Gravity Shifts Suggest New Approach to Quake Warnings
A new study of high-sensitivity gravimeter and seismometer data—recorded just before and during the magnitude-9.0 earthquake on March 11, 2011, in Tohoku-Oki, Japan—describes a promising new data collection and analysis methodology that could give authorities time to issue advance warnings and secure vulnerable systems before earthquakes strike.
Rather than focus on the propagation of fast-traveling seismic P-waves, which can arrive seconds before ground movement begins, research by an international group of seismologists and physicists correlated seismic wave patterns with subtle shifts in gravity signals at the point of rupture in the days leading up to and during the earthquake.
The team examined data from the 2011 quake, which generated a tsunami that killed nearly 16,000 people, left more than 2,500 missing, 230,000 homeless and damaged several nuclear plants, including Fukushima Daiichi.
Most quakes are too small to generate gravity signals strong enough to register with any but the most sensitive gravimeters, and the signals are difficult to separate from background noise. But the 2011 quake was strong enough that the scientists were able to detect gravity-field changes that began to register before the quake, says Caltech seismologist Jean Paul Ampuero, one of the study authors.
For the new analysis, the researchers studied data captured by a highly sensitive, superconducting gravimeter about 510 kilometers away from the earthquake epicenter, together with data gathered by five broadband seismometers from Japan’s F-Net, a broadband seismograph network.
“While prompt gravity signal detection with state-of-the-art gravimeters and seismometers is challenged by background seismic noise, its robust detection with gravity gradiometers under development could open new directions in earthquake seismology and overcome fundamental limitations of current earthquake early-warning systems imposed by the propagation speed of seismic waves,” the study authors concluded in the Nov. 22 issue of Nature Communications.
Ampuero says that even having a few extra seconds of warning could allow automated systems—such as those used to control fire-station gates, elevators, public transit and heavy machinery—to stop operating before the earthquake hits.
However, he notes that even the most highly sensitive gravimeters have limitations. Development of a more sensitive prototype will take about 10 years and more funding, he says.
Scientists also are interested in using GPS technology to measure ground shaking, says University of Washington seismologist Yong Wei. But GPS also has its limitations. “It’s difficult to get the data out” of the devices and transmit the information in a useful and timely way to the public, he says.
Adding GPS technology to the existing network of seismometers could “improve the quality of information,” in that there would be a more robust collection of data, but it is not certain that the information will be provided “any faster,” Ampuero notes.
A magnitude-7.4 earthquake struck the same area of Japan on Nov. 22, although it did not set off a significant tsunami. Yet the event demonstrated to authorities and seismologists that, since the 2011 quake, improvements have been made in both the seismic monitoring network and the J-Alert communication system. Those improvements could help to save lives in a future seismic event.
Robert Weiss, associate professor of geosciences at Virginia Tech, says Japan has added more sensors to its F-Net and created more redundancy in its J-Alert system, using cell phones, texts, email and other systems to get the word out about an imminent earthquake and tsunami risk. “Japan has learned from failures from the 2011 tsunami,” says Weiss.