One day after Christmas 2004, the most deadly natural disaster in recent history struck a portion of the earth at precisely 07:58:53 local Pacific Standard time.
The Dec. 26 Sumatra-Andaman earthquake produced a huge tsunami that killed more than 283,100 people, 229,886 of which died or went missing from the tidal wave alone.
Recently, a group of scientists at Scripps Institution of Oceanography at the University of California, San Diego helped to pinpoint the full dimensions of the fault rupture that caused that devastating quake.
Conclusions from the team of geologists, geodesists and geophysicists were published in the March 2 issue of the journal Nature.
Findings suggest that previous ideas about where large earthquakes are likely to occur need to be revised. Regions of the planet previously thought to be immune to such major events may actually be at high risk of experiencing them.
California, however, is less prone to large tsunamis because it is in a different tectonic setting than areas like Sumatra and the Pacific Northwest, which are in precarious subduction zones.
San Diego’s coast is in a tectonic regime where plates slide past each other laterally, rather than subducting, or moving vertically.
The 2004 earthquake occurred on what is called a subduction megathrust. It is an earthquake fault where the Indian and Australian tectonic plates are diving beneath the margin of Southeast Asia.
Here, the fault surface lies several kilometers deep inside Earth’s crust.
This fault, much of it beneath the ocean, caused movements at the surface that released a great amount of energy trapped within the fault.
Scripps researchers and other scientists from around the world studied the shift in location of GPS (global positioning system) monuments that had been accurately tracked prior to Dec. 26, 2004.
Yehuda Bock, research geodesist and senior lecturer at Scripps Orbit and Permanent Array Center and director of the California Spatial Reference Center, co-authored the recently published study with eight colleagues. and said that these surveys provide valuable new data that help scientists to better understand major earthquake events.
“These studies provided us with an invaluable record of observations prior to the great Sumatra-Andaman earthquake,” Bock said. “We mobilized quickly after the earthquake to re-survey these monuments. The Indonesian surveys were completed under extremely difficult field conditions in the regions of greatest devastation within several weeks after the event.”
Researchers also studied giant coral heads on island reef and compared satellite images of island lagoons and reefs taken before and after the earthquake. Noted were changes in the color of the seawater near these reefs that indicated a change in the water’s depth and ultimately a rise or fall of the crust at that location.
Researchers concluded that this mighty 2004 earthquake was one of the longest ruptures of the megathrust ever recorded. At a magnitude of 9.15, the quake is the third largest of the past 100 years.
Findings also point toward concern that another earthquake of this magnitude is waiting to happen in the area off the densely populated island of Java.
“Before the Sumatra-Andaman event, this tectonic environment was not thought to produce great earthquakes,” Bock said. “The findings of our paper should provide concern to Indonesian disaster planners.”
The main threats to San Diego coastal communities are from earthquakes where plates slide side to side, like the Rose Canyon fault in San Diego, and earthquakes offshore that could generate tsunamis.
But Bock also said that none of these new findings, nor the fact that earthquake-prone regions need to be re-evaluated affect the probability of a major earthquake along California’s San Andreas Fault, which includes all the faults that make up the plate boundary between North America and Pacific Plates. This zone is several hundred kilometers wide in Southern California. Relative motion is about 50 mm per year.
“In San Diego, we move almost completely with the Pacific Plate, but there is some motion offshore, for example on the San Clemente Fault,” Bock said. “A tsunami could be generated by offshore faults in two ways. One is from rapid uplift of the sea floor such as occurred in Sumatra, and the second from landslides generated by an earthquake.”
With these Sumatran findings, Southern California’s tsunami probability status is about the same as it was before the 2004 earthquake. However, the researchers now know from the three types of studies that conditions for a giant earthquake exist off the southern coast of Chile, where a large earthquake occurred in 1960.
Scientists are now revisiting areas where a large rupture could occur and cause a possibly devastating tsunami surge near the coast. They note possible concerns for the Ryukyu Island area between Taiwan and Japan, where a quake could create a tsunami off the south coast of China. The Pacific Northwest of the United States, where a giant earthquake occurred in the 1700s, and the Caribbean subduction zone from Trinidad to Barbados and Puerto Rico are also concerns.
All subduction zones, even quiet ones, are capable of producing heavy seismic activity resulting in great earthquakes. Scientists have begun better monitoring systems by placing $1 million recording GPS station networks more prominently around the world.
Bock said that Scripps has started working with scientists at the Canadian Pacific Geocsciences Center in Vancouver, British Columbia, to use new real-time GPS monitoring techniques. These techniques developed by Scripps provide the basis for a tsunami warning system for the Southwestern Canadian coastline.
Though the United States participates in a tsunami warning and watch system, there is little formal technology in place to provide San Diego with this type of a sophisticated warning system.
“A 20-station real-time GPS monitoring system that we have recently completed with San Diego County could serve as one component of a tsunami warning system, by measuring mostly vertical ground displacements,” Bock said. “We are working more closely with Canadian scientists on a warning system in the Vancouver area, which is much more at risk than Southern California.”
Site measurements are obtained in British Columbia almost instantaneously and evaluated to provide early warnings when necessary. Part of this Canadian system will soon be expanded to the U.S. portion of the same zone.
“Although we do not expect to experience a great earthquake in San Diego, such as the one that occurred in Sumatra, it is important to recognize the earthquake hazards in our region and prepare accordingly,” Bock added. “The lesson from Hurricane Katrina, and other natural disasters, is that families need to be fully prepared and not expect immediate help from the authorities.”