Sumatra Earthquake & Indian Ocean Tsunami

2004 Sumatra Earthquake & Indian Ocean Tsunami

Lecture Notes

GSC 350: Natural Disasters

(last updated: January 10, 2005)

Lecture notes by:

Dr. Jeff Marshall

Geological Sciences Department

Cal Poly Pomona University

Pomona, California, USA 91768

marshall@csupomona.edu

http://www.csupomona.edu/~marshall

(Note: These lecture notes are provided for educational purposes only.

Any other use may constitute a violation of copyright.)

I. Overview

- On the morning of 12/26/04 at 7:58:49 am local time (00:58:49 UTC)

- M 9.0 earthquake, ~100 km (60 mi) off W. coast Northern Sumatra, Indonesia

- 1200 km section of Earth’s crust shifted ~10-20 m beneath Indian Ocean

- quake released stored elastic energy = ~ 23,000 Hiroshima atomic bombs

- uplifted seafloor several meters, displacing billions of gallons of seawater

- set off massive waves (“tsunami”) that raced out toward the west and east

- over period of 10 hours, tsunami struck coasts of over 12 countries

- major damage and deaths around Indian Ocean from SE Asia to East Africa

- one of the most devastating natural disasters in modern human history

- > 150,000 people killed directly by earthquake & tsunami, millions injured

- one third of dead are children (also left tens of thousands of orphans)

- over 5 million homeless and without basic needs (water, food, shelter)

- threat of exposure, poor sanitation, & disease could kill as many as tsunami

- World has mobilized one of greatest disaster relief efforts in history

II. Why was this such a tremendous disaster?

The earthquake & tsunami:

- The earthquake and tsunami themselves are not unusual events

- While Great Earthquakes of ≥ M 9.0 are infrequent on human time scale, they are a normal part of the way the Earth works (Plate Tectonics)

- This quake 4^th largest since accurate seismic records began in late 1800’s

- 5 earthquakes of ≥ M 9.0 have all occurred in last 52 years:

o This event: 2004 M 9.0 Sumatra – Andaman Islands, Indian Ocean

o 1964 M 9.2 Alaska - Prince William Sound (“Good Friday Quake”)

o 1960 M 9.5 Chile - Pacific Coast

o 1957 M 9.1 Alaska - Andreanof Islands

o 1952 M 9.0 Kamchatka (Eastern Russia)

- The four prior quakes all set off deadly tsunami in Pacific basin

- These ≥ M 9.0 quakes only occur on subduction zones: where Ocean crust slides beneath edge of Continental plates or Oceanic plates

- while these plates are continually moving toward each other (cm/yr), their edges (“margins”) can remain “stuck” to one another due to friction for long periods of time (centuries)

- ongoing stress of plate motion causes plate edges to bend (or “strain”)

- elastic energy builds until sudden slip (“rupture”) between the plates releases the energy in an earthquake

- the sudden lurch of seafloor, displaces ocean water, causing tsunami

The disaster itself:

- While the quake & tsunami were not unusual from a geologic perspective, the resulting disaster was unprecedented

- unusually widespread, w/ death & destruction across 1000’s of km

- disaster affected people in over 12 nations across two continents

- also 1000’s of foreign tourists (worst disaster in history for Sweden)

- However, it is important to keep this event in perspective:

- while this is indeed a major disaster, it is certainly not the worst disaster in history in terms of lives lost or impact on nations. For example:

o M 7.6 Earthquake in China in 1972 killed 240,000 in city of Tangshan

o Hurricane & flood in Bangladesh in 1970 killed 400,000

o Chernobyl Nuclear Power Plant accident in 1986 killed 165,000

o Millions of people are dying right now in slowly unfolding disasters of famine (e.g., East Africa), disease (e.g., AIDS), and war / genocide

Five main reasons why this disaster was so deadly:

o high population density on low-lying coastal areas of Indian Ocean (world population has doubled since time of Alaska & Chile quakes in the 1960’s; Asia has one of the fastest rates of population growth)

o short distance from tsunami source to populated low lying coasts, leaving little time for warning ( tsunami struck Sumatra within 20 min. of quake, killing 94,000; Sri Lanka & Thailand were hit within 2 hours)

o No tsunami warning system in Indian Ocean (very effective system in Pacific Ocean where 85% of tsunamis occur, but lack of science funding and disinterest have impeded implementation elsewhere)

o Poor and developing countries with vulnerable infrastructure and minimal disaster preparedness

o little public awareness of tsunami hazard (Indian Ocean countries had little prior experience with large tsunami; last major disaster of this type was 1883 eruption of Krakatoa, Indonesia which set off tsunamis that killed 35,000 people)

III. The Earthquake

Sumatra-Andaman Islands earthquake

- rupture initiated: 12/26/04, 7:58:49 am local time (00:58:49 UTC)

- M 9.0, centered ~100 km (60 mi) off W. coast Northern Sumatra, Indonesia

- result of NE subduction of India plate beneath Burma microplate (6.1 cm/yr)

