Monday, May 10, 2010

Peruvian tectonic plates move by earthquakes and non-seismic slip

LIVERMORE, Calif. – Just a few years ago, Dan Farber happened to be doing field work in Peru with students when the 8.0 Pisco earthquake struck.

As a scientist working in the active tectonics of the Peruvian Andes — funded through the Lawrence Livermore National Laboratory’s Institute for Geophysics and Planetary Physics — Farber was asked by colleagues if he could participate in a rapid response team to map the damage of the seismic deformation and install a system of geodetic stations.

Views of the city of Pisco, Peru following the Aug. 15, 2007 magnitude 8.0 earthquake. In much of the city, the earthquake resulted in an almost a complete leveling of all unreinforced masonry structures and left much of the city without electricity and water for weeks.

He jumped at the opportunity to install a Global Positioning System (GPS) network to capture the post-seismic response and collected critical geological data for the understanding of the inter-plate dynamics of one of the Earth’s largest subduction zones — the Central Peru Megathrust.

In a new paper appearing in the May 6 edition of the journal Nature, Farber and international colleagues determined that the seismic slip on the Central Peru Megathrust is not dependent on earthquakes alone. As it turns out, movement along this subduction zone is caused by earthquakes as well as non-seismic (aseismic) related slip from steady or transient creep between or directly after earthquakes.

“Active faults are made up of areas that slip mostly during earthquakes and areas that mostly slip aseismically,” Farber said. “The size, location and frequency of earthquakes that a megathrust can generate depend on where and when aseismic creep is taking place.”

The 8.0 Pisco earthquake that occurred in 2007 ruptured the subduction interface — where load-bearing flat surfaces butt up — between the Nazca plate and the South American plate, an area that subducts about 6 centimeters per year. In this event, two distinct areas moved 60 seconds apart in a zone that had remained locked in between earthquakes. The event also triggered aseismic frictional afterslip on two adjacent areas.

The most prominent afterslip coincides with the Nazca ridge subduction, which seems to have repeatedly acted as a barrier to seismic rupture propagation in the past.

To sum up, aseismic (non-earthquake producing) slip accounts for as much as 50 percent to 70 percent of the slip on this portion of the megathrust in central Peru. Because much of the interface displacement is taken up aseismically, an earthquake the size of the 2007 earthquake is estimated to occur only every 250 years.

Other collaborators included those from: Institut de Recherche pour le Développement, the Instituto Geofisico del Peru, California Institute of Technology, Géosciences Azur, University of California Santa Cruz and Université Paul Sabatier/CNRS/IRD.

Founded in 1952, Lawrence Livermore National Laboratory is a national security laboratory that develops science and engineering technology and provides innovative solutions to our nation's most important challenges. Lawrence Livermore National Laboratory is managed by Lawrence Livermore National Security, LLC for the U.S. Department of Energy’s National Nuclear Security Administration.

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