Sunday, April 10, 2005

What to Expect: Aftershock Sequences

Aftershocks to the Sumatran Earthquake

April 11, 2005
Lucy Jones
Scientist-in-charge for Southern California, U. S. Geological Survey

Like all big earthquakes, the December 26, 2004 and March 28, 2005 Sumatran earthquakes have been followed by a sequence of aftershocks. Scientists have shown that aftershocks follow predictable patterns and have developed equations to describe those patterns. The Sumatran aftershocks are following those patterns and from this we can predict how many aftershocks can be expected over the next year.

Aftershocks usually occur geographically near their mainshock. The stress on and around the mainshock's fault changes during the mainshock and that fault and its nearest neighbors produce most of the aftershocks. Aftershocks are other earthquakes triggered either on the mainshock fault or at a distance from the mainshock fault no greater than the length of that fault. Because the December fault segment was so long (1200 km), any earthquake within 1,200 km of the fault fits the standard definition of aftershock. The March 28 earthquake occurred on an adjacent segment of the same fault that produced the December event,

The rate of aftershocks dies off quickly with time so even the second day will have many fewer aftershocks than the first. Approximately, the second day has half the number of the first, the third day has one-third the number of the first, etc. We call an earthquake an aftershock as long as the rate of earthquakes in the area is greater than it was before the mainshock. How long this lasts depends on the size of the mainshock (bigger earthquakes have more aftershocks) and how active the region was before the mainshock (if it was quiet, the aftershocks continue to occur above the previous rate for a longer time). It could be a decade or more for the Sumatran earthquakes.

Aftershocks come in all magnitudes, but smaller magnitudes are much more common. The relative number of small to large aftershocks in the Sumatran sequence is very close to the worldwide average and implies 1 magnitude 6 event for every 100 magnitude 4 events. By fitting an equation to the 3 months of aftershocks we have already recorded, we can estimate the probability of