- shallow rupture of northern 1200-1300 km of Sunda Trench

- hypocenter depth = 10 km, dip angle 10^o, down-dip rupture width 100 km

- slip of 10 - 20m (variable along trench)

- greatest slip concentrated in southern 400 km near epicenter

- Magnitude of quake function of: Rupture Area (l x w) and Slip Length

- rupture propagated northward from epicenter off northern Sumatra along Sunda Trench beneath Nicobar and Andaman Islands

- These islands displaced 10 m to the SW and uplifted several meters

- elastic energy released = 20X10^17 Joules = 475,000 kilotons (475 megatons) of TNT = 23,000 Hiroshima atomic bombs

Prior large earthquakes in this area:

- Prior historic Sunda Trench quakes (all to the south along Sumatra):

o largest since 1900: M 7.9 in 2000

o M 8.4 in 1797, M 8.7 in 1833, M 8.5 in 1861

o 1797 & 1833 quakes ruptured same area only 36 years apart

o paleoseismic data (Kerry Sieh, Caltech Univ.) show great earthquakes or earthquake couplets with 230 year recurrence interval in that area

Challenges in determining Quake Magnitude:

- early reports gave Magnitude of M 8.0 - 8.5 for the 2004 quake

- while quake location (epicenter) can be determined rapidly

- accurate Magnitude is more problematic ( esp. w/ very large earthquakes)

- location based upon seismic wave arrival times at seismographic stations

- magnitude usually based on seismic wave amplitude (height)

- this data involves significant variation and uncertainty

- larger earthquakes have lower characteristic frequency for seismic waves

- must use Surface Wave arrivals (lower frequency energy than Body Waves)

- For great earthquake (>8.0), several hours of data needed for accurate Mag.

- M 9.0 Sumatra-Andaman Islands earthquake:

o standard methods inadequate for measuring very low freq. energy

o delayed the final determination of the magnitude until the next day

IV. The Tsunami

Tsunami basics:

- large waves that inundate coastal areas far beyond normal tidal range

- generated by large vertical displacements of seafloor that move water: such as earthquakes, undersea landslides, volcanic eruptions, meteor impacts

- not just one wave, but a series of waves w/ multiple crests & troughs

- in open ocean, very long wavelength (100’s km) w/ short amplitude (cm’s)

- difficult to detect except with pressure gauges & current meters

- move extremely fast in open ocean: up to 700 km/hr (500 mph: speed of jet!)

- when wave front encounters shallow water, it slows down

- fast moving back of wave compresses into slower front, rapidly increasing height (amplitude) of wave, pushing large volumes of water onshore

- wave strikes coast w/ much shorter wavelength and great height

Shape of Indian Ocean tsunami:

- waves generated along 1200 km long “linear” trench

- SW verging thrust motion generated two waves, heading West & East:

- wave heading West toward Sri Lanka & India began with crest (high water)

- wave heading East toward Sumatra & Thailand began w/ trough (low water)

Height of Indian Ocean tsunami:

- waves near epicenter striking Sumatra coast: 10-15 m high (30-45 ft.)

- waves that swept over Nicobar Islands near the Sunda Trench: > 5 m (15 ft)

- Thailand: 3-5 m (9-15 ft), Sri Lanka: 5-10 m (15-30 ft), Kenya: 2-3 m (6-9 ft)

- local shape of coast & constrictions can greatly enhance wave height

Time sequence for Indian Ocean tsunami (see Kenji Satake model):

- 10 min: swept across Nicobar & Andaman Islands (territories of India)

- 20 min: slammed into adjacent western coasts of Northern Sumatra, and city of Banda Aceh (in Iowland valley on Sumatra’s northern tip)

- continued at ~700 kph (500mph) for 1000’s km across open water

- 1.5-2.0 hrs: strikes Thailand and Burma (Myanmar) to the East

- 2.0-2.5 hrs: to the West across Indian Ocean to hit Sri Lanka and India

- 3-4 hrs: continues West across island nations of Maldives & Seychelles

- 8-10 hrs: hits coast of E. Africa - Somalia, Kenya, Tanzania, Madagascar

Lack of Tsunami Warning System:

- Indian Ocean has no tsunami warning system in place

- 85% of historical tsunami have occurred in Pacific Ocean

- U.S. NOAA Pacific Tsunami Warning Center established in 1949

- buoys and ocean bottom sensors w/ satellite links continuously monitor Pacific Basin seismic activity & ocean surface level

- the Center sends early tsunami warnings to Pacific Rim countries

- UN set up Int’l Tsunami Information Center in 1965 after tsunami disasters in Hawaii, Japan, California caused by 1960 Chile & 1964 Alaska quakes

- 12/26/04 PTWC recognized that a large, potentially tsunamigenic earthquake had occurred offshore of Sumatra, Indonesia (gave it M 8.2 magnitude)

- PTWC released a warning to Pacific nations, but were unable to communicate with Indian Ocean nations before the wave struck

- Warning systems are relatively inexpensive, but limited government science funding and general apathy had excluded Indian Ocean until now