EL CERRITO, CA, January 9, 2007 -- The great Sumatra-Andaman earthquake of 2004 is the best recorded large earthquake in history and has revealed the extent of study still necessary to understand such devastating events. New data overturn the commonly held view that great earthquakes only occur in fast, young subduction zones.
The 2004 earthquake is the focus of the January special issue of the Bulletin of the Seismological Society of America (BSSA), in which scientists present research and analyses about the current state of earthquakes and tsunamis, as learned from the Sumatra--Andaman event.
"The 2004 earthquake necessitates a revisitation of commonly accepted views on the relationship between the size of great earthquakes and physical characteristics of subduction zones," write guest editors Susan L Bilek, Ph.D., of New Mexico Tech; Kenji Satake, Ph.D., of Active Fault Research Center in Japan; and Kerry Sieh, Ph.D., of California Institute of Technology.
A great earthquake is defined as having a moment magnitude greater than 9.0. The moment magnitude scale refers to the physical size of fault rupture and the movement across the fault, thereby measuring the strength of the earthquake. Prior to the 2004 earthquake, scientists assumed that great earthquakes only occurred at the site of a young subduction zone, where one tectonic plate is pushed quickly underneath another one. Scientists are rethinking their theories, based on data collected after the earthquake.
The 2004 Sumatra-Andaman earthquake is the best recorded seismic event in history. With a Moment magnitude of 9.1 - 9.3, it ranks as the third largest earthquake in recorded history and the first to supply sufficient data to allow for a detailed analysis of what exactly occurred at the source of rupture. This issue's articles by Rhie et al., and Chlieh et al., combine several different datasets to present a more complete picture of the fault rupture.
In order to evaluate the 2004 great earthquake, scientists took advantage of new technology, such as GPS, satellite telemetry, field tests, and unique datasets developed to understand this giant earthquake and subsequent tsunami. Both the duration of the earthquake, nearly 600 seconds, and the length of the rupture, between 1250 km and 1600 km, exceeded any previously recorded. Limited historical data had suggested that such a powerful earthquake was not possible at this site.
"Because of the long duration and size of the earthquake, scientists have developed new techniques to analyze earthquakes such as these," explains Bilek. "Many of these new techniques incorporate a range of the new technologies, thus giving a better picture of what happened on the fault."
The special issue features research on the key aspects of the great earthquake. Highlights include an article by authors Stein and Okal, who suggest that the correlation between great earthquakes and fast, young subduction zones disappears when a longer time frame is considered. The short earthquake history sampled doesn't address the rarity of earthquakes with a magnitude >9.0, making it difficult to assess risk of great earthquakes in subduction zones elsewhere.
Scientists seek to understand this great earthquake and tsunami in order to better prepare for similar events. Authors Rajendran et al., suggest that a similar event in the exact location is unlikely. Results from preliminary study of the Andaman Islands and the coast of India suggest that great earthquakes and resulting tsunami are quite rare. Based on the evaluation of sand layers mixed with archeological ruins on the Indian coast, authors suggest that the previous great earthquake and tsunami would likely have occurred 1000 years ago.
The guest editors write of the potential new understanding that will result from focusing on the 2004 earthquake: "Hopefully, the legacy of the science presented in this volume will be a greater understanding of earthquake and tsunami processes that will be useful in advancing the resilience of our communities to Nature's violence."
The Bulletin of the Seismological Society of America (BSSA) is the premier journal of advanced research in earthquake seismology and related disciplines. It first appeared in 1911 and was issued on a quarterly basis until 1963. Since 1963, it has appeared bimonthly (in February, April, June, August, October, and December).