THE DECEMBER 26, 2004, SUMATRA EARTHQUAKE

The process of subduction that has created Indonesia through volcanic activity, also makes it dangerous.

Burma Microplate Sumatra

Indian Plate

INDIAN PLATE MOVES NORTHCOLLIDING WITH EURASIA

COMPLEX PLATE

BOUNDARY ZONE IN

SOUTHEAST ASIA

Northward motion of India deforms all of

the region

Many small plates (microplates) and

blocks

Note the split in the seismicity into separate thrust and strike-slip faulting.

The Indian plate subducts beneath

the Burma microplate

at about 50 mm/yr

Earthquakes occur at the plate interface along the Sumatra arc (Sunda trench)

These are spectacular &

destructive results of many years of

accumulated motion

Courtesy of W.-Y. Kim

STAGE 1: INTERSEISMIC:

India subducts beneath Burma microplateat about 50 mm/yr

Fault interface is locked

STAGE 1: INTERSEISMIC:

India subducts beneath Burma microplateat about 50 mm/yr

Fault interface is locked

STAGE 2: EARTHQUAKE (COSEISMIC):

Fault interface slips, overriding plate rebounds, releasing accumulated motion

STAGE 1: INTERSEISMIC:

India subducts beneath Burma microplateat about 50 mm/yr

Fault interface is locked

STAGE 2: EARTHQUAKE (COSEISMIC):

Fault interface slips, overriding plate rebounds, releasing accumulated motion

HOW OFTEN MIGHT THEY OCCUR?

Fault slipped ~ 10 m = 10000 mm

10000 mm / 50 mm/yr ≈ 200 yr (Last BIG one was in 1833)

Faults aren’t exactly periodic for reasons we don’t understand.

Examples of seismograms, showing the ground motions recorded at different locations around the world as a function of time.

Note how long in duration these P waves are.

MODELING THESE SEISMOGRAMS shows how slip varied along the fault plane.

Most slip along an area ~400 km long

Maximum slip ~ 20 m

[Contours show time, in seconds]

Courtesy of C. Ji

TWO VIEWS OF THE PART OF THE SUMATRA SUBDUCTION ZONE THAT SLIPPED

Seismogram analysis shows most slip in southern 400 km

Aftershocks show slip extended almost 1200 km

Courtesy of C. Ji

For comparison, a magnitude 5 earthquake would rupture a patch roughly the size of New York City's Central Park.

Seismograms show the seismic waves wrapping around the Earth many times.

Courtesy of R. Aster

For a large earthquake, seismic waves will continue to circle the Earth for weeks, making it reverberate like a gong.

Courtesy of J. Tromp

For a large earthquake, seismic waves will continue to circle the Earth for weeks, making it reverberate like a gong.

(Earth’s lowest “fundamental” note, with a period of ~54 minutes, is an E, twenty octaves below middle E on a piano.)

Courtesy of J. Tromp

Courtesy of M. Van Camp

Raw Gravimeter Record

Filtered to Show 0S0 and 0S2 0S0 = Blue 0S2 = Green

0S0 “Beats” are due to mode splitting.

Courtesy of M. Van Camp

Courtesy of S. Stein and E. Okal

Courtesy of S. Stein and E. Okal

The increasing energy with ultralong periods suggests an enormous “slow” earthquake.

EARTHQUAKE MAGNITUDE 9.3

Energy released ≈ 65 × 1017 Joules, or 1,550,000 kilotons (1,550 megatons) of TNT, or the equivalent of 75,000

Hiroshima bombs.

Courtesy of IRIS

 Location Date Magnitude

1.Chile 1960 05 22 9.5

2.West Coast of Northern Sumatra 2004 12 26 9.3

3.Prince William Sound, Alaska 1964 03 28 9.2

4.Andreanof Islands, Alaska 1957 03 09 9.1

5.Kamchatka 1952 11 04 9.0

6.Off the Coast of Ecuador 1906 01 31 8.8

7.Rat Islands, Alaska 1965 02 04 8.75

8.Assam – Tibet 1950 08 15 8.6

9/10.Kamchatka 1923 02 03 8.5

9/10.Banda Sea, Indonesia 1938 02 01 8.5

Top Ten List of Earthquakes in the Past 100 Years.

