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 estimate.

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)
1. 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.
2. 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.
3. 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.
4.  1861/02/16 Exceptionally strong earthquake affected all the western coast of Sumatra. Several thousand fatalities.
5. 1883 Krakatau explosion 36,000 fatalities

Question: How have tsunamis affected the United States?

Answer: The 1964 great Alaska earthquake (M9.2) tsunami caused damage and loss of life across the Pacific. The West Coast & Alaska Tsunami Warning Center, Palmer Alaska (http://www.wcatwc.gov/64quake.htm) indicates that the 1964 tsunami was the mostdisastrous tsunami to hit the U.S. West Coast and British Columbia in Canada. The largest wave height for this tsunami was reported at Shoup Bay, Valdez Inlet (67 meters). Summary of lives lost and damage for Alaska, Canada, Washington, Oregon and California are: Alaska- 106 deaths and $84 million; British Columbia- $10 million; Washington- minor damage throughout the coast; Oregon- 4 deaths and $0.7 million, with much of the damage away from the coast where rivers overflowed; California- 13 deaths and $10 million damage.

The Pacific Disaster Center reports that tsunamis have accounted for more lost lives in Hawaii than the total of all other local disasters (www.pdc.org). In the 20th century, an estimated 221 people have been killed by tsunamis on the islands of Hawaii. Most of these deaths occurred on the Big Island during the tsunamis of 1946 and 1960, two of the largest tsunamis to strike in the Pacific. The 1946 Aleutian Island earthquake (M8) killed 159 residents and caused more than $26 million in damage. The May 23, 1960 Chile earthquake (M 9.5) generated a 35 foot wave causing 61 deaths and $23 million in damage. Other significant tsunamis in Hawaii include: 1952 (M9) Kamchatka, USSR earthquake ($1 million damage), 1957 (M9.1) Aleutian Islands earthquake ($5 million damage), 1975 (M7.5) earthquake off the Big Island of Hawaii (2 deaths).