2011 Tōhoku Earthquake and Tsunami
MODIS satellite image on 26
FEB, before the tsunami. Scale
bar is 10 km.
MODIS satellite images on 13 MAR,
after the tsunami. The red circle
marks a fire. Scale bar is 10 km.
The Earth’s Crust consists of a number of continent size plates that move slowly. Most
Earthquakes and Volcanoes occur along or near the boundaries of the plates. As the
plates continue moving, rocks are locked together at the boundaries until stresses
accumulate past the breaking point. Earthquakes occur when the rocks suddenly crack
and slip along lines called faults (often at or near plate boundaries).
Velocities (mm/yr) and boundaries of the Plates Near Japan. When plates converge, the
denser oceanic plate sinks or subducts below the less dense continental plate. The line,
studded with triangles (same as for cold fronts) indicates the collision zone (usually an
ocean trench). After the oceanic plate sinks enough, rock melts and rises to form a line
of volcanoes about 100 km from the collision point.
Illustrated Sequences of Earthquakes and Tsunamis. In this sequence, the subducting
plate forces the overriding continental plate to distort until stress becomes too great. At
this point the overriding plate springs back. When any vertical motions occur in the
ocean, the water is disturbed vertically and tsunami waves are produced.
Animation at http://www.youtube.com/watch?v=4Xebwzb3dDE&feature=related
Second Sequence
Out at sea, the tsunami wave is about 100 km long and usually less than 1 meter high.
But as the wave approaches the shore, it slows down, its wavelength gets shorter, and
the wave amplitude increases as the water alternately piles up much higher and is
depressed much lower.
Three factors affect the propagation and amplitude of the tsunami.
1. Slowing of the wave speed
2. Refraction and focusing
3. Lateral constriction into a narrowing bay (funnel effect, a form of focusing)
“Shallow” Water Wave Speed
“Shallow” Waves
c
  h
gh
g = 10 ms-2 (acceleration of gravity)
h = depth (m)
Typical Mid-Ocean Tsunami Speed
Tsunamis are “Shallow” Waves since
  100 km >> Ocean Depth, h  4 km
c  gh  10  4000
1
 200 ms  400 mph
Why Tsunamis Become Deadly as They Approach Shore
As tsunamis move into shallower water near shore, they slow down (c gets smaller)
but their frequency, f, remains the same because just as many waves pass any
point. As a result their wavelength also gets smaller because =c/f.
Water Wave Energy
(per wavelength, )
Amplitude = a
Width = w
E 
1
2
ga  w
2
Waves conserve their energy as they
move into shallow water, so when 
decreases, amplitude, a must increase
to compensate. Also, if the waves are
squeezed into a narrowing bay, width,
w decreases, so amplitude, a, must
increases to compensate.
Storms can generate enormous waves, which can
travel across the ocean to break on distant shores
Tsunami Amplitude over the Pacific Ocean. Ribbons of greater height are caused
by focusing due to refraction. The tsunami caused one death in northern California
when a man standing on a pier photographing the event was swept away and
drowned.
Once a major earthquake occurs,
the rest of the plate boundary
adjusts. This causes aftershock
earthquakes that are usually at
least one order of magnitude
smaller. The magnitude of the
2011 Tōhoku Earthquake (9.0)
was 2.1 higher than the Haiti
quake or over 1000 times more
powerful.
Earthquake and Aftershocks
Circle Size indicates Magnitude
Except for refraction,
Sendai would have been
safe from a tsunami
Wave Refraction
Waves always bend toward the region where they move more slowly.
Speed
Fast
Air = Fast
Water = Slow
Slow
Dispersion
Waves always move at right
angles to their crests or fronts.
So, when one part of the wave
front moves more slowly than
another, the entire wave turns
(refracts).
Refraction of Thunder
Thunder cannot be heard far from the storm at
ground level because sound waves travel refract
upward as they move faster in the warm air near
ground level than in the cold air aloft.
COLD
HOT
ZONE OF SILENCE
Refraction of Waves Approaching Shore Obliquely
Funneling or Focusing
Effect in Narrowing Bay
SHALLOW = SLOW
DEEPER = FASTER
Light contour lines (20 m intervals)
show land elevation. This town was
inundated because it was barely
above sea level.
Weather has largely been
favorable for radiation
released into the air. The
winds are mostly blowing
out to sea (except for a
few troubling hours at
first).
12 Hour Forecast
24 Hour Forecast