Scientists at a Tsunami Warning Center are called Watchstanders, but that doesn't mean they continually watch the world's oceans for evidence of tsunamis. Instead, their job requires first watching measurements of seismic energy, which is the energy released when rock suddenly breaks or moves within the earth.

These seismic events, including earthquakes and landslides, are what trigger tsunamis by lifting and moving large amounts of water. Knowing when an earthquake occurs is straightforward based on seismic measurements, but deciding if a particular seismic event could have triggered a tsunami takes a little more detective work.

Energy from a seismic event will show up as rapid, higher-than-normal oscillations on a seismometer.

"Body" waves travel through the body or inner part of Earth and include Primary or P waves and Secondary or S waves. Surface waves travel along Earth's surface like ripples across a pond.

Surface waves from an earthquake in northeastern Nevada, U.S.A.

Seismic data being monitored by one of NOAA's Tsunami Warning Centers. Each line represents data for one station.

Seismic data from all over the world come to the Warning Center continuously.

When one or more seismometers record a signal from a strong seismic event, an alarm is triggered and the Watchstanders get to work analyzing the event to see if it could have generated a tsunami.

As you saw in the exercise, the first waves to show up on a seismograph are the Primary, or P, waves. The Secondary, or S, waves follow.

The difference between the P-wave arrival time and the S-wave arrival time provides an estimate of the seismometer's distance from the earthquake epicenter, or the location at the surface above the earthquake.

Once earthquake data are recorded at three seismometer locations, the P-wave and S-wave arrival information can help locate the earthquake epicenter. Complete the next exercise to see why three seismometer locations are needed.

Magnitude numbers are indicated along the bottom scale of the graph. These recent significant earthquakes that triggered tsunamis were all magnitude 7.0 or greater.

At the Tsunami Warning Centers, computer systems enable Watchstanders to find the earthquake location very quickly. The Watchstanders also need to know how large the earthquake was before they can decide if it could have generated a tsunami. A large earthquake generates more movement of the earth and therefore has a higher magnitude. Watchstanders worry a lot about earthquakes with a magnitude of 7.0 or higher, because if these bigger quakes occur beneath the sea, they are more likely to have large rupture areas and lift more sea water, causing a tsunami.

At the Warning Center, Watchstanders use different scales to estimate magnitude, including a value called moment magnitude. Moment magnitude estimates the amount of energy released by an earthquake, which has a direct relationship to how much the earth moves.

The energy released also corresponds to a quake's destructive power, so big magnitude numbers are worthy of concern.

The quake occurring near Chile in February 2010, which launched a tsunami across the Pacific Ocean, had a magnitude of 8.8.

A computer model animation of the 2010 Chile tsunami.

A Watchstander's job is challenging because not all big quakes will cause a tsunami. The Watchstanders keep looking at the seismic data to try to find out the type of earthquake that occurred, precisely where it occurred, and how big the rupture area was.

Piecing all this information together, they can decide whether a large amount of water could have been displaced and if a tsunami is possible. If it is, the next step will be getting Tsunami Warnings out to people who need them.