9.0 - OFF THE WEST COAST OF NORTHERN SUMATRA
2004 December 26 00:58:53 UTC
Preliminary Earthquake Report
U.S. Geological Survey, National Earthquake Information Center
World Data Center for Seismology, Denver
Magnitude 9.0 OFF W COAST OF NORTHERN
Sunday, December 26, 2004 at 00:58:53 UTC
Preliminary Earthquake Report
U.S. Geological Survey, National Earthquake Information Center
World Data Center for Seismology, Denver
The devastating megathrust
earthquake of December 26,
2004, occurred on the interface of the India and Burma plates
and was caused by the release of stresses that develop as the India plate subducts beneath the overriding Burma plate. The India plate
begins its descent into the mantle at the Sunda
trench, which lies to the west of the earthquake's epicenter. The trench is the
surface expression of the plate interface between the Australia and India plates,
situated to the southwest of the trench, and the Burma and Sunda
plates, situated to the northeast.
In the region of the earthquake, the India plate moves toward the northeast at a rate of about 6 cm/year relative to the Burma plate. This results in oblique convergence at the Sunda trench. The oblique motion is partitioned into thrust-faulting, which occurs on the plate-interface and which involves slip directed perpendicular to the trench, and strike-slip faulting, which occurs several hundred kilometers to the east of the trench and involves slip directed parallel to the trench. The December 26 earthquake occurred as the result of thrust-faulting.
Preliminary locations of larger aftershocks following the megathrust earthquake show that approximately 1200 km of the plate boundary slipped as a result of the earthquake. By comparison with other large megathrust earthquakes, the width of the causative fault-rupture was likely over one-hundred km. From the size of the earthquake, it is likely that the average displacement on the fault plane was about fifteen meters. The sea floor overlying the thrust fault would have been uplifted by several meters as a result of the earthquake. The above estimates of fault-dimensions and displacement will be refined in the near future as the result of detailed analyses of the earthquake waves.
The world's largest recorded earthquakes have all been megathrust events, occurring where one tectonic plate subducts beneath another. These include:
the magnitude 9.5 1960 Chile earthquake, the magnitude 9.2 1964 Prince William Sound, Alaska, earthquake, the magnitude 9.1 1957 Andreanof Islands, Alaska, earthquake, and the magnitude 9.0 1952 Kamchatka earthquake. As with the recent event, megathrust earthquakes often generate large tsunamis that cause damage over a much wider area than is directly affected by ground shaking near the earthquake's rupture.
Physics of Tsunamis
The phenomenon we call a tsunami (soo-NAH-mee)
is a series of waves of extremely long wave length and long period generated in
a body of water by an impulsive disturbance that displaces the water. Tsunamis
are primarily associated with earthquakes in oceanic and coastal regions.
Landslides, volcanic eruptions, nuclear explosions, and even impacts of objects
from outer space (such as meteorites, asteroids, and comets) can also generate
As the tsunami crosses the deep ocean, its length from crest to crest may be a hundred miles or more, and its height from crest to trough will only be a few feet or less. They can not be felt aboard ships nor can they be seen from the air in the open ocean. In the deepest oceans, the waves will reach speeds exceeding 600 miles per hour (970 km/hr). When the tsunami enters the shoaling water of coastlines in its path, the velocity of its waves diminishes and the wave height increases. It is in these shallow waters that a large tsunami can crest to heights exceeding 100 feet (30 m) and strike with devastating force.
The term tsunami was adopted for general use in 1963 by an international scientific conference. Tsunami is a Japanese word represented by two characters: "tsu" and "nami". The character "tsu" means harbor, while the character "nami" means wave. In the past, tsunamis were often referred to as "tidal waves" by many English speaking people. The term "tidal wave" is a misnomer. Tides are the result of gravitational influences of the moon, sun, and planets. Tsunamis are not caused by the tides and are unrelated to the tides; although a tsunami striking a coastal area is influenced by the tide level at the time of impact. Also in the past, the scientific community referred to tsunamis as "seismic sea waves". "Seismic" implies an earthquake-related mechanism of generation. Although tsunamis are usually generated by earthquakes, tsunamis are less commonly caused by landslides, infrequently by volcanic eruptions, and very rarely by a large meteorite impact in the ocean.