This one earthquake comprises more than ½ of the total energy released by earthquakes during the past 30 years!

Courtesy of G. Ekstrom

New Okal & Stein value

VERTICAL UPLIFT HORIZONTAL MOTION

Courtesy of C. Ji

TSUNAMI - water wave generated by earthquake

IN DEEP OCEAN tsunami has long wavelength, travels fast, small amplitude - doesn’t affect ships

AS IT APPROACHES SHORE, it slows. Since energy is

conserved, amplitude builds up - very damaging

TSUNAMI SPEED IN DEEP WATER of

depth d

c = (gd)1/2

g = 9.8 m/s2 d = 4000 m

c = 200 m/s = 720 km/hr = 450 m/hr

Tsunami generated along fault, where sea floor displaced, and

spreads outward

Reached Sri Lanka in 2 hrs, India in 2-3

Courtesy of K. Sataki

Tsunami waves bend around “obstacles” like islands and peninsulas in a process called DIFFRACTION.

The Tsunami Eventually Crossed All Oceans

SLOW

FAST

Tsunami waves also bend in a process called REFRACTION (according to “Snell’s Law”) as water depth & thus speed changes.

RAYS BEND AS WATER DEPTH CHANGES

DENSITY OF RAYS SHOWS FOCUSING & DEFOCUSING

1 hour

Seen safely from high ground, a wave of the 1960 Chilean tsunami pours into Onagawa, Japan.

1. 1797: A magnitude 8.4 earthquake near the central part of the western Sumatra generated a tsunami that flooded Padang. More than 300 fatalities.  

2. 1833: A magnitude 8.7 earthquake near the south coast of the western Sumatra triggered a huge tsunami that flooded the southern part of western Sumatra. Numerous victims.

3. 1843: A tsunami that came from the southeast and flooded the coast of the Nias Island. Many fatalities.

4. 1861: A magnitude 8.5 earthquake affected all the western coast of Sumatra. Several thousand fatalities.

5. 1881: A magnitude 7.9 earthquake in the Andaman Island region generated a 1 m high tsunami on India’s eastern coast.

6. 1883: Krakatau explosion. 36,000 fatalities, primarily on the islands of Java and Sumatra.

7. 1941: A magnitude ~7.7 Adaman Islands earthquake. Anecdotal accounts exist of a tsunami, however, no official records exist.

Previous Tsunamis in the Sumatra Region

Giant Eruption of Krakatau in 1883.

Tsunamis are highly DISPERSIVE.

NY Times

….this has very bad consequences!

Aftermath of the 1960 Chilean tsunami in Hilo, Hawaii, where the tsunami caused 61 deaths.

Waterfront area in Crescent City, California, flooded by the 1960 Chilean tsunami. Here, the tsunami caused more than $30,000 in damage. However, damage was much more severe 600 miles to the south in the Los Angeles area, where the tsunami killed one person and caused from $500,000 to $1,000,000 in damage, including the sinking of 30 boats.

Because seismic waves travel much faster (km/s) than tsunamis, rapid analysis of seismograms can identify earthquakes likely to cause major tsunamis and predict when waves will arrive.

TSUNAMI WARNING SYSTEM

Deep ocean buoys can measure wave heights, verify tsunami and reduce false alarms.

Atmospheric Angular Momentum (AAM, in red) and VLBI length of day (LOD, in blue) measurements are strongly correlated over periods of one week to several years (storms, atmospheric currents, El Nino, etc.).

Ocean tides and currents modulate the Earth's orientation on periods of a day and shorter. The model (red) follows the VLBI measurements (blue) closely.

Predicted change in length of day due to the Sumatra earthquake:    -2.676 microseconds (not verifiable?)

Changes in Length of Day (due to mass redistribution)

The change in polar motion excitation from the earthquake is predicted to be very small, about 0.82 milliarcseconds (also not observable?).

“CIVILIZATION EXISTS BY GEOLOGICAL

CONSENT”

The same geologic processes that make our planet

habitable also make it dangerous