models, but inversions using tide gauge data from the 1944 Tonankai event were similarly unable to prefer a model based on splay slip, detachment slip or a combination of the two (Baba et al., 2006). We suggest that the satellite altimetry data can be used as a constraint on splay fault activation. We model the seafloor deformation based on a variety of splay fault locations and slip distributions to determine the criteria for a double peaked arrival at Sri Lanka. The Jason-1 track data indicates that the waves are dispersive (Kulikov, 2005), so we compare the results of linear wave propagation using shallow water theory (c = \sqrt{gh}) versus including wave dispersion (c = \sqrt{g \tanh(kh) / k}), albeit for a model with uniform sea depth h; here k is wave number. The distance between and the relative magnitude of the peaks is determined by the fault position and amount of moment transferred to the splay. Aftershock distribution following the 2004 Sumatra Andaman earthquake hints that splay faults were activated during the event at 70 km and 110 km inward of the trench (Araki et al., 2005). A splay fault location of 70 km inboard does not result in a double peaked wave matching the observed wave, but a fault located 110 km inboard results in a modeled spacing of 106 km between peaks which correlates well with the observed ~115 km from the Jason-1 data. When the effects of dispersion are accounted for in the wave propagation, the shape of the wave changes with the distance from the source. An initially sharp rise in the wave front becomes broader due to the lag of the high frequency components, and the waveform can evolve into two peaks of approximately the same wavelength and magnitude. Such a wave structure is similar to that observed by the satellite. We conclude that dispersion is important to the shape of the propagating wave, if not to its arrival time, and that the source process likely involved a significant component of splay faulting.
DE: 4564 Tsunamis and storm surges
DE: 8170 Subduction zone processes (1031, 3060, 3613, 8413)
SC: Seismology [S]
MN: 2007 Fall Meeting