Earthquakes generate tsunamis when the sea floor abruptly deforms and displaces the overlying water from its equilibrium position. Waves are formed as the displaced water mass, which acts under the influence of gravity, attempts to regain its equilibrium. The main factor which determines the initial size of a tsunami is the amount of vertical sea floor deformation. This is controlled by the earthquake's magnitude, depth, fault characteristics and coincident slumping of sediments or secondary faulting. Other features which influence the size of a tsunami along the coast are the shoreline and bathymetric configuration, the velocity of the sea floor deformation, the water depth near the earthquake source, and the efficiency which energy is transferred from the earth's crust to the water column.
A tsunami can be generated by ANY disturbance that displaces a large water mass from its equilibrium position. Submarine landslides, which often occur during a large earthquake, can also create a tsunami. During a submarine landslide, the equilibrium sea-level is altered by sediment moving along the sea-floor. Gravitational forces then propagate the tsunami given the initial perturbation of the sea-level. Similarly, a violent marine volcanic eruption can create an impulsive force that displaces the water column and generates a tsunami. Above water (subarial) landslides and space born objects can disturb the water from above the surface. The falling debris displaces the water from its equilibrium position and produces a tsunami. Unlike ocean-wide tsunamis caused by some earthquakes, tsunamis generated by non-seismic mechanisms usually dissipate quickly and rarely affect coastlines far from the source area.
Tsunamis are characterized as shallow-water waves. Shallow-water waves are different from wind-generated waves, the waves many of us have observed on a the beach. Wind-generated waves usually have period (time between two sucessional waves) of five to twenty seconds and a wavelength (distance between two sucessional waves) of about 100 to 200 meters (300 to 600 ft). A tsunami can have a period in the range of ten minutes to two hours and a wavelength in excess of 300 miles (500 km). It is because of their long wavelengths that tsunamis behave as shallow-water waves. A wave is characterized as a shallow-water wave when the ratio between the water depth and its wavelength gets very small. The speed of a shallow-water wave is equal to the square root of the product of the acceleration of gravity (32ft/sec/sec or 980cm/sec/sec) and the depth of the water. The rate at which a wave loses its energy is inversely related to its wavelength. Since a tsunami has a very large wave length, it will lose little energy as it propagates. Hence in very deep water, a tsunami will travel at high speeds and travel great transoceanic distances with limited energy loss. For example, when the ocean is 20,000 feet (6100 m) deep, unnoticed tsunami travel about 550 miles per hour (890 km/hr), the speed of a jet airplane. And they can move from one side of the Pacific Ocean to the other side in less than one day.
As a tsunami leaves the deep water of the open sea and propagates into the more shallow waters near the coast, it undergoes a transformation. Since the speed of the tsunami is related to the water depth, as the depth of the water decreases, the speed of the tsunami diminishes. The change of total energy of the tsunami remains constant. Therefore, the speed of the tsunami decreases as it enters shallower water, and the height of the wave grows. Because of this "shoaling" effect, a tsunami that was imperceptible in deep water may grow to be several feet or more in height.
When a tsunami finally reaches the shore, it may appear as a rapidly rising or falling tide, a series of breaking waves, or even a bore. Reefs, bays, entrances to rivers, undersea features and the slope of the beach all help to modify the tsunami as it approaches the shore. Tsunamis rarely become great, towering breaking waves. Sometimes the tsunami may break far offshore. Or it may form into a bore: a step-like wave with a steep breaking front. A bore can happen if the tsunami moves from deep water into a shallow bay or river. The water level on shore can rise many feet. In extreme cases, water level can rise to more than 50 feet (15 m) for tsunamis of distant origin and over 100 feet (30 m) for tsunami generated near the earthquake's epicenter. The first wave may not be the largest in the series of waves. One coastal area may see no damaging wave activity while in another area destructive waves can be large and violent. The flooding of an area can extend inland by 1000 feet (305 m) or more, covering large expanses of land with water and debris. Flooding tsunami waves tend to carry loose objects and people out to sea
when they retreat. Tsunamis may reach a maximum vertical height onshore above sea level, called a run-up height, of 30 meters (98 ft). A notable exception is the landslide generated tsunami in Lituya Bay, Alaska in 1958 which produced a 525 meter (1722 ft) wave.
