Researchers in California have developed a system that can rapidly determine the size of an earthquake and the extent of its impacts within a fault zone, including its potential for triggering a devastating tsunami. The researchers have used the system – which is based on GPS measurements – to accurately model two historic earthquakes in Japan and northern Mexico.
The 2011 Japanese earthquake disaster showed that the first few minutes after an earthquake are critical. When the Tōhoku earthquake struck, it took geophysicists more than 20 min to compute that the earthquake was magnitude 9.0 on the Richter scale. Had the authorities known the full extent of the earthquake sooner, it would have given them valuable time to activate early-warning systems to help prepare people for the large tsunami that would inevitably follow.
The speed of response in current systems is constrained by the fact that instruments in seismic stations close to major earthquakes tend to be saturated by periods of intense shaking. Therefore, to determine the size and extent of earthquakes, seismologists need to look at data from a range of stations further afield. What is more, because instruments in seismic stations cannot measure the full range to which tectonic plates are displaced, earthquake magnitudes are often underestimated in the first minutes following an earthquake.
The view from above
An alternative approach that has been developed over the past two decades is to use GPS data to monitor the Earth’s crust from space. The basic principle is to create a regional network of GPS stations where geoscientists can then track the position of the stations within a given geographical area. Following an earthquake, scientists can examine the movements of these GPS stations relative to each other in order to work out the degree to which the land has shifted.
Brendan Crowell, Yehuda Bock and Diego Melgar at the Scripps Institute of Oceanography at the University of California, San Diego have now developed this approach into a system for modelling the extent of earthquakes in detail. The system is based on a mathematical model that enables the researchers to use regional GPS data to recreate fault planes and to characterize the activity within these zones during an earthquake. They claim that their system can ascertain the magnitude of an earthquake significantly faster than is possible with traditional seismic methods. It would have enabled seismologists to identify the magnitude-9 Japanese earthquake within 2 to 3 min, Crowell told physicsworld.com.
Crowell and his team say that the model can be run in two different ways. The first is to apply the model to a region where the network of faults is already well known to geoscientists. In the second approach, the model is applied to a region where the details of the fault zone are not so well established. In this second instance, the model is capable of using the GPS data to recreate the network of faults beneath the Earth’s surface by using a mathematical function known as a centroid moment tensor.
Recreating major quakes
Publishing their findings in a paper to appear in Geophysical Research Letters, Crowell’s team demonstrates its model for two large earthquakes. The first example used GPS data from 356 GEONET stations to build a picture of the magnitude-8.3 Tokachi-oki earthquake that struck 100 km offshore from Japan’s Hokkaido island. The second case used GPS data from 95 stations in the California Real Time Network (CRTN) to recreate the magnitude-7.2 El Mayor-Cucapah earthquake that struck the Baja California region of northern Mexico in 2010.
In both cases Crowell’s group was able to determine the earthquake magnitudes in less than 2 min, improving on traditional seismic methods by a factor of 10. The researchers found that in the case of the Japanese quake, the method of predefining the faults worked better because the fault system gets more complicated with depth. In the Mexican case study, the method of recreating the fault zone worked better too, partly because the dip angle of the fault does not change much with depth, allowing for a simple representation of the fault. Crowell told physicsworld.com that his group has also used this GPS method to model the 2011 Tōhoku earthquake and it is currently working on a separate paper to describe this work.
The research is funded in part by NASA, and Crowell says that his group is developing a prototype working system that will be deployed within the next year and monitored from the Scripps Institute. He cautions that the system should not be viewed as a true early-warning system, because that would require a response within seconds of an event. “It can, however, assist first-responders to locate the regions of greatest damage more accurately. For tsunami modelling, this method would work perfectly and could speed up the current modelling by tens of minutes,” he says.
