One year after Solomon Islands, scientists learn barrier to earthquakes weaker than expected
On the one year anniversary of a devastating earthquake and tsunami in the Solomon Islands that killed 52 people and displaced more than 6,000, scientists are revising their understanding of the potential for similar giant earthquakes in other parts of the globe.
Geoscientists from The University of Texas at Austin鈥檚 Jackson School of Geosciences and their colleagues report this week that the rupture, which produced an 8.1 magnitude earthquake, broke through a geological province previously thought to form a barrier to earthquakes. This could mean that other sites with similar geological barriers, such as the Cascadia Subduction Zone in northwestern North America, have the potential for more severe earthquakes than once thought.
In an advance online publication in the journal Nature Geoscience, the scientists report that the rupture started on the Pacific seafloor near a spot where two of Earth鈥檚 tectonic plates are subducting, or diving below, a third plate.
The two subducting plates鈥攖he Australian and Woodlark plates鈥攁re also spreading apart and sliding past one another. The boundary between them, called Simbo Ridge, was thought to work as a barrier to the propagation of a rupture because the two plates are sliding under the overriding Pacific plate at different rates, in different directions, and each is likely to have a different amount of built-up stress and friction with the overlying rock. But the boundary did not stop the rupture from spreading from one plate to the other.
鈥淏oth sides of that boundary had accumulated elastic strain,鈥� says Fred Taylor, a researcher at the university鈥檚 Institute for Geophysics and principal investigator for the project. 鈥淭hose plates hadn鈥檛 had an earthquake for quite a while and they were both ready to rupture. When the first segment ruptured, there was probably stress transferred from one to the other.
鈥淲hat our work shows is that this is a barrier, but not a reliable one,鈥� says Taylor. In other words, it resists rupturing, but not insurmountably. The work has implications for earthquakes in other parts of the world.
鈥淐ascadia is an important boundary because of its potential for a great earthquake in the future,鈥� says Taylor. 鈥淵ou have these transform faults separating the plates鈥擩uan de Fuca, Gorda and Explorer. If such boundaries are not a barrier to rupture in the Solomons, there鈥檚 no reason to believe they are in Cascadia either.鈥�
The last great earthquake along the Cascadia Subduction Zone was in the year 1700. The intensity of the quake has been estimated at around magnitude 9. If it happened today, it could be devastating to people living in the northwestern U.S. and western Canada. The geological record suggests such great quakes occur there every few hundred years.
The scientists were able to piece together where and how the fault near the Solomons ruptured by observing how it affected corals living in shallow water around the islands. Because corals normally grow right up to the low-tide water mark, scientists can readily measure how far they have been displaced up or down by an earthquake. In the case of uplift, scientists measure how far the coral dies back from its previous height as a result of being thrust up out of the water. In the case of subsidence, scientists measure how deep the coral is compared to its usual maximum depth below sea level.
鈥淚n many ways the corals are much better than manmade instruments as you don't need to deploy corals or change their batteries鈥攖hey just go on measuring uplift and subsidence for you anyhow,鈥� says Taylor.
With funds from the Jackson School of Geosciences, Taylor was able to travel to the Solomons just 10 days after the earthquake to make observations, an extremely swift trip in the world of scientific field work. It was part of a new rapid response capability the Jackson School is developing for research that cannot wait several months for government or foundation grants to be approved.
鈥淭he trip wouldn鈥檛 have happened without the Jackson School support,鈥� said Taylor. 鈥淲e are extremely grateful for that.鈥�
Source: University of Texas at Austin