Since science cannot predict when earthquakes will occur, they cannot determine exactly when a tsunami will be generated. But, with the aid of historical records of tsunamis and numerical models, science can get an idea as to where they are most likely to be generated. Past tsunami height measurements and computer modeling help to forecast future tsunami impact and flooding limits at specific coastal areas. There is an average of two destructive tsunamis per year in the Pacific basin. Pacific wide tsunamis are a rare phenomenon, occurring every 10 - 12 years on the average.
What is a tsunami?
A tsunami is a series of waves with a long wavelength and period (time between crests).
Time between crests of the wave can vary from a few minutes to over an hour.
Tsunamis are often incorrectly called tidal waves; they have no relation to the daily ocean tides.
How are tsunamis generated?
Tsunamis are generated by any large, impulsive displacement of the sea level.
Earthquakes generate tsunamis by vertical movement of the sea floor. If the sea floor movement is horizontal, a tsunami is not generated.
Tsunamis are also triggered by landslides into or under the water surface, and can be generated by volcanic activity and meteorite impacts
How often do tsunamis occur?
On the average, there are two tsunamis per year somewhere in the world which cause damage near the source
Approximately every 15 years a destructive, Pacific-wide tsunami occurs.
What does the word “tsunami” mean?
Tsunami (soo-NAH-mee) is a Japanese word meaning harbor wave.
How fast does a tsunami travel?
Tsunami velocity is dependent on the depth of water through which it travels (Velocity equals the square root of water depth times the gravitational acceleration).
Tsunamis travel approximately 475 mph in 15,000 feet of water. In 100 feet of water the velocity drops to about 40 mph.
For example, a tsunami travels from the central Aleutian Is. to Hawaii in about 5 hours and to California in about 6 hours.
How big is a tsunami?
Tsunamis range in size from inches to over a hundred feet.
In deep water (greater than 600 feet), tsunamis are rarely over 3 feet and will not be noticed by ships due to their long period (time between crests).
As tsunamis propagate into shallow water, the wave height can increase by over 10 times.
Tsunami heights can vary greatly along a coast. The waves are amplified by certain shoreline and bathymetric (sea floor) features.
A large tsunami can flood land up to a mile from the coast.
What does a tsunami look like when it reaches shore?
Normally, a tsunami appears as a rapidly advancing or receding tide.
It some cases a bore (wall of water) or series of breaking waves may form.
How is a tsunami different from a wind-generated wave?
Wind-generated waves usually have periods (time between crests) of 5 to twenty seconds. Tsunami periods are usually between 5 minutes and an hour.
waves break as they shoal and lose energy offshore. Tsunamis act more
like a flooding wave. A twenty foot tsunami is a twenty foot rise in
Question: Can we expect many aftershocks to this earthquake?
Answer: There have been numerous aftershocks detected following the recent magnitude 9 megathrust earthquake. The U.S. Geological Survey National Earthquake Information Center (USGS/NEIC) continues to record many newly occurred aftershocks. As of 1:00PM, MST, December 29, sixty-eight aftershocks have been cataloged. The largest occurred about three hours after the main shock and is now assigned a magnitude of 7.1. Thirteen of the aftershocks thus far cataloged have magnitudes of 6.0 or larger. There have been no reports of tsunamis being generated from the aftershocks. We know from past experience that the number of aftershocks will decrease with time. However, the number of aftershocks can be quite variable. There might be short episodes of higher activity as well as lulls in activity, but the overall trend will be for fewer aftershocks as time goes by. Seismologists are not able to predict the timing and sizes of individual aftershocks.
