GPS data help warn of rare tsunamis

Using data from GPS receivers and seismographs, three seismologists may have found a way to identify tsunami earthquakes in time to warn people

A few times a century, a medium-sized earthquake causes a large and devastating tsunami. The most recent occurrence was in 2010, when a magnitude 7.8 earthquake off the Mentawai Islands in Indonesia set off a tsunami that was more than 50 feet high in some places, killing 509 people and displacing 15,000.

While rare, these tsunami earthquakes are particularly dangerous because they can hit coastal communities within five to 15 minutes, before officials can issue a warning. Now, however, using data from GPS receivers and seismographs near the 2010 Mentawai event, three seismologists — Valerie Sahakian and Diego Melgar at the University of Oregon and Muzli Muzli at the Earth Observatory of Singapore — may have found a way to identify tsunami earthquakes in time to warn people.

Very large earthquakes under an ocean break both the deeper part of a subduction zone, where one tectonic plate is sinking beneath another, as well as its shallow part, in a rapid motion that creates a tsunami. Tsunami earthquakes, on the other hand, happen almost entirely in the soft, weak section of a fault, moving slower and creating much more movement on or near the sea floor compared to earthquakes of the same size that happen in rigid rock. This creates much larger tsunamis than expected. A tsunami earthquake might have the same magnitude as an earthquake that occurs in rigid rock but produces much less of what seismologists call high-frequency energy.

Currently, officials issue tsunami warnings within tens of minutes of detecting an earthquake above a certain magnitude within a certain distance of a coastal area. This method, however, fails in the case of tsunami earthquakes, which produce tsunamis that are disproportionate to their magnitude.

Indian Ocean (Jan. 2, 2005): A village near the coast of Sumatra lays in ruin after a tsunami struck South East Asia. (Photo: U.S. Navy/Photographer’s Mate 2nd Class Philip A. McDaniel)

Traditionally, scientists have detected tsunami earthquakes by comparing their seismic magnitude with the amount of high-frequency energy they radiate, both recorded by distant stations. Tsunami earthquakes have a very low ratio of energy to magnitude; their energy, instead of strong shaking, produces a large slow movement of the seafloor.

In the past, scientists had to measure this ratio using seismic waves that had traveled from the earthquake’s epicenter to seismographs hundreds or thousands of miles away. This did not give them enough time to identify tsunami earthquakes and warn people before the tsunami’s wave hit the coast.

The recent analysis, however, enabled scientists to figure out a faster way to identify these rare tsunami earthquakes by using two proxies:

• data from seismic stations onshore near the epicenters of 16 earthquakes that measured directly how much the ground shook in each case, to determine the amount of high frequency energy in each earthquake, and
• data from GPS stations close to the earthquakes, to measure the magnitude of each one on the basis of how much it moved the ground.

The GPS stations used in this study were from the Badan Informasi Geospasial (BIG) network from Indonesia. The data were acquired in real-time but processed with final orbits and clocks using precise point positioning (PPP). The scientists averaged the 3-component displacement, using centimeter-level solutions, and saw 3-10 centimeter vertical displacement.

This methodology, using data available during and immediately after an earthquake, enables scientists to compare the amount of energy in each earthquake with its magnitude, without waiting for their seismic waves to travel to distant measuring stations. Seismologists will be able to use this approach to identify tsunami earthquakes immediately and warn nearby coastal communities before a tsunami wave reaches them.

Citation. Sahakian, V. J., Melgar, D., & Muzli, M. (2019). “Weak near-field behavior of a tsunami earthquake: Toward real-time identification for local warning.” Geophysical Research Letters, 46(16), 9519–9528.