3 Chapter Tsunamis
3.1 INTRODUCTION TO TSUNAMI
- Tsunamis are produced by the sudden displacement of ocean, lake or river water.
- Some causes of tsunamis are large earthquakes, landslides, explosive volcanic eruption,
or an impact in the ocean of an asteroid or comet.
Earthquake-Triggered Tsunami
- Displaces the seafloor or the floor of a large lake, or by triggering a large landslide.
- When seafloor displaces a block of earths crust moves rapidly up or down during an
earthquake. (earthquake 7.5 or higher to cause damaging tsunami)
- upward or downward movement of the seafloor displaces the overlying water and
initiates a four stage process
FOUR STAGE Process:
1. Displacement of seafloor causes oscillatory waves transmitting energy outward
and upward from source. Waves then intercept surface spreading outward
2. In deep ocean waves move more rapidly and farther apart. Velocity equals square
root of the product of acceleration of gravity and water depth. Could not notice a
passing tsunami in the deep ocean (1 metre or less)
3. As tsunami reaches land velocity and water depth decrease. The lengths between
waves decrease but wave height increases.
4. First tsunami wave transforms into turbulent, surging mass of water that moves
quickly inland. During some tsunamis the trough of the wave arrives first,
exposing the sea floor.
-Tsunami waves can be hours after each other, which is the reason why in Crescent City,
California during the great Pacific tsunami of 1964 many people died after returning to
their homes before the second wave came.
- The run-up of the tsunami is the maximum horizontal and vertical distances that the
largest wave of a tsunami reaches as it travels inland.
- Second or third wave can be larger than the first if edge waves interact with the tsunami
wave.
-Distant Tsunami travels thousands of kilometers across the open ocean and strikes
remote shorelines with little loss of energy. (mostly caused by great subduction
earthquakes)
- Local Tsunami affects shorelines near the source of the earthquake
- Earthquake driven tsunamis travel fast inland, example one is Japan came 2-5 minutes
after earthquake occurred.
Landslide-Triggered Tsunami
-cause tsunami by landslides happening on the seafloor or in a lake
- also large landslides falling from a mountain into a large body of water can cause
tsunami.
-largest in 20th century occurred in Lituya Bay, Alaska (large earthquake triggered
rockslide at head of bay.
Volcano-Triggered frequent
3.2 Tsunami
-
less REGIONS AT RISK
-coats in proximity to a major subduction zone, directly across the ocean basin from
subduction zone capable of generating M9 earthquakes are at greatest risk.
-85% of tsunamis have been in the Pacific Ocean because of association with
earthquakes at subduction zones that surround much of the Pacific.
-research of tsunami involves excavation and radiocarbon dating of the rings of trees
killed by tsunami, computer modeling of tsunami propagation and run-up.
3.3 EFFECTS OF TSUNAMI AND LINKAGES WITH OTHER NATURAL HAZARDS
-tsunamis have primary and secondary effects
Primary Effects- related to the impact of the onrushing water and its entrained debris, and
to the resulting flooding and erosion. Much damage is the result from the return of water
to the ocean.
Secondary Effects- are those effects that occur in the hours, days, and weeks following
the event.
3.5 MINIMIZING THE TSUNAMI HAZARD
Detection and warning
o First detection when an earthquake of M7.5 happens in an offshore area.
o Three types of warning systems exist for tsunami in the:
Pacific Ocean: Pacific-wide system
Hawaii: regional systems
Japan and Chile: Local systems
Structural control
o Structures are elevated to higher levels to provide greater protection but
costs may be higher
o Dykes and walls prevent waves from reaching threatened residential and
commercial areas
Construction of tsunami inundation maps
o Run-up or inundation map after a damaging tsunami
o Risk to communities can be assessed through the maps
Land use
o Protective buffer of coastal vegetation
Probability analysis
o Identification and specification of potential earthquake sources and their
associated uncertainties
o Specification of factors that will attenuate or reduce tsunami waves as they
travel from the source area
o Statistical analysis of past tsunami, their sources, attenuation, and other
factors, similar to what is done in an earthquake hazard analysis
Education
o People must respond to earthquake in orderly and intelligently manners.
o Tsunami watch- a notification that an earthquake that could cause a
tsunami has occurred
o Tsunami warning- indicating a tsunami has been detected and is moving
across the ocean towards the area
Tsunami readiness
o Have emergency operation center with 24 hr capacity
o Have ways to receive tsunami warnings
o Have ways to alert public
o Have tsunami preparedness plan
o Promote community awareness