Waves and Tsunamis
Ralph Stephen, WHOI
29th Annual MME Conference
April 30, 2005
NE-COSEE OSEI-2 Project with Plymouth Schools
Teachers:
Mary Lavin, PCIS
Derek Strohschneider, PCIS
Richard Maichle, PSMS
Facilitor:
Kim Frashure, Umass Boston
Science Coordinator:
Nick Micozzi, Plymouth Schools
Curriculum Vitae
• B.A.Sc. - University of Toronto, 1974
• Ph.D. - University of Cambridge, 1978
• Woods Hole Oceanographic Institution -1978 to the
present - Senior Scientist - Marine Seismology
• Research: To observe and explain sound and
vibration (0.001 to 100Hz) in the sea (passive and
active).
JOIDES Resolution
Teacher at Sea Program
Some Tsunami Fundamentals
http://www.ess.washington.edu/tsu
nami/index.html
The December 26, 2004 tsunami that was
generated from the M9.0 Sumatra earthquake has
raised public awareness worldwide of the disastrous
consequences of tsunami waves. There are at least
five lessons that we can learn from tsunamis.
First, all school children, and particularly school
children who live near the coast, should be taught to
run to high ground away from the beach if they feel an
earthquake or if they observe that the water level
lowers dramatically.
Onset of October 9, 1995 Manzanillo, Mexico tsunami.
For many local tsunamis, the ocean initially recedes
before inundation. Photo from Tsunami Field Survey
Photographs site maintained by Tsunami Hazard
Mitigation Program (Costas Synolakis, Director) at USC.
http://walrus.wr.usgs.gov/tsunami/cascadia.html
Second, tsunamis are an excellent example of the
property of waves to transport energy without
transporting mass. The water that impacted the
beaches in Sri Lanka, for example, did not "come
from" Sumatra; just the energy "came from" Sumatra.
Third, a ship at sea in deep water is unlikely to feel the
tsunami at all. There are two reasons for this. The
first reason is that the amplitude of tsunamis in the
deep ocean is quite small, only a few centimeters. For
example the NOAA maximum amplitude map for the
December 26 tsunami shows a maximum amplitude of
about 50cm in the deep Indian Ocean to the west of
the Andoman and Nicobar Islands.
http://www.ngdc.noaa.gov/spotlight/tsunami/image/pmelmaxcomputedamplitude.gif
In deep water the energy of the tsunami is
distributed throughout the water column
which is typically 4 to 5 kilometers deep.
Since the effective mass is quite large the
same energy can be transported with small
displacements. As the tsunami approaches
shallow water the mass of available water
becomes less and the amplitude becomes
larger in response to conservation of energy.
The second reason that ships at sea do not feel
tsunamis is that the time it takes the sea surface to rise
and fall during the passage of the tsunami is from 5 to
20minutes. Such a small change in amplitude over
such a long time is unlikely to be felt by a ship.
Fourth, tsunamis provide an interesting demonstration of the
relationships between period (P) and frequency(f):
P=1/f,
and between frequency(f), wavelength (w) and velocity (v):
v=w*f.
For a tsunami wave with a period of 40minutes the frequency is
about 0.0004Hz (cycles per second). The wavelength of a
tsunami in deep water is about 500km (see the NOAA
animation). From this we can compute the tsunami velocity to
be about 200m/s or 450 miles an hour - about as fast as a
commercial jet liner.
QuickTime™ and a
Photo decompressor
are needed to see this picture.
http://www.noaanews.noaa.gov/video/tsunami-indonesia12-2004.qt
Fifth, many people might think that the NOAA
tsunami buoys in the Pacific respond in some way to
the sea surface response of the tsunami. The buoy,
however, is just the platform for communicating the
real time data to a satellite. The actual tsunami is
measured by a pressure detector on the seafloor. The
bottom pressure sensors detect pressure fluctuations
with periods longer than about 2minutes and they
measure a change in sea level to better than 1mm
(compared to a typical tsunami period of 6minutes and
a small tsunami amplitude of about 3cm).
http://www.pmel.noaa.gov/tsuna
mi/Dart/dart_ms1.html
http://www.pmel.noaa.gov/tsuna
mi/Dart/dart_ms1.html
http://www.ndbc.noaa.gov/Dart/milburn_1996.shtml
Plymouth Wave Lab
http://msg.whoi.edu/String_Lab/New_String_Movies.html