Restless Plates in the Peruvian Andes
One of my articles last month was about the devastating earthquakes that have plagued many parts of the world this year, such as Haiti, Chile, China, and the Baja. These events cause catastrophic destruction, and are the result of the movement of tectonic plates. The Earth's crust is restless, constantly in motion, and earthquakes represent the most violent shifts. However, even when there are no earthquakes, the tectonic plates have the ability to creep unnoticeably. A scientist from California studied one of the largest subduction zones on Earth, The Central Peru Megathrust in the Peruvian Andes. Through his efforts, the truth behind this phenomenon was determined.
The scientist's name is Dan Farber, and his work in Peru was funded through the Lawrence Livermore National Laboratory Institute for Geophysics and Planetary Physics. Several years ago, his colleagues asked if he could join a rapid response team to map out the damage of a recent earthquake, the 8.0 magnitude Pisco earthquake of 2007. The task also included installing a Global Positioning System (GPS) network at an array of geodetic stations. The GPS system would be used to capture the post-seismic tectonic motion and collect critical geological data to understand the Central Peru Megathrust.
A megathrust refers to megathrust earthquakes which occur at destructive plate boundaries where one plate is forced under another. Often, large sections of the plates get stuck, and their subsequent release produces some of the world's largest earthquakes. Since 1900, all five earthquakes of magnitude 9.0 or greater have occurred at these megathrust fault zones.
Farber's results showed that the seismic slip at this particular subduction zone is not dependent on earthquakes alone. Movement is also caused by non-seismic (aseismic) related slip from a steady or transient creep that occur between or directly following 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 Pisco quake in 2007 ruptured the subduction interface between the South American continental plate and the Nazca oceanic plate. The subduction refers to the oceanic plate sliding underneath the continental plate, pushing it up (Andes Mountains are the result). The area subducts an average of six centimeters per year. The Pisco quake moved two distinct areas in a span of sixty seconds, areas that had been locked in place between earthquakes. The event triggered aseismic afterslip coinciding with the Nazca ridge subduction. In other wordsd, the plates were able to move after they became "un-stuck" during the megathrust earthquake.
The research was summed up by stating that the aseismic slips in the central Peru area account for between fifty and seventy percent of the tectonic movement. Because most of the interface displacement is aseismic, it is estimated that earthquakes the size of the 8.0 Pisco quake occur only every 250 years.
Dan Farber of the Lawrence Livermore National Laboratory was assisted in his studies by a number of international colleagues. Their research has been published in the May 6 edition of the journal, Nature.
Link to article in Nature