Tsunamis aren’t subtle, but they do still manage to be surprising. They’re created by earthquakes under the sea, sometimes so far from a coast that people will have no idea any seismic activity occurred. Then, once the surge of water reaches a shoreline, anyone there has very little time to react and escape the area. As we get better at monitoring the ocean floor for earthquakes, these events are becoming slightly easier to predict, but the sea floor is so vast that it’s not the most practical endeavor. However, new research is suggesting that the key to catching tsunamis earlier may come down to listening to the sea, and acoustic gravity waves in particular, in just the right way.
Massive amounts of movement
When an earthquake occurs in the ocean, there’s obviously a lot of shaking and vibrating going on. In addition to massive amounts of displaced water, a quake will send out acoustic gravity waves (AGWs) in every direction. These waves are a bit like a hybrid of sound waves moving laterally through the air, and the gravity-sensitive waves you see shaping fluids like the average waves near a beach. This has made AGWs tricky to study and model, since they don’t follow the exact patterns we see in more common wave activity. One trait that has stood out, however, is that an AGW can move through the ocean at the speed of sound across huge distances. Because of their impressive sizes and speeds, researchers have long hoped that they could be detected well in advance of a tsunami’s arrival, buying people more time to get to safety.
The difficulty hasn’t been detecting the AGWs, but making sense of them. Fortunately, scientists from the University of Cardiff are now suggesting that this kind of analysis is not only possible, but practical even with only a single hydrophone sensor in the ocean to detect the wave. The distinct shape and speed of any AGW should reveal various aspects about the earthquake that created them. With more information in the system, such as details about the suspected fault location, researchers state that the tsunami’s amplitude and potential impact on a shoreline can be predicted. Once compiled, these data could then be used to trigger tsunami alarms in the tsunami’s path, giving people crucial time to find safety.
Heard through single hydrophone
On a basic level, this is similar to the tsunami alarms we have today. Devices known as dart buoys are anchored at sea, and can then detect unusual pressure changes in the water below them. This works if the buoys are in the tsunami’s path, which then requires that they’re located in all the right locations at all the right times. Measuring AGWs, however, don’t require that kind of specific placement. Because AGWs expand in multiple directions from an earthquake’s epicenter, hydrophones in any direction could detect clues about the formation of a tsunami. This then leads to a much more practical system for early warnings, increasing the chances that an alarm will reach people with enough time to get away from the water.
Source: Could underwater sound waves be the key to early tsunami warnings? by Cardiff University, Science Daily