M A S T E R P A G E
Teacher Background Reading
Seismic SeaWave
2.4f
A seismic sea wave is created when a fault in the ocean floor moves vertically. The energy of the lifted or lowered
water radiates outward as very long shallow water waves commonly (and erroneously) called “tidal waves.”
Because these waves have nothing to do with the attraction of the Moon or the Sun, scientists prefer the Japanese
word tsunami, which means “wave in the harbor,” or the English term seismic sea wave.
Most seismic sea waves, like most earthquakes, occur around the Pacific Ocean, but there have been great seismic
sea waves in most regions of the Earth.
The great destructive power of these waves comes from the huge energy imparted to the water by fault
movement. To equate this energy to mechanical work, we can imagine the work needed to lift an average volume
of ocean water a distance (d) of 1 meter. The average depth of the ocean (h) is 3.8 km. The average surface area
(A) of the ocean floor moved up or down by such an event, according to seismic sea wave research, is 20,000
square kilometers—a 200 km x 100 km piece of seafloor about the size of New Jersey. The volume (V) of
seawater lifted would then be 76,000 cubic kilometers. If we take 1.03 kg/m
3
as the density (D) of seawater, the
mass (m) of that seawater would be about 78 billion metric tons. To lift this much water by 1 m would take 7.6 x
10
14
Joules, the energy of 183 kilotons of TNT. See Master 2.4j for a complete quantitative analysis.
This energy radiates outward from the epicenter as a wave train of low waves, not as a single large wave. Each of
these seismic sea waves has an average amplitude of 1 meter, wavelengths over 100 km, and periods of 7-15
minutes (for short-period tsunami) or over 40 minutes (for long-period tsunami). These waves travel at speeds
between 550 and 800 kilometers per hour before encountering land.
Speed=550 to 800 km per hour
Period=7-15 min or > 40 min
Seismic Sea Wave Characteristics
Seismic sea waves are very different from wind-generated sea waves. Normal wind waves rarely have
wavelengths over 300 meters, and generally travel under 100 km/hr. A medium-sized tsunami can have
wavelengths of 150 km and travel at 550–800 km an hour. Tsunami are like tides in that a low tide is followed by
a high tide, but in the case of a tsunami dramatic high and low tides can be only tens of minutes apart. This may
be the origin of the expression tidal wave.
As seismic sea waves encounter land, they cause rapid tide-like motion. The trough of the waves causes very low
tides, while the wave crests may cause a run as high as 32 meters. Sometimes seismic sea waves can cause
enormous breaking waves. The mechanism for these waves is very similar to that of wind waves; the friction of
the ocean bottom slows the troughs and the crests move over them and break. Thus the characteristics of seismic
sea waves when they hit land, like those of wind waves, are very much related to the characteristics of the near-
shore ocean bottom and the shoreline.
A G U
/
F E M A
113
S
E I S M I C
S
L E U T H S
