1. The devastating impact of seismic sea waves
Tsunami
The devastating impact of seismic sea waves

2. Tsunami (harbor wave) Seismic sea waves (NOT tidal waves) earthquake
Caused by processes that abruptly move large volumes of ocean water:
earthquake
submarine volcanic eruption
coastal/submarine landslide or rockfall
extraterrestrial impact

3. How do EQ cause tsunami?

4. Tsunami causes
Unless there is an underwater landslide, strike-slip EQ WILL NOT cause tsunami
Most tsunami generated by subduction zones
Chile,
Alaska,
Japan,
Cascadia,
Philippines,
New Zealand

5. Tsunami wavelength Long wavelengths (over 100 km)
Periods longer than 1 hour
316,800 ft = 60 miles

6. Tsunami wave speed
Travel at high speeds : 400 to 500 mph (~200 yards/sec)
Alaska to CA 4 to 7 hrs
Alaska to Hawaii 4 to 6 hrs
Chile to Hawaii 14 to 15 hrs
Chile to Japan 22 to 33 hrs

7. Tsunami wave speed g = acceleration of gravity (9.8 m/sec2)
v = speed ~
g = acceleration of gravity (9.8 m/sec2)
d = depth of ocean (m)
deeper water means higher speed
For d = 4,600 m, v = 763 km/hr (speed of jet plane)

8. What happens when tsunami gets near shore?
Tsunami slows down (shallower water)
Example: d = 100 m, v = 113 km/hr
Wave gets taller
λ (wavelength)gets shorter, T gets shorter

9. Tsunami nears shore
As wave gets into shallow water bottom of wave drags along ocean floor
Top of wave still moving fast: can cause cresting of wave, and breaking onto shore

10. Tsunami run-up
Run-up = measurement of height of water onshore observed above a reference sea level
Generally don’t get big gigantic wave
Water comes as a fast moving rise in tide that rapidly moves inland
NOT JUST ONE WAVE…multiple waves coming in about ½ hour or so apart
See tsunami wave simulator

11. Energy in tsunami
Loss of energy in a wave is inversely proportional to λ
Since λ very long, little energy lost
Waves can travel great distances and still be very destructive

12. Damage due to tsunami
Waves often full of debris (trees, cars, pieces of wood etc.)
As the wave recedes, the debris drags more stuff with it
Can recede as much as a km out to see, leaving shoreline empty with flopping fish, boats, etc. on the bottom

13. Detecting a tsunami Pressure recorder on bottom of ocean
Buoy to communicate readings via satellite
Tsunami Warning Centers issue warning

14. Tsunami Warning Centers
Hawaii and Alaska
When EQ considered capable of generating tsunami, send warning with estimated arrival time
Once tsunami hits somewhere, tsunami watch established to monitor tide gauges and ocean buoys

15. Tsunami Warning When warning is issued – low lying areas are evacuated
U.S. Coast guard issues warnings over marine frequencies
Some places have sirens

16. How could you evaluate the level of risk
Tsunami
How could you evaluate the level of risk
due to tsunami?

17. Possible tsunami
“run-up” zones

18. Seiche
standing wave
Standing wave in an enclosed or partially enclosed body of water (similar to sloshing in a bath tub)
Generated by wind or seismic activity
Often swimming pools experience a seiche during EQ

19. Seiche potential in Lake Tahoe
Scientists at UNR have determined the seiches have occurred on Lake Tahoe in the past
Low probability – on average once every 2000 to 3000 years
Good page on seiches w/great animation