Tsunami! Vicki M. Harder

Introduction A tsunami, or seismic sea wave, is a very long ocean wave that is generated by the displacement of the sea floor. Displacement is most usually caused by earthquakes but submarine landslides, volcanoes, and meteorites can also generate tsunamis. Characteristics of Tsunamis In describing a tsunami, the terms wavelength, amplitude, peak, and trough are often used. The relationships between these terms are shown in the figure at the right.

Tsunamis can reach 950 km/hr or more in the open ocean, the velocity being dependent upon water depth. Once the tsunami reaches shore and water depth shallows, the tsunami slows abruptly and the amplitude of the wave increases dramatically to levels of 20-40 m above normal sea level. If the front of the peak of the tsunami reaches shore first the first movement of water may be a rise. Since tsunami wavelengths are so long it may be several minutes in-between the fall and subsequent rise of the next peak. However, if the trough of the tsunami reaches shore first the water will recede from shore and then rise rapidly with the arrival of the peak.

The water level reached by a tsunami once it hits shore is called its run-up and is determined from the height in meters above normal high tide. Measurements of tsunami magnitudes are calculated in terms of the logarithm of the maximum wave amplitude observed locally (analogous to the earthquake Richter scale). The magnitude of an earthquake is not the sole determinant of the magnitude of a tsunami. The degree, direction, and disposition of crustal deformation are also important. However, if an earthquake meets certain criteria with respect to location and magnitude, local tidal gauges are monitored for signs of a tsunami. Once detected, arrival times for the tsunami are calculated and areas warned of the impending wave.

Tsunami Homepage

To learn more about tsunamis and to help you answer the following questions read the sections found under "The Physics of Tsunamis." Go to the "Survey of Great Tsunami" and the "Recent Tsunami Events" sections and determine the minimum earthquake magnitude it takes to generate a tsunami. Why aren't tsunamis generated along divergent plate margins?

Access Current Seismicity

Locate any large magnitude, potentially tsunami-generating earthquakes in the Pacific Ocean region. Using the Pacific Warning map determine the time it would take for a potential tsunami to reach Hawaii.

Hawaii Tsunami Page

Go to the Forecast Offices of the Tsunami Warning Center to determine whether the earthquake did generate a tsunami.