Factors Controlling Damage and Fatality Rate from an Earthquake
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Earthquake-related factors:
- size, magnitude of EQ
- depth of EQ
- distance to EQ
- type of EQ (edge of hanging wall of shallow thrust EQs experiences large accelerations)
- location of EQ (ocean, land, type of plate boundary): certain types of EQs in the ocean can trigger tsunami
- time of day (in combination with human activity)
- bad times in areas with poorly build residential housing: night times when most people are at home (e.g. 2003 Bam, Iran)
- bad times in areas with extensive transit systems: rush hour times
- local geology
- sedimentary basins enhance shaking (site amplification)
- area with certain geology (e.g. sedimentary cover) prone to liquefaction (see below)
- EQs can trigger mass movements in areas with unstable geology (e.g. faulted hillsides, cliffs)
Human factors:
- population density
- people's routines: an earthquake in the very early morning in a rural area with adobe buildings can have high fatalities as sleeping people are inside collapsing buildings. A few hours later, when people are busy outside that same earthquake would kill far less people. But that same earthquake occurring in the Los Angeles basin could kill many people when they are in rush hour traffic and freeways collapse.
- local building codes
- brick-and-mortar buildings typically prone to collapse, while wood-frame structures can sway with EQ motion
- lower stories in cheaply built concrete buildings can collapse
- apartment buildings with garages below particularly prone to collapse (e.g. 1989 Loma Prieta Earthquake)
- poorly built or poorly anchored freeways and bridges can collapse (e.g. 1994 Northridge Earthquake)
- ruptured water lines can trigger mass movements
- ruptured gas lines can fuel fires (e.g. 1906 San Francisco Earthquake)
- time of day (see earthquake-related factors)
Ocean Wind Waves
- unlike tsunami, ordinary ocean waves (ocean gravity waves) are generated by winds blowing across the sea surface
- wave height depends on
- wind speed
- duration of winds
- fetch: size of area over which winds blow consistently
- wave height usually less than 3m
- wavelength: usually between 60 and 150 m
- deep-water waves: Waves don't sense the bottom. When water depth is greater than 1/2 wavelength l. Wave speed depends on wavelength: v=l/T, where T is the wave period. It turns out that if T is known, v can be obtained through v=g x T/(2π), where g is the gravitational acceleration (9.81 m/s2)
- shallow-water waves: Waves sense the bottom. When water depth is less than 1/20 wavelength l. Wave speed depends on water depth D: v=sqrt(D x g) (see tsunami below)
- largest wave heights are typically less than 18m but can approach this height in the Southern Oceans (around Antarctica); some rogue waves have been observed to be 34m high
Tsunami (an example of EQ related hazard)
- from Japanese "tsu" for harbor and "nami" for wave; singular/plural both tsunami
- name may come from the fact that waves are not noticeable in the open ocean but only when they approach the coast, or a harbor
- potentially one of the great killers during an earthquake
- generated by initial displacement of incompressible water
- triggered by any type of vertical movement in the ocean's water column: e.g. earthquakes, collapsing volcanoes or landslides
- falsely called "tidal waves"; tsunami have nothing to do with tides;
NB: waves in the oceans are also called gravity waves because the propagation speed of these waves is controlled mainly by gravitational forces
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- in the open ocean, tsunami are hard to detect:
- the amplitude of tsunami is less than a meter
- the wavelength is so large that a ship does not notice a passing tsunami
- upon approaching the shore tsunami "built up": trailing parts, traveling in deeper water, are faster and catch up with leading parts traveling in shallower parts (see equation above)
- the largest tsunami can be 10m high or even higher after crossing an ocean basin (e.g. Chile 1960 tsunami arriving in Hawaii was nearly 18m in some bays)
- due to the long wavelength, the time between a wave crest and the next crest is 15-30 min (time=l/v), in the open ocean; approaching the coast, the waves slow down and this interval becomes longer
- depending on the geometry of the beach, a tell-tale sign of a coming tsunami is often that the water recedes in the space of a few minutes; a person knowing the tide calendar would therefore know that something unusual is about to happen
- water recedes faster than the usual low tide
- timing of receding water is wrong
- how long before I can go back to the beach?
two more important facts about tsunami:- tsunami bounce back from coasts (like balls on a pool table) and go back into the ocean, and back to the beach. They can do this multiple times.
