## Magnitude 9.1 Sumatra-Andaman Islands Earthquake FAQ

Question: Is the magnitude of the December 26th, 2004 Sumatra-Andaman Islands earthquake greater than 9.1?

Answer: Many detailed studies on the size of the December 26th, 2004 Sumatra-Andaman Islands earthquake have been conducted. One careful study reports a magnitude as high as magnitude 9.3. Determining the magnitude of earthquakes larger than 9 is difficult, so several techniques have been used and new techniques were developed. Since determining the magnitude of the Sumatra-Andaman Islands earthquake is complicated, there is no firm community consensus on the precise "correct" magnitude of this earthquake. The USGS derives it magnitude from a study by Park et al. [2005] and the IASPEI moment-magnitude conversion [`Mw= (2/3)*(log(Mo)-16.1)`, where Mo is moment in dyne-cm].

Question: What was the background seismicity in the region before the M9.1 earthquake?

Answer: This table represents the number of earthquakes in the aftershock zone of the magnitude 9.1 earthquake on 12/26/04 for the ten previous years.

The numbers of earthquakes located in 2004 does NOT reflect the main shock or aftershocks from the 9.1. The region encompasses a rectangular box which extends from 2N to 14N and from 92E to 98E. These statistics were obtained from the USGS PDE earthquake catalog search page.

 YEAR Magnitude 5.5 & larger events Magnitude 5.0 & larger events Magnitude 4.5 & larger events 1995 2 7 35 1996 2 9 36 1997 2 11 37 1998 1 8 38 1999 3 11 34 2000 5 12 44 2001 4 9 36 2002 11 25 91 2003 6 20 64 2004 4 14 67

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: How come the 12/23/04 M8.1 Macquarie Island earthquake didn't produce a tsunami? What was the difference?

Answer: A tsunami is a sea wave of local or distant origin that can be generated when the sea floor abruptly deforms and vertically displaces overlying water.   Such a displacement can occur when an earthquake ruptures oceanic lithosphere.  When the opposite sides of a fault are inclined and have a vertical component of motion, we have an earthquake with dip-slip faulting.  When the opposite sides of a fault are vertical and move horizontally, we have an earthquake with strike-slip faulting.  Given two earthquakes of the same size, the one that has greater vertical fault motion is likely to displace a greater amount of overlying water.  Indeed, the Sumatra and Macquarie Ridge earthquakes occurred on different plate boundaries and had different faulting mechanisms.  The Macquarie Ridge forms part of the Pacific-Australian plate boundary and the faulting mechanism of this earthquake is predominantly strike-slip.  The Sumatra earthquake occurred on the interface of the India and Burma plates and its faulting mechanism was predominantly thrust with vertical slip.

However, tsunamis can also arise from strike-slip earthquakes.  A strike-slip Macquarie Ridge earthquake on May 1989, which had a similar magnitude (Mw 8.1) to the December 2004 earthquake, generated a small tsunami.  A strike-slip earthquake in the Gulf of Alaska (November 1987, Mw 7.9) generated a 0.8 m tsunami while a strike-slip earthquake off the coast of northern California (Aug 1991, Mw 7.1) generated a 0.5 m tsunami.  Although the fault displacements produced by these earthquakes were predominantly horizontal they may have had a slight vertical component.  A combination of horizontal and vertical motion across a fault plane is called oblique slip.  Strike-slip earthquakes can also cause underwater landslides that can generate tsunamis. Thus, another major reason that the Sumatra earthquake generated a tsunami is its sheer size, a magnitude (Mw 9.1) that was so much larger than that of the Macquarie Ridge earthquake (Mw 8.1).

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 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 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: 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 Nagasaki bombs.

Question: How long did the earthquake shake? (What was the duration?)

Answer: The actual rupture duration on the fault (the time it took for the earthquake to take place on the fault and rupture the entire length) was approximately 3 to 4 minutes. The exact length of time that people felt the shaking varied from place to place, depending on their distance to the fault, and other factors, such as what type of bedrock they were on, what the crustal structure was below them and between them and the fault, etc. In northern Sumatra , which lies almost above the fault, shaking may have been experienced for up to several minutes.

Question: What effect did this earthquake have on the rotation of the earth?

Answer: While this question is a little outside the earthquake role of the USGS, scientists at NASA's Jet Propulsion Laboratory (JPL) who work with the USGS, have told us that the effects on the Earth's rotation from an earthquake even of this magnitude is much too small to be observed. The length of the day can be measured with an accuracy of about 20 microseconds and calculations of the source properties of the earthquake showed the change in the length of the day to be -2.676 microseconds, or in other words, less than can be effectively measured.

If you want a more complete and technical answer to this question, Richard Gross at JPL offers the following:

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: 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.

Question: What other significant tsunamis have occurred in the region?

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. (http://cires.colorado.edu/~bilham/Oldham1881account.htm)

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.

References:
Tsunami Laboratory, Institute of Computational Mathematics and Mathematical Geophysics
National Geophysical Data Center
R. Bilham
Oritz and Billham, 2003 JGR, VOL. 108, NO. B4,  pp 2215

Question: Can earthquakes trigger volcanic eruptions?

Answer: Volcano eruptions have occurred shortly after earthquakes and they may be linked, but scientists are still debating the topic.  Notably, an Andean volcano (Cordon Caulle) began erupting 2 days after the magnitude 9.5 1960 Chile earthquake.

Eruptions of mud volcanoes have occurred in the Andaman Islands following the magnitude 9.1 megathrust earthquake. Mud volcanoes consist of surface mud extrusions that vary in size from meters to several kilometers. They sometimes resemble magmatic volcanoes in appearance but they generally consist of low lying mud flows.  Mud volcanoes do not involve magma. They emit mud at significantly cooler temperatures than lava, well below the ~800 degrees Celsius temperatures that characterize volcanic eruptions. Eruptions from mud volcanoes can reach heights of several hundred meters and consist of mud and sometimes burning hydrocarbon gasses.  They are often associated with gas and oil fields. Mud volcanoes were known to exist in the Andaman Islands before the earthquake and in many other regions of the world.

Deadly mud volcano eruptions are extremely rare because their eruptions generally do not affect large areas. One deadly eruption in Bozdagh, Azerbaijan reportedly killed six shepherds who were camping in the caldera of a mud volcano and about 2,000 of their sheep.