tsunami and the depth of the ocean solution set ============ martin b. farley department of geology & geography
Tsunami and the Depth of the Ocean
Solution Set
============
Martin B. Farley
Department of Geology & Geography
University of North Carolina at Pembroke
Pembroke, NC 28372
[email protected]
(910) 521-6478
As noted below, much solution information is in tsunami lab.xlsx.
Question answers are in italics.
Solution Set
Tsunami and the Depth of the Ocean
These seismically generated sea waves have wavelengths from 100-200
km. Where in the ocean would these be a deep-water wave? Intermediate?
Shallow-water wave?
The average depth of the oceans is less than 4000 m and even the
maximum depth of the ocean is less than 12 km in ocean trenches (and
these are not areally large). With these wavelengths, tsunamis will be
shallow water waves as long as the water is less than 5-10 km deep
(i.e., travel in water < 1/20 their wavelength), that is, basically
everywhere in the ocean.
In 1855, A.D. Bache estimated the average depth of the ocean by
comparing the arrival times of a tsunami at various points across the
Pacific. How might he have done this?
Shallow water celerity C = sqrt (gD), where D=water depth and
g=acceleration due to gravity
so if you know the starting time, arrival time, and distance, you can
calculate D:
D=C**2/g
See tsunami lab.xlsx for solutions for necessary routes, distances,
and calculation of water depths.
Here is a table of places associated with the Good Friday Alaskan
Earthquake of 1964 and its ensuing tsunami.
Using an available globe and string, lay out the Great Circle route
from the earthquake epicenter (near Valdez, Alaska, which is in the
bay east of Anchorage) to each of the other places. If the Great
Circle route isn’t practical, you need to explain why and work out a
route the tsunami could have taken.
Location
Great Circle route comments
Epicenter of Good Friday Earthquake
61° 6' N 147° 30' W
---------
Whidbey Bay, AK
(very close to epicenter)
59° 58' 148° 57' W
Kodiak AK
(large island SW of epicenter)
57° 50' N 152° 30' W
Tofino BC
(west coast Vancouver Island)
49° 09' N 125° 54' W
Crescent City CA
(just south of border with Oregon)
41° 46' N 124° 13' W
Hilo, HI
(NE coast of big island)
19° 44' N 155° 4' W
Kushiro, Japan (E Coast Hokkaido)
43° 00' N 144° 22' E
Single Great circle traverses Alaska, so need corner south of
Aleutians
(I use 53N 165W)
Kwajalein, Marshall I.
8° 45’ N 167° 45' E
La Punta, Peru
12° 05' S 77° 10' W
Single Great circle traverses North America/GOM, so need corner west
of Los Angeles
(I use 30N 120W)
Sydney, Australia
33° 51' S 151° 14' E
Using this latitude-longitude calculator
(http://williams.best.vwh.net/gccalc.htm) and your notes on the likely
route, determine the distance in km from each place to the epicenter
of the 1964 “Good Friday” Alaska earthquake.
Location
Distance (km)
Epicenter of Good Friday Earthquake
61° 6' N 147° 30' W
0
Whidbey Bay, AK
59° 58' 148° 57' W
Kodiak AK
57° 50' N 152° 30' W
Tofino BC
49° 09' N 125° 54' W
Crescent City CA
41° 46' N 124° 13' W
Hilo, HI
19° 44' N 155° 4' W
Kushiro, Japan (E Coast Hokkaido)
43° 00' N 144° 22' E
Kwajalein, Marshall I.
8̊ 45’ N 167̊ 45' E
La Punta, Peru
12° 05' S 77° 10' W
Sydney, Australia
33° 51' S 151° 14' E
Here are the arrival times for the tsunami. All times are Greenwich
Mean Time (GMT), so no time adjustments are necessary.
Arrival (GMT, 24-hr clock)
D
Epicenter Good Friday EQ
0336
-----
-----
-----
-----
-----
Whidbey Bay, AK
0355
Kodiak AK
0424
Tofino BC
0700
Crescent City CA
0739
Hilo, HI
0900
Kushiro, Japan (E Coast Hokkaido)
1031
Kwajalein, Marshall Islands
1200
La Punta, Peru
1911
Sydney, Australia
2045
Do there seem to be any geographic trends in the depth estimates? Are
any places conspicuously different? Can you think of an explanation?
What average depth do you calculate?
Some differences in analysis are to be expected. My classroom has a
large Heezen and Tharp Physiographic Map of the Ocean that students
can look at. Here is what I see:
Epicenter to
Alaska shallow seafloor/continental shelf
British Columbia wave apparently travels close to North America in
relatively shallow water
Mid-Pacific (Hilo, Kwajalein) deepest water depths over very open
ocean (i.e., into center of Pacific)
Japan, California, Peru, Sydney Intermediate depths (3300-3800 m)
May travel through water depths like those of mid-Pacific but has
enough shallower water on either end to slow wave
Japan/Australia, Peru are the main paths that traverse trenches, but
these are so narrow they don’t seem to affect the tsunami much.
Average depth ~3600 m