Geography 2152F/G Lecture 3: Week 3 - Tsunamis + Models of Human Response
22 Oct 2018
School
Department
Course
Professor
24/09/18
Tsunamis/Models of Human Response
• Tsunami is Japanese for “harbour wave”
• They are produced by the sudden displacement of water
• Events capable of triggering tsunamis:
o Earthquakes that cause uplift of the seafloor (underwater earthquake)
▪ Most common cause
o Landslides
o Volcano flank collapse
o Underwater volcanic eruptions
o Meteorites
HISTORIC TSUNAMIS
Date
Cause
Approximate
Human Casualties
1755
Lisbon Earthquake (M 9.0), Portugal
20,000
1883
Krakatoa Volcanic Eruption, Indonesia
36,000
2004
Sumatra Earthquake (M 9.1), Indonesia
230,000
2011
Tohoku Earthquake (M 9.0), Japan
16,000
EARTHQUAKE-TRIGGERED TSUNAMIS
• Earthquakes can cause tsunamis in two ways:
o By displacement of the seafloor
o By triggering a landslide that enters water
• Generally, an earthquake must be of at least M7.5 in order to trigger a tsunami
• Tsunamis develop in a 4-stage process
Stage 1
• Displacement of the seafloor sets waves in motion that transmit energy
upward and outward
• When the waves reach the surface of the water, they spread outward
Stage 2
• The waves move rapidly across the open ocean (they can reach speeds of over
500 km/hr)
• The spacing of the wave crests is very large (it can be more than 100 km)
• The height (amplitude) of the waves is often small (less than 1 m)
• Passengers on ships in the ocean rarely notice tsunamis passing beneath them
Stage 3
• As the tsunami approaches land, the water depth decreases
• This results in the water ‘piling up’ and causes these effects:
o A decrease in wave speed
o A decrease in spacing of the waves
o An increase in wave amplitude
24/09/18
Stage 4
• As the tsunami impacts land, waves can reach heights of dozens of metres
• Wave speed at this time can be up to 50 km/hr making it impossible to outrun
• During some tsunamis, the water first recedes from the shore and exposes the
seafloor
TSUNAMI EVENT
• A tsunami event consists of a series of large waves reaching shore that can
last for several hours
• Run Up: The maximum vertical distance that the largest wave of a tsunami
reaches as it travels inland
TYPES OF TSUNAMIS
• Distant Tsunami: A tsunami that travels thousands of kilometres across the
open ocean
o On remote shorelines across the ocean, reduced energy lessens impact
o Also called tele-tsunamis
• Local Tsunami:
o Because of this short distance, local tsunamis provide little warning
REGIONS AT RISK
• Coasts located near subduction zones or across oceans from subduction zones
are most at risk
• Areas at greatest risk are the Pacific Ocean and the Mediterranean Sea
EFFECTS OF TSUNAMIS
• Primary effects:
o Flooding and erosion destroy beaches, coastal vegetation, and
infrastructure
o After the tsunami retreats to the ocean, scattered debris is left behind
o Most tsunami deaths are from drowning
▪ Injuries result from physical impacts with debris
• Secondary effects:
o These are effects that generally occur after the event is over
o Fires may develop due to ruptured gas lines or from ignition of
flammable chemicals
o Water supplies may become contaminated and water-borne diseases
(cholera) may spread
INDIAN OCEAN TSUNAMI OF 2004
• This catastrophic event occurred on Dec. 26th
• The source was a M 9.1 earthquake off the west coast of Sumatra (an island in
Indonesia)
• It was the 3rd strongest earthquake in world history
24/09/18
• The earthquake occurred in a subduction zone between the Burma and
Indian-Australian plates (*don’t need to know plate names*)
• These plates had been locked for over 150 years thus allowing strain to build
• Rupture caused some land areas along the coastline to subside below sea level
• Tsunami reached nearby Indonesian islands within minutes of the earthquake
• Many coastal communities in Indonesia and surrounding countries were
heavily damaged during the event
• Countries bordering the Indian Ocean did not have a tsunami warning system
like those bordering the Pacific Ocean
• People were caught by surprise and over 230,000 died
• Many were unfamiliar with tsunamis and some were intrigued by the
approaching waves
• Most people in the area were ignorant of an early warning sign (receding sea)
LESSONS FROM THE 2004 TSUNAMI
• Effective tsunami warning systems are needed around all oceans where
tsunamis can occur
• In 2006, a new warning system became operational in the Indian Ocean
• A warning system by itself is not enough, why?
o Emergency officials must have an organized plan for evacuating
residents during a warning
o Earthquake and tsunami education is necessary for people who live
along or visit coastlines
DETECTING TSUNAMIS
• The Pacific Ocean warning system uses a network of seismographs to
estimate earthquake magnitude
• Sensors electronically connected to buoys verify that a tsunami was produced
• They rest on the seafloor and measure changes in water pressure passing over
them
• These sensors are called tsunameters
STRUCTURAL CONTROL
• Damage can be minimized through regulations on buildings and structures
• Some cities in Hawaii require floor proofing measures such as basement
window sealing and bolting homes to their foundation
• Concrete levees (also known as a flood wall) are other preventative measures
but can be very expensive
• Offshore barriers are only feasible outside cities with very large populations
INUNDATION MAPS
• Maps showing the geographic area that can be potentially impacted by
tsunamis are created to help plan for future events
• Historical records, geologic data, aerial photography aid in making the maps
Document Summary
Stage 1: displacement of the seafloor sets waves in motion that transmit energy upward and outward, when the waves reach the surface of the water, they spread outward. Stage 2: the waves move rapidly across the open ocean (they can reach speeds of over. Tsunami event: a tsunami event consists of a series of large waves reaching shore that can last for several hours, run up: the maximum vertical distance that the largest wave of a tsunami reaches as it travels inland. Regions at risk: coasts located near subduction zones or across oceans from subduction zones are most at risk, areas at greatest risk are the pacific ocean and the mediterranean sea. Effects of tsunamis: primary effects, flooding and erosion destroy beaches, coastal vegetation, and infrastructure, after the tsunami retreats to the ocean, scattered debris is left behind, most tsunami deaths are from drowning.
