Oregon State University

College of Earth, Ocean, and Atmospheric Sciences

Revisiting the Site of the 2010 Megathrust Earthquake off the Coast of Chile

Anne Trehu aboard R/V Melville

May 16, 2012

Among the hazards that arise from large earthquakes are the direct effects of great earthquakes, as evidenced by the devastating earthquake off Japan in 2011, and local and distant effects of tsunamis. Though earthquakes and tsunamis pose enormous risks to life and property in highly populated areas, neither of these phenomena is as yet well understood.

The recent great earthquake in Chile did not generate a large tsunami, probably because the rupture did not reach the seafloor. Available evidence suggests that slip did not occur at depths shallower than about 30 km down-dip from the trench. Researchers infer that the outer accretionary wedge that lies along the Chile coast above the rupture zone behaves as a poro-elastic medium that will adjust over time to the new stress regime caused by the earthquake.

Anne Trehu is leading a research cruise sponsored by the National Science Foundation to study the seafloor off Maule, Chile, to monitor the post-seismic response following a megathrust earthquake which occurred there on February 27, 2010.

Researchers are deploying a focused array of ocean bottom seismographs equipped with flow meters. The scientists propose to evaluate how the outer accretionary prism, where sediments are accreted onto the non-subducting tectonic plate, responds to the change in tectonic stress that resulted from slip of the subduction fault during the earthquake. In particular, scientists will monitor for seismic tremor and for low frequency earthquakes as well as for normal earthquakes in the study area and underlying subducting crust and for slow fluid flow out of the seafloor that can be modeled to derive volumetric strain in the underlying sediments.

Map of the Chile subduction zone

The principal broader impact of this effort is considerably improved understanding of megathrust earthquakes and tsunamis. This subduction zone setting is typical of numerous locations around the world, and the results of the proposed survey will have broad application. These are settings that generate the world's largest and most destructive earthquakes and tsunamis, and the results of this study will have broad implications for geohazards studies and societal benefit.

Another important aspect of the research cruise is education. Six students will be processing seismic and bathymetric data on board as well as standing watch, and shipboard discussions will include informal lectures about the principles underlying the procedures.

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