Earthquake acoustics offer hint that a tsunami is imminent, researchers find

Jun 05, 2013 by Bjorn Carey

(Phys.org) —On March 11, 2011, a magnitude 9.0 undersea earthquake occurred 43 miles off the shore of Japan. The earthquake generated an unexpectedly massive tsunami that washed over eastern Japan roughly 30 minutes later, killing more than 15,800 people and injuring more than 6,100. More than 2,600 people are still unaccounted for.

Now, by Stanford scientists reveal that in the ocean produced by the earthquake probably reached land tens of minutes before the . If correctly interpreted, they could have offered a warning that a large tsunami was on the way.

Although various systems can detect undersea earthquakes, they can't reliably tell which will form a tsunami, or predict the size of the wave. There are ocean-based devices that can sense an oncoming tsunami, but they typically provide only a few minutes of advance warning.

Because the sound from a will reach land well before the water itself, the researchers suggest that identifying the specific of tsunami-generating earthquakes could lead to a faster-acting warning system for massive tsunamis.

Discovering the signal

The finding was something of a surprise. The earthquake's epicenter had been traced to the underwater Japan Trench, a about 40 miles east of Tohoku, the northeastern region of Japan's larger island. Based on existing knowledge of earthquakes in this area, seismologists puzzled over why the earthquake rupture propagated from the underground fault all the way up to the , creating a massive upward thrust that resulted in the tsunami.

Direct observations of the fault were scarce, so Eric Dunham, an assistant professor of geophysics in the School of , and Jeremy Kozdon, a postdoctoral researcher working with Dunham, began using the cluster of supercomputers at Stanford's Center for Computational Earth and Environmental Science (CEES) to simulate how the tremors moved through the crust and ocean.

This video is not supported by your browser at this time.
Eric Dunham, assistant professor of geophysics in the School of Earth Sciences, talks about the research.

The researchers built a high-resolution model that incorporated the known geologic features of the Japan Trench and used CEES simulations to identify possible histories compatible with the available data.

Retroactively, the models accurately predicted the seafloor uplift seen in the earthquake, which is directly related to tsunami wave heights, and also simulated sound waves that propagated within the ocean.

In addition to valuable insight into the seismic events as they likely occurred during the 2011 earthquake, the researchers identified the specific fault conditions necessary for ruptures to reach the seafloor and create large tsunamis.

The model also generated acoustic data; an interesting revelation of the simulation was that tsunamigenic surface-breaking ruptures, like the 2011 earthquake, produce higher amplitude ocean acoustic waves than those that do not.

The model showed how those sound waves would have traveled through the water and indicated that they reached shore 15 to 20 minutes before the tsunami.

"We've found that there's a strong correlation between the amplitude of the sound waves and the tsunami wave heights," Dunham said. "Sound waves propagate through water 10 times faster than the tsunami waves, so we can have knowledge of what's happening a hundred miles offshore within minutes of an earthquake occurring. We could know whether a tsunami is coming, how large it will be and when it will arrive."

Worldwide application

The team's model could apply to tsunami-forming fault zones around the world, though the characteristics of telltale acoustic signature might vary depending on the geology of the local environment. The crustal composition and orientation of faults off the coasts of Japan, Alaska, the Pacific Northwest and Chile differ greatly.

"The ideal situation would be to analyze lots of measurements from major events and eventually be able to say, 'this is the signal'," said Kozdon, who is now an assistant professor of applied mathematics at the Naval Postgraduate School. "Fortunately, these catastrophic earthquakes don't happen frequently, but we can input these site specific characteristics into computer models – such as those made possible with the CEES cluster – in the hopes of identifying acoustic signatures that indicates whether or not an has generated a large tsunami."

Dunham and Kozdon pointed out that identifying a tsunami signature doesn't complete the warning system. Underwater microphones called hydrophones would need to be deployed on the seafloor or on buoys to detect the signal, which would then need to be analyzed to confirm a threat, both of which could be costly. Policymakers would also need to work with scientists to settle on the degree of certainty needed before pulling the alarm.

If these points can be worked out, though, the technique could help provide precious minutes for an evacuation.

The study is detailed in the current issue of the journal The Bulletin of the Seismological Society of America.

Explore further: Massive debris pile reveals risk of huge tsunamis in Hawaii

More information: pangea.stanford.edu/~edunham/p… am_Tohoku_BSSA13.pdf

Related Stories

GPS solution provides 3-minute tsunami alerts

May 17, 2013

Researchers have shown that, by using global positioning systems (GPS) to measure ground deformation caused by a large underwater earthquake, they can provide accurate warning of the resulting tsunami in ...

Calculating tsunami risk for the US East Coast

Apr 19, 2013

The greatest threat of a tsunami for the U.S. east coast from a nearby offshore earthquake stretches from the coast of New England to New Jersey, according to John Ebel of Boston College, who presented his findings today ...

New study highlights California tsunami risk

Mar 26, 2013

More than a quarter of a million Californians live in coastal areas which could be hit by devastating floods from a major tsunami in the quake-prone US state, a new study says.

Breaking the rules for how tsunamis work

Mar 07, 2013

The earthquake zones off of certain coasts—like those of Japan and Java—make them especially vulnerable to tsunamis, according to a new study. They can produce a focusing point that creates massive and devastating tsunamis ...

Recommended for you

The ocean's living carbon pumps

2 hours ago

When we talk about global carbon fixation – "pumping" carbon out of the atmosphere and fixing it into organic molecules by photosynthesis – proper measurement is key to understanding this process. By ...

User comments : 0