Download ISNS 4359 Earthquakes and Volcanoes

ISNS 4359 Earthquakes and Volcanoes
(aka shake and bake)
Lecture 7: Tsunami
Fall 2005
Tsunami
• Created most often by earthquakes
– Vertical shift of ocean floor that offsets water mass,
transmitted throughout ocean in tsunami
– Usually vertical fault motions at subduction zones,
mostly in Pacific Ocean
• 70,000 people killed by tsunami in 20th century
• Seiche (tsunami in enclosed area)
– Largest ever known was caused by rockfall in Lituya Bay
in Alaska, 1958 – up to 520 m high
• Largest tsunami possible probably from asteroid or
comet impact into ocean – never witnessed by
humans
Tsunami vs. Wind-Caused Waves
• Wind waves
– Velocities depend on period
• 17 mph for 5-second wave
• 70 mph for 20-second wave
• Tsunami
– Velocity: v = (g x D) ½
•
•
•
•
g – acceleration due to gravity
D – depth of ocean water
For average D = 5,500 m, v = 232 m/sec
Will slow down as nearing shoreline but still freeway speeds
– Height: ~1 m in open ocean, 6 to 15 m in shallow water,
higher in narrow topography
Tsunami vs. Wind-Caused Waves
• Tsunami at the shoreline
– Not a gigantic version of breaking wave
– Very rapidly rising tide, rushing inland
– Height of wave is typically less important than volume of
water flooding area, before retreating to form next wave
– Tsunami waves are usually separated by 10-60 minutes
Tsunami Warnings
• Warnings
– Feel the earthquake
– See sea level draw down significantly
– Hear wave coming
• What to do
– Seek high ground immediately
– Go upstairs in well-built building
• Warning system
– First sensors activated in 2003
– Tsunami warning center in Honolulu
for Pacific Ocean
Fig. 4.40
Tsunami hazard zones
Tsunami travel times in hours
Tsunami Case Histories
• Alaska, 1 April 1946
• Two large subduction zone earthquakes in
Aleutian Islands created 30 m high tsunami
• Destroyed local lighthouse
• Raced across Pacific Ocean to Hilo, Hawaii – killed
159 people
Fig. 4.36
Hilo, HA 1946
Alaska quake
Fig. 4.39
Hilo 1946
Alaska earthquake
Tsunami Case Histories
• Chile, 22 May 1960
• Magnitude 9.5 subduction event was most
powerful earthquake ever recorded, created large
tsunami
• Three waves, each successively larger, hit Chilean
coast, killing 1,000 Chileans
• Adequate warning was given in Hawaii but 61
people killed
• Tsunami continued to Japan, killing 185 people
• Could continue to be measured in Pacific Ocean for
a week
Hilo—note distance to shore
From 1960 Chile earthquake
Tsunami Case Histories
• Alaska, 27 March 1964
• Magnitude 9.2 subduction earthquake killed 122
people in sparsely populated Alaskan coast
• Tsunami hit Vancouver Island, then California
• Series of waves, with fifth one largest
• Which wave in series will be largest is not
predictable
Tsunami Case Histories
• Nicaragua, 1 September 1992
• Very slow but large (magnitude 7.6) earthquake
shook ground very little so was barely noticed
• 100 km segment of plate moved 1 m over two
minutes
• Generated 10 m high tsunami to hit 300 km of
coastline without warning
Tsunami Case Histories
• Papua New Guinea, 17 July 1998
• Magnitude 7.1 earthquake 20 km offshore,
triggered underwater landslide that caused
tsunami
• Hit coastline of Papua New Guinea about 5 minutes
later, washing four villages on barrier beaches into
lagoons
• Caused re-thinking of tsunami threat, as caused not
just by large earthquakes but also by landslides
triggered by moderate earthquakes
Tsunami Hazards from Landslides
• Southern California:
– Protected from distant tsunami by offshore islands
– Offshore islands were formed by active faults, that could
cause landslides to generate local tsunami
• Atlantic coastline:
– Off east coast of North America, images show scars of
big submarine landslides
• 1929 magnitude 7.2 Grand Banks earthquake triggered landslide
that caused three tsunami waves to hit Newfoundland
• Lake shores are at risk from seiche waves also
– Active faults run under Lake Tahoe
Fig. 4.42
Tsunami swept sandbar, New Guinea,1998
Sumatra Tsunami, December 26, 2004
Sumatra
earthquake,
2005
At 00:58:53 UTC on 26 December 2004,
Great earthquake off northeast coast of Sumatra, Indonesia.
Location map of Indonesia from the National Earthquake Information Center
(http://neic.usgs.gov/neis/bulletin/neic_slav_l.html)
Earthquake caused by subduction of
Indian plate beneath the Burma
microplate
Relative motion of Indian plate to
Burma microplate = 0.06 m per year
(= 6 cm or 2.4 inches per year)
Other tectonic elements in region:
Yangtze Plate
Eurasia Plate
Sunda Trench
Sunda Plate
Australian Plate
Ninety East Ridge
Tectonic Setting of Earthquake of 26 December 2004 from United States Geological Survey
(http://earthquake.usgs.gov/eqinthenews/2004/usslav/)
Mw = 9.0
Hypocenter = 30 km depth
Focal mechanism = thrust
Largest quake since 1964
Good Friday event, Alaska
Large magnitude quakes at
subduction zones are often
tsunamigenic
Local tsunamis propagate
toward nearest shoreline
Teletsunamis spread out
across ocean basins
Map showing epicenter (star) of earthquake from the National Earthquake Information Center
(http://neic.usgs.gov/neis/bulletin/neic_slav_l.html)
Animation of Before and After Images of Banda Aceh, Indonesia
Seismicity 1990-2004
Star is 26 Dec 2004 event
Well-defined Benioff Zone
associated with subduction
of Indian plate beneath
Burmas microplate
Thus, main event occurred in
seismically active area
Many earthquakes annually
Many have Mw > 6.5
(damaging magnitudes)
Map showing historical seismicity (1990 - 2004) from the National Earthquake Information Center
(http://neic.usgs.gov/neis/bulletin/neic_slav_l.html)
Est. rupture length = 1200 km
Est. rupture width = 100 km
Est. thrust fault offset = 15m
Est. seafloor uplift = m?
