By Marci Wills, March 6, 2010
The enormous magnitude 8.7 earthquake which struck Chile last Saturday (February 27th) in startling proximity to the Haiti disaster has lead some of my friends and family to an apocalyptic level of speculation. They want to know what freak force of nature caused these earthquakes to occur so close together? But it is important to remember that earthquakes are a normal occurrence on Earth that can happen at any time. In fact, last week’s upheaval in Chile came as no surprise to the scientific community and to two geophysicists in particular who expected it to happen.
Jian Lin of the Woods Hole Oceanographic Institute (Woods Hole, MA) and Ross Stein of the United States Geological Survey (Menlo Park, CA), had anticipated an earthquake in the location of the Feb 27th event since the completion of their research in the region 6 years ago. In a paper published in the Journal of Geophysical Research in February of 2004, they warned that this area was at increased risk of a large earthquake due to after effects of the world’s largest recorded quake, a magnitude 9.5 event, which occurred in Chile in 1960.
Both the 1960 and 2010 Chilean earthquakes occurred just off the west coast of Chile along a tectonic plate boundary where the Nazca plate moves beneath the South American plate. Stress accumulates along this boundary until it is suddenly released by movement of the plates during an earthquake. The portion of the plate boundary that moves, called the earthquake’s rupture zone, can stretch hundreds of kilometers. Stress is relieved in this zone following an earthquake but it may increase elsewhere.
Jian Lin and Ross Stein used GPS measurements of tectonic plate motion to estimate changes in stress from the 1960 event and found that stress had greatly increased just north of the 1960 rupture. They predicted that this region would be the next portion of the plate boundary to produce a large earthquake in Chile. Sure enough, last Saturday the 2010 earthquake picked up where the 1960 earthquake rupture left off.
This sort of progression also occurred after the December 26th, 2004 magnitude 9.0 earthquake in Sumatra, when it was followed by a magnitude 8.7 earthquake on the southern end of the rupture zone just 3 months later on March 28, 2005. “The only difference is that it took 50 years for the northern neighboring section of the 1960 [Chile] earthquake to rupture, while it took only 3 months for the southern adjacent segment to rupture in Sumatra”, Lin noted.
Lin believes that the Haiti earthquake similarly transferred stress further east along the Enriquillo Fault which broke on Jan. 12th, but it would have had no effect on the stress state in Chile.
Lin and Stein also used their method of measuring stress changes to assess which faults in the San Andreas region of southern California are most likely to move next, but only time will reveal the accuracy of their predictions there. It seems as though we are another step closer to understanding and anticipating earthquakes, rather than thinking of them as freak events.
Lin, J, and Stein, R.S., 2004, Stress triggering in thrust and subduction earthquakes and stressinteraction between the southern San Andreas and nearby thrust and strike-slip faults: Journal of Geophysical Research, v. 109.