- the first tsunami wave may not be the highest. Because bouncing tsunami waves can add up, a later tsunami wave can actually be the highest
- a tsunami warning system was put in place in the Pacific Ocean after a fatal tsunami generated by the 1946 Aleutian Islands earthquake killed people in Hawaii
- a tsunami warning system was incomplete/not functioning in the Indian Ocean during the 2004 Sumatra-Andaman Earthquake. Tsunami in the Indian Ocean are less common than in the Pacific Ocean. As a consequence of this, there is less collective memory/education about tsunami risks.
- see a USGS YouTube video in the Indian Ocean Tsunami Warning System (IOTWS)
- for tsunami warning websites, see below "recommended reading" The 26 December 2004 Sumatra-Andaman tsunami The 11 March 2011 Tohoku Japan tsunami
- NHK coverage of the tsunami on YouTube
- CNN coverage of the tsunami on YouTube
- better LiveLeak coverage of the same tsunami video on YouTube
- a short amateur video
- another amateur video
- another amateur video; note that it snows (i.e. this happened during the cold season!); note the tsunami reached over large buildings; also shows aftermath debris
- another amateur video; very graphic version of how tsunami surged into Kamaishi city
- recent, very disturbing and long, but must-see Youtube video as the tsunami runs up the Okawa river and devastates the city of Kesennuma
Liquefaction (an example of a site-related hazard)
- significantly enhances effects from even relatively small EQs
- ground may seem solid/stable under normal conditions
- structures build on such ground lose support, may topple over as complete structure
- unconsolidated sediments
- clay
- weak sediments underlying mechanically strong layers particularly treacherous as risk often not recognizable on the surface
Some Example events:
- the MW=6.9 17 October 1989 Loma Prieta Earthquake caused liquefaction in San Francisco's Marina District 60 mi from the epicenter. Other effects included sand volcanoes and landslides. The Cypress Viaduct of Interstate 880 collapsed killing 42 people. Some parts of the San Francisco - Oakland Bay Bridge also collapsed. 63 fatalities.
- the MW=6.8 17 January 1995 (Great Hanshin) Kobe, Japan earthquake caused widespread liquefaction on Awaji Island, killing 6434 people (4600 in Kobe) and leaving 300,000 homeless, and causing $102 billion in damage. Artificial islands in Kobe suffered subsidence caused by liquefaction, where water reached the surface. This quake was a major wake-up call for Japan's disaster prevention authorities. Buildings were placed further apart to avoid a falling-domino effect and rubber shock absorbers were installed under bridges. Although located in a subduction zone, this earthquake was a strike-slip earthquake (see lecture 6).
- the MW=8.0 19 September 1985 Mexico City earthquake killed 10,000 people (number somewhat uncertain, and some source cite 40,000). The city is vulnerable to extensive liquefaction because it is built on a sedimentary basin in an old lake bed. The sediments can have a high water content in some locations and the lake bed resonates with seismic waves at certain periods (around 2.5 s) that also cause certain 6-15-story buildings to resonate and collapse. The quake caused 412 building to collapse and damaged 3124 more.