Uplift of seafloor is
responsible for excitation of
tsunamis
Map showing historical seismicity (1900 - 2004 ≥ M6) from the National Earthquake Information Center
(http://neic.usgs.gov/neis/bulletin/neic_slav_l.html)
http://earthquake.usgs.gov/eqinthe
news/2004/usslav/rupture_areanw.html
Areas potentially affected by tsunami inundation and run-up in red
Map showing areas potentially inundated by tsunami on 26 December 2004 from UNOSAT.
(http://cern.ch/unosat/freeproducts/Tsunami/JRC/Asia_Tsunami_07January_landcover.pdf)
Potential population affected by tsunami in red zone
Potential population affected by tsunami on 26 December 2004 from UNOSAT
(http://cern.ch/unosat/freeproducts/Tsunami/JRC/Asia_Tsunami_04January.pdf)
Areas potentially affected by tsunami inundation and run-up in yellow
(i.e. shore zones < 20 m above sea-level)
Areas Potentially Affected by Tsunami of 26 December 2004 from UNOSAT
(www.unosat.org)
Predicted Arrival Time of First Tsunami Wave (in hours after earthquake)
QuickTime™ and a
Cinepak decompressor
are needed to see this picture.
Numerical simulation (computer model) of tsunami
Dr. A. Piatanesi of the National Institute of Geophysics and Volcanology (INGV) in Rome, Italy.
Numerical simulation (computer model) of tsunami
http://www.noaanews.noaa.gov/stories2005/s2365.htm
Areas potentially affected by tsunami inundation and run-up in yellow
(i.e. shore zones < 20 m above sea-level)
http://www.nasa.go
v/vision/earth/looki
ngatearth/tsunamiimages.html
Sri Lanka
http://www.nasa.go
v/vision/earth/looki
ngatearth/tsunamiimages.html
QuickTime™ and a
Video decompressor
are needed to see this picture.
http://earthquake.usgs.gov/eqinthenews/2004/usslav/andaman_fig1.gif
http://earthquake.usgs.gov/eqinthenews/2004/usslav/andaman_fig2.gif
http://earthquake.usgs.gov/eqinthenews/2005/usweax/
Animation of Before and After Images of Banda Aceh, Indonesia
Animation produced at the Earth Visualization Laboratory, University of Arkansas, USA
By Dr. Stephen K. Boss
Countries Reporting Direct Casualties from Earthquake and Tsunami
Indonesia
Thailand
Andaman Islands (India)
Nicobar Islands (India)
Myanmar
Bangladesh
India
Sri Lanka
The Maldives
The Seychelles
Kenya
Somalia
Tanzania
In addition, many western nations are reporting fatalities and missing among
tourists who were visiting the affected countries
Fundamental Features of Water Waves
Wave Crest: is the highest portion of the wave.
Wave Trough: is the lowest portion of the wave.
Wavelength: is the linear distance separating wave crests (or separating wave troughs).
Wave Amplitude: is the displacement of a crest or trough about the mean position or water level.
Wave Height: is the total vertical distance from crest to trough (equal to twice the amplitude).
Wave Period: is the time required for successive wave crests (or troughs) to pass a fixed point.
from An Introduction to the World's Oceans, 8th edition by K.A. Sverdrup, A.C. Duxbury, and A.B. Duxbury (2004)
deep-water waves: water depth is greater than one-half the waves' length.
shallow-water waves: depth becomes shallower than 1/20th of the wavelength
Tsunamis have wavelengths ranging from 100 – 200 km!
Tsunamis are, therefore, shallow-water waves (even in the open ocean)!
Though tsunamis have very long wavelengths, their amplitude in the open ocean
is often relatively small - commonly only a meter or two - and this amplitude is
distributed over the very long wavelength so that tsunamis are quite
imperceptible on the surface of the ocean.
The speed of tsunamis is called celerity and is equal to:
3.31 x D0.5, where D = water depth (in meters)
In the deep ocean, tsunamis travel at speeds up to 750 km per hour (465 mi/hr)!
Wave Refraction occurs when waves enter shallow water over an irregular bottom
Wave traveling in shallowest water will also move with the slowest celerity
Waves in deeper water continue moving with relatively greater celerity
Wave crests will curve or refract as the wave moves forward
Waves spread or disperse upon passing through the gaps in barriers.
This process is called "wave diffraction“
Barriers with many gaps scatter wave energy, diminish wave height and power
Tsunami Triggered by Explosive Volcanic Eruption
Krakatau, Indonesia, 1883
On 26-27 August 1883, Krakatau volcano erupted
Among most violent volcanic eruptions in last 200 years
Tsunamis triggered by volcanic explosion and caldera collapse
Destroyed 165 coastal Indonesian villages on Java and Sumatra
36,000 Fatalities
Prior to 26 December 2004, most destructive tsunamis on record
Tsunami Triggered by Impacts of Large Extraterrestrial Objects
Chicxulub, Mexico
Asteroid impact site and tsunami trigger terminating the Cretaceous Period
Asteroid believed to have had 10 km diameter (6.2 miles)
Impact in shallow tropical seas
Tsunami deposits widespread across Caribbean basin & Gulf of Mexico
Impact event also associated with mass extinction of terrestrial and marine biota