2009年03月01日

SEISMLOGY : Greatness thrust upon them

The latest research seems to imply that all earthquakes are born eaual. But combining that insight with earlier, seemingly contradictory, work could help us to tell which tremors grow to become more equal than others.

In an analysis on page 358 of this issue, Steven Wesnousky provides strong evidence that the ultimate size of a seismic ruputure is largely controlled by the structure of the underlyng fault, and therebefore that big earthquakes do not differ from small earthquakes in their beginnings.(Wesnousky,S.G et Nature444,358-360/2006)

These data might seen to conflict with earlier observations implying that the size of an earthquake is determined by the dynamics of ruputure onset.

In fact, both conclusions could be true, and combining these two data sets in future analyses of seismic hazards might result in a better prediction of the eventual size of an earthquake before the shaking stops.

Many Earth scientists have long suspected that the limit of an earthquake rupture, and therefore the magnitude of an event, is largely controlled by the structure of the fault zone and variations in stress along the fault itself.(Segall,P. & Pollard,D.D.J. Geophys.Res.85,4337-4350/1980)(Sibson,R.H. Nature316,248-251/1985)(Harris,R.A & Day,S.M,Geophys. Lett.26,2089-2092/1999)

In this view, all earthquakes begin in the same way, and continue to propagate until they reach a barrier, in the from of a structural complexity or a part of the fault on which the stresses are sufficiently low to stop rupture. Bur the alternative standpoint - that large earthquakes are in some way born differenty from their smaller brethren - is attractive becouse it holds thepromise of determining the evevntual size of anearthquake during the first fewseconds of the rupture.(Ellsworth, W.L. & Beroza, G. C. Science 268,851-855/1995)(Olsen,E.L. & Allen, R.M. Nature438,212-215/2005)

That could provide the basis for an wffective eary-waring system.


### DataBace ###
nature Vol.444 243-400 Issue no.7117 16 November 2007
News & Views p.276 / SEISMLOGY : Greatness thrust upon them / James F. Dolan
posted by 0≠素子 at 02:47| Comment(0) | earth | このブログの読者になる | 更新情報をチェックする

Rotation at faults

An early finding of the San Andress Fault Ovservatory at Depth (SAFOD) project, currently drilling an inclined borehole across the San Andreas Fault Zone to a 4-km depth, is that strees is rotated in the vicinity of the fault zoone.

Why this should occur here and at other similar 'weak' faults is not yet clear. A new model to help explain the phenomenon has been developed, based on field structure date of a large fault zone, lab measurements of rock elasticity, and numerical modelling.

The model suggests that is re-oriented around faults by the change in rock elasticity resulting from microscale damage produced by the fault.

### DataBace ###
nature Vol.454 789-970 Issue no.7121 14 December 2007
Letter p.922 / Slip on 'weak' faults by the rotation of regional stress in the fracture damage zone / D R Faulkner et al.(University of Liverpool)


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posted by 0≠素子 at 01:08| Comment(0) | earth | このブログの読者になる | 更新情報をチェックする
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