Earth Sciences 240A - Lecture 17 - Tsunami

Terminology/Character

“Seismic sea wave”: common but wrong

Period – Wavelength:

Normal wave: 10 sec; 150 m; Tsunami: 1 hr; 200 km

Velocity: Varies with ocean depth; Pacific Ocean: 200 m/sec or 700 km/hr

Magnitude Estimate

Attempted scale like Richter: Imamura-Iida Scale : Calculated as function of max. wave height in open ocean

Not very satisfactory!

Generation

Quick elevation/subsidence of ocean floor blocks

Most effective in subduction zones

Quick displacement of large water volume

Submarine/supermarine landslide

Violent submarine volcanic eruption

Cosmic body impact

Approach to land

Bottom of wave on ocean bottom

Wave amplitude increases with shallowing sea floor (‘shoaling’)

Wave speed/energy remains high

Land-Fall

Commonly, first moderate wave followed by water ‘draw-down’

Reduction of energy:

Bottom friction

Land erosion

Turbulence

Reflection/interference

‘Run-up’

Wave height on-shore

10, 20, 30 meters measured

Examples:
1929
Grand Banks

EarthquakeLandslideTsunami

Broken telephone submarine cables

29 deaths

Registered in South Carolina; Portugal

1952 research on area: First documentation of turbidity currents

1964 Prince William Sound

EarthquakeSubmarine landslideTsunami

122 deaths (106 in Alaska; run-up: 67 m)

5.4 hrs later: Hilo, Hawaii (run-up: 3 m)

Alaska disaster prompted creation of warning system

1960: Chile

Huge earthquake (M 9.5)I-I scale: 8.6 tsunami

490-2290 deaths; >$0.5 B damage

Off-shore boats lost

Run-up est. 25 m

Hilo: 14.8 hrs later: 61 deaths; $24 M damage; Run-up at Hilo: 10.7 m

1755 Lisbon

Earthquake (8.5) Azores-Gibraltar3 tsunami; waves 5-13 m high

60,000 deaths in Lisbon area alone

Many deaths around Mediterranean

Recent research: Contributing submarine landslide

Tsunami Warning System

Circum-Pacific; 26 participating countries

Headquarters: near Honolulu, Hawaii

Two (NOAA) warning centers:

ATWC in Palmer, Alaska

PTWC in Honolulu

Based on fault break type/magnitude

Warning steps

Earthquake data interpretation

NOAA Weather Radio System

Direct VHF broadcast of danger

US Coast Guard

Track/broadcast marine data to ships, shore facilities

Local authorities/emergency managers

Execute emergency plans

“All Clear”

 

Next: Volcanoes

IT IS ESSENTIAL THAT YOU READ THE PREREQUISITE MATERIAL BEFORE NEXT LECTURE