Magnitude 9.0 OFF W COAST OF NORTHERN SUMATRA
Sunday, December 26, 2004 at 00:58:49 UTC
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
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
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: How much energy was released by this earthquake?
Answer: Es 20X10^17 Joules, or 475,000 kilotons (475 megatons) of TNT, or the equivalent of 23,000 Hiroshima bombs.
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.