The number of cataloged aftershocks will be constantly changing, as new aftershocks occur and as USGS/NEIC analysts newly locate aftershocks from the first few days after the earthquake. Magnitudes assigned to individual aftershocks may change somewhat as new data come in. An up-to-date catalog of analyst-processed USGS/NEIC epicenters and magnitudes is at http://earthquake.usgs.gov/recenteqsww/Quakes/quakes_all.html
Question: How has the occurrence of this earthquake affected the probability of another great earthquake?
Answer: The occurrence of this earthquake will have produced a redistribution of tectonic stresses along and near the boundary between the India plate and the Burma plate. In some areas, this redistribution of stresses will be such as to shorten the time to the next big earthquake compared to what would have been the case if the earthquake had not happened. In other areas, the redistribution of stresses will be such as to increase the time to the next big earthquake. Once the distribution of slip along the earthquake fault has been mapped, it will be possible to estimate the areas that were moved closer to future failure and those that were moved farther from future failure. It is not yet possible, however, to reliably estimate when the future failure will occur in a given area or how large will be the resulting earthquake.
Question: This earthquake occurred within three days of a magnitude 8.1 earthquake in the Macquarie Islands. Is there any relation between the two earthquakes?
Answer: The occurrence of two great earthquakes within such a short space of time is indeed striking. However, even in retrospect, we do not yet see evidence for a strong causal relationship between the two earthquakes.
It seems clear that long-term stress changes associated with one earthquake may trigger other earthquakes on the same fault or on nearby faults. In fact, the aftershocks that occur around the source of a large earthquake are triggered by such stress changes. But the long-term stress changes caused by an earthquake decrease rapidly with distance away from the earthquake source. The Macquarie Ridge earthquake was very far from the site of the yet-to-occur Sumatra-Andaman Islands earthquake, and occurred on a different plate boundary. The hypothesis that long-term stress changes associated with the Macquarie Ridge earthquake triggered the Sumatra-Andaman Islands earthquake therefore does not seem compelling.
There is also strong evidence that the shaking of the ground caused by a great earthquake, such as the Macquarie Ridge earthquake, can trigger small earthquakes in sensitive tectonic environments at large distances from the great earthquake. The evidence for such triggering is most convincing when the earthquakes that are thought to be triggered occur near the time of strongest shaking from the triggering earthquake, which would be within several hours following the triggering earthquake. However, the Sumatra/Andaman-Islands earthquake occurred about two-and-a-half days after the Macquarie Ridge earthquake.
An alternative to the hypothesis that the Macquarie Ridge and Sumatra/Andaman Islands earthquakes are causally related is that the occurrence of the two, widely separated, great earthquakes within three days was a probabilistic coincidence.
Question: What was the size of the fault that produced the earthquake?
Answer: An initial
estimate of the size of the rupture that caused the earthquake
is obtained from the length of the aftershock zone, the dimensions of historical earthquakes, and a study of the elastic waves generated by the earthquake. The aftershocks suggest that the earthquake rupture had a maximum length of 1200 -- 1300 km parallel to the Sunda trench and a width of over 100 km perpendicular to the earthquake source. An early estimate from the study of elastic waves show the majority of slip was concentrated in the southernmost 400 km of the rupture.
Question: What was the maximum displacement on the rupture surface between the plates ?
Answer: The maximum displacement estimated from a preliminary study of the seismic body waves is 20 meters.
Question: What was the maximum displacement of the sea bottom above the earthquake source?
Answer: The displacement of the ground surface will be related to, but somewhat less than, the displacement on the earthquake fault at depth. In places, the block of crust beneath the sea floor and overlying the causative fault is likely to have moved on the order of 10 meters to the west-southwest and to have been uplifted by several meters.
Question: What is the angle of subduction of the India plate beneath the Burma plate?