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the MW= 7.6 16 June 1964 Niigata, Japan earthquake caused liquefaction over large parts of the city. In reclaimed land by the Shinona River, reinforced concrete buildings that withstood shaking tilted and one toppled over on liquefied soil. Similar buildings founded on piles bearing on a firm layer at 20 m depth did not suffer damage. The quake destroyed over 3500 buildings and damaged 11,000 more. This earthquake also caused a 6m-high tsunami and 26 casualties. |
Photo: USGS
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the MW= 9.2 27 March 1964 "Good Friday" Alaska earthquake caused wide-spread liquefaction. In Turnagain Heights, a large area failed where a strong layer was underlaid by a weak liquefying water-saturated clay layer; 131 fatalities. This earthquake generates a 8.2-foot tsunami in Prince William Sound. It traveled down the west coast of North America and killed 12 people in Crescent City, CA. Minor damage to boats reached as far south as Los Angeles. This tsunami also caused damage in Hawaii. |
Earthquakes and other Disasters
- landslides: e.g. earthquakes can trigger landslides along previously unstable hillsides
- floods: e.g. when quakes damages a dam, water mains
- fire: e.g. when gas lines rupture; e.g. 1906 San Francisco EQ
- volcanoes: EQs can be the final trigger of an imminent eruption
- 1) an earthquake on Mt. St. Helens on 18 May 1980 triggered a landslide on the volcano's northern flank. The sudden relief in pressure allowed the volcano to go off.
- 2) the great 22 May 1960 Valdivia, Chile earthquake triggered the eruption of Puyehue-Cordon Caulle volcano two days later
- 3) the magnitude 7.9 03 November 2002 Denali, Alaska earthquake changed geyser activity in Yellowstone National Park (see also Lecture 7)
- earthquakes: earthquakes change the stress field in the greater area around a fault; this change can trigger earthquakes further down on the same fault or neighboring faults; e.g. the magnitude 7.2 28 June 1992 Landers Earthquake in the Mojave desert triggered a magnitude 6.5 earthquake in Big Bear 3 hours later
- diseases: e.g. when access to clean drinking water is disrupted; e.g. when dust bearing pathogens is shaken up; the 1994 Northridge EQ made a fungus airborne that causes Valley Fever, a respiratory disease.
Just how much an earthquake can shake up dust and make bacteria, pathogens and toxins airborne was dramatically captured in cell-phone videos during the 4 April 2010 El Mayor (Easter) earthquake in Baja California: - pollution: e.g. when chemical, bio-hazardous and nuclear material from damaged human structures enter environment. An example of this would be the meltdown of the Fukushima-Daiichi nuclear power plant after the 11 March 2011 Tohoku, Japan earthquake (go to Wikipedia). The plant withstood the shaking of the earthquake. But the tsunami topped the protective seawall and caused flooding. The subsequent power outage and lack to get adequate backup power running led to explosions in three of the 4 reactor units, subsequent release of radioactive material, and a losing battle to contain the radioactive material. This has been the largest nuclear disasters since the 1986 Chernobyl disaster.
- power outage, which in turn can draw many other problems with it, e.g.,
- lack of cooling -> food spoilage, industrial accidents (e.g. Fukushima meltdown)
- no traffic guidance -> evacuation breakdown
- no phone connection of cordless phones are used with landlines
- loss of cell phone coverage
- no gas pumps -> gas stations are shut down
- no water pumps -> waste water backs up and contaminates drinking water -> diseases
Recommended Reading
- "A Crack in the Edge of the World", by Simon Winchester; Harper Collins, 2005 ISBN:0-06057199-3 (hardcover); 2006 ISBN: 0-06057200-0 (paperback); describes the 1906 San Francisco earthquake
- "The Raging Sea - The Powerful Account of the Worst Tsunami in U.S. History", by Dennis M. Powers; Citadel Press, 2005 ISBN: 0-8065-2682-3 (paperback); describes the tsunami of the "Good Friday" 1964 Alaska Earthquake and how it damaged and affected Crescent City, California
- the National Oceanographic and Atmospheric Administration (NOAA) tsunami website; click here
- the Pacific Tsunami Warning Center (PTWC) click here
- the West Coast and Alaska Tsunami Warning Center (WCATWC) click here