HR: 13:45h
AN: U43A-02 INVITED     [Abstracts]
TI: Ultra-long period seismic moment of the Sumatra earthquake: implications for the slip process and tsunami generation
AU: * OKAL, E A
EM: emile@earth.nwu.edu
AF: Northwestern, Univ., Evanston, IL 60208 United States
AU: STEIN, S
AF: Northwestern, Univ., Evanston, IL 60208 United States
AB: We have estimated the seismic moment at ultra-long periods using split singlets of the normal modes 0s2, 0s3, and 0s4, as well as the radial modes 0s0 and 1s0. Consistent estimates of moment and Q for the split modes result from fitting both amplitude spectra and the decay of narrow-band filtered singlets (we leave Q constrained for the radials); Q estimates are consistent with previously reported values. We find that seismic moment increases steadily with period from 4E29 dyn-cm inverted by the CMT project at T = 300 s to 1.3E30 dyn-cm (Mw = 9.3) at the period of 0s2 (3231 s), making the Sumatran earthquake the second largest ever recorded instrumentally. The larger magnitude likely reflects slow slip along the entire rupture zone suggested by aftershocks. The systematic increase in moment with increasing period is consistent with this idea, and has not been previously observed for other earthquakes, raising important issues notably at what period the moment ultimately stabilizes. The variation of moment with frequency follows an f**(-1/2) law, thus indicating a source time function more complex than a simple boxcar. The larger moment is fit by a slip of 13 m on a fault 1200 km long by 200 km wide, a larger area than implied by body wave inversions that limited slip to its southern part. The new source model is supported by the successful modeling of the tsunami on the high seas, as detected by the JASON satellite, using a source including the northern segment [V. Titov, pers. comm.], and by the directivity of the tsunami towards Sri Lanka and East India. If the entire aftershock zone slipped, strain accumulated on the northern part of the rupture has also been released, leaving no immediate danger of a comparable tsunami being generated on this segment of the plate boundary. However, the danger of a local tsunami due to a large aftershock or of a large tsunami from a great earthquake on segments to the south remains.
DE: 7209 Earthquake dynamics and mechanics
DE: 7255 Surface waves and free oscillations
SC: Union [U]
MN: 2005 Joint Assembly