Answer: At the source of the earthquake, the interface between the India plate and the Burma plate dips about 10 degrees to the east-northeast. The subducting plate dips more steeply at greater depths.
Question: What effect did this earthquake have on the rotation of the earth?
Answer: Richard Gross at
JPL has modeled the coseismic effect on the Earth's rotation of the December 26
earthquake in Indonesia by using the PREM model for the elastic properties of
the Earth and the Harvard centroid-moment tensor solution for the source
properties of the earthquake. The result is:
change in length of day: -2.676 microseconds
polar motion excitation X : -0.670 milliarcseconds
polar motion excitation Y: 0.475 milliarcseconds
Since the length of the day can be measured with an accuracy of about 20 microseconds, this model predicts that the change in the length-of-day caused by the earthquake is much too small to be observed. And, since the location of the earthquake was near the equator, this model predicts that the change in polar motion excitation is also rather small, being about 0.82 milliarcsecond in amplitude. Such a small change in polar motion excitation will also be difficult to detect.
Question: Why did the magnitude of this earthquake change?
Answer: While earthquake
location can be determined fairly rapidly, earthquake size is somewhat more
problematic. This is because location is mainly based upon measurements of the
time that seismic waves arrive at a station. Magnitude, on the other hand, is
based upon the amplitude of those waves. The amplitude is much more variable
than the arrival times, thus causing greater uncertainty in the magnitude
For larger earthquakes, the problem is compounded by the fact that the larger the earthquake, the lower the characteristic frequency of the seismic waves. This means that surface wave arrivals, which contain lower frequency energy than the body waves, must be used to determine the magnitude. For a great earthquake, several hours of data must be recorded in order to accurately determine the magnitude.
Thus, accurate estimates of the magnitude can follow an accurate estimate of the location by several hours. In the case of the M 9.0 Sumatra-Andaman Islands earthquake, the standard methods were inadequate for measuring the very low frequency energy produced and had to be modified. This delayed the final determination of the magnitude until the next day.
Question: Is there a system to warn populations of an imminent occurrence of a tsunami?
Answer: The Pacific
Tsunami Warning Center is responsible for tsunami monitoring in
the Pacific Basin. Their website is at http://www.prh.noaa.gov/ptwc/. Tragically, no such system exists for the Bay of Bengal where the recent disaster occurred.
Question: What other great (M > 8) earthquakes have occurred in the region?
Answer: Since 1900 and prior to the December 26 earthquake, the largest earthquake along the subduction zone from southern Sumatra to the Andaman Islands occurred in 2000 and had a magnitude of 7.9. A magnitude 8.4 earthquake occurred in 1797, a magnitude 8.5 in 1861 and a magnitude 8.7 in 1833 . All three ruptured sections of the subduction zone to the south of the recent earthquake. Interestingly, the 1797 and 1833 quakes are believed to have ruptured roughly the same area with only 36 years separating the events. Paleoseismic evidence shows that great earthquakes or earthquake couplets occur about every 230 years (http://www.gps.caltech.edu/~sieh/publications/a10.html).
Question: What other significant tsunamis have occurred in the region?
The following destructive tsunamis are listed on a data base maintained by the Tsunami Laboratory, Institute of Computational Mathematics and Mathematical Geophysics (http://tsun.sscc.ru/tsulab/20041226tsun.htm)
1797/02/10 Central part of the western Sumatra. The quake was most felt near Padang and in the area within +/-2 deg of equator. Padang was flooded by powerful waves. More then 300 fatalities.
1833/11/24 South coast of the western Sumatra, estimated rupture from 1 S to 6 S latitude. Huge tidal wave flooded all southern part of the western Sumatra. Numerous victims.
1843/01/05 Strong earthquake west of the central Sumatra. Terrible wave came from the south-east and flooded all the coast of the Nias Island. Many fatalities.
1861/02/16 Exceptionally strong earthquake affected all the western coast of Sumatra. Several thousand fatalities.
1883 Krakatau explosion 36,000 fatalities