2009年02月27日

Sesismic strees at Mentawai

In March 2005, just south of the rupture area of the 2004 great Sumatora-Andaman earthquake, the mangnitude 8.6 Sunde megathrust earthquake occurres. Concern then focused further south on the Mentawai area, wherelarge earthquakes had occurred in 1797 and 1833. A magnitude 8.4 and, twelve hours later, a magunitude 7.9 earthquake did indeed occur in September 2007. Konca et al. now show that the 2007 seismic crisis released only a fraction of the elastic strain that had accumulated since the events of 1797 and 1833, with the same protion of the megathrust rupturing in diffrent patterns depending on local factrors. The potential for a large megathrust event in the Mentawai area is therefore still high.

### DataBace ###
nature Vol.456 545-674 Issue no.7222 4 December 2008
Letter p.631 / Partrial rupture of a locked patch of the Sumatra megathrust durring the 2007 earthquake sequence / A O Konca et al. (California Institute of Technology)

The great Smatora-Andaman earthquake and tsunami of 2004 was a dramatic reminder of the importance of understanding the seismic and tsumami hazards of subduction zones. (Lay,T et al. The great Smatora-Andaman earthquake of 26 December 2004. Science 308,1127-113 /2005)(Chlieh,M. et al. Coseismic slip and afterslip the great Mw 9.15 Sumatra-Andaman earthquake of 2004. Bull. Seismol. Soc. Am. 97, S152-S173/2007)(Ammon,C.J. et al. Rupture process of the 2004 Sumatora-Andaman earthquake. Science 308, 1133-1139/2005)(Subarya,C. et al. Plate-boundary deformation associated with the great Sumatra-Andaman earthquake. Nature 440,46-51/2006).

In March 2005, the Sunda megathrust ruptured agein, producing an event of moment nagnitude (Mw)8.6 south of the 2004 ruputer area (Briggs,R.W. et al. Deformation and slipalong the Sunda Meathrust in the great 2005 Nias-Simeulue earthquake. Science 311,1897-1901/2006) , whish was the site of a similar event in 1861.

Concern was then focused on the Mebtawai area, where large earthquakes had occurred in 1797 (Mw=8.8) and 1833 (Mw=9.0).(Newcomb,K. & McCann,W. Seismic history and seismotectonics of the Sunda arc. j. Geophys. Res. 92,721-439/1987)(Natawidjaja,D.H. et al. Source parameters of the gerat Sumatran megathrustearthquakes of 1797 and 1833 inferred form coral microatolls. J.Geophys. Res. 111, doi:10.1029/2005JB004025/2006)

Two earthquakes, one of Mw=8.4 and, twelve hours later, one of Mw=7.9, indeed occurred there on 12 September 2007.

Here we show that these earthquakes ruptured only a fraction of the area ruptured in 1833 and consist of distinct asperities within a patch of the megathrust that had remainned locked in the interseismic pariod.

This indicates that the same portion of a megathrust can rupture in different patterns depending on whether asperities break as isolated seismic events or cooprate to produce a larger rupture.

This variability probably arises from the influence of mon-permenent barries, zones with locally lower pre-stress due to the past earthquakws.

The stress state of the portion of the Sunda megathrust that had ruptured in 1833 and 1797 was probably not adequate for the developemnt of a single large rupture in 2007.

The moment released in 2007 amounts to only a fraction both of that released in 1833 and of the defict of moment that had accumulated as a result of interseimic strain since 1833.

The potential for a large megathrust event in the Mentawai area thus remains Large.

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