2005 IRISH SCIENTIST YEAR BOOK

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University of Ulster
Sandy Steacy, John McCloskey, Suleyman Nalbant
Earthquake hazard in Sumatra

Research by the Geophysics Research Group on the rapid computation of stress changes following large earthquakes is improving our ability to make short-term estimations of seismic hazard. Recent world-wide research has demonstrated that very small stress changes can greatly influence the location of subsequent events; regions that experience positive stress changes of as little as 0.1 bar become much more likely to have earthquakes while a temporary cessation of seismic activity is often observed in areas experiencing stress decreases of a similar magnitude.

On St. Stephen's Day, 2004, a great earthquake occurred in the Indian Ocean. At magnitude 9.3, this earthquake was the second largest ever recorded; the tsunami generated by it had tragic consequences for Indonesia, Thailand, Sri Lanka, India, and other countries in the region. Following this earthquake, we calculated the stress change on nearby faults and found that both the Sumatra fault and the Sunda Trench (see figure) had experienced large stress increases, up to 8 bars. Both of these structures are capable of producing large earthquakes; the Sumatra Fault as large as about M=7.5, the Sunda trench as large as M=9. This work was published in Nature on 17 March 2005.


On 28 March 2005, a second great earthquake occurred on the Sunda Trench. This earthquake measured 8.7 on the Richter scale but fortunately did not produce a tsunami. As shown in the figure, the earthquake nucleated at a location where the stress had increased as a result of the St. Stephen's Day event (black star) but not at the place where the stress increase was greatest. The rupture plane of the earthquake (shown by the black rectangle) did, however, include all the highly stressed portion of the Sunda Trench. Unfortunately, like its predecessor, this earthquake has redistributed stresses but not relieved them, hence the seismic hazard in Sumatra remains high.

More information about the Geophysics Research Group and its work on earthquake physics, fluid flow in fractured porous media, and aeolian sediment transport can be found at http://www.science.ulster.ac.uk/geophys/ or by emailing Dr. Sandy Steacy at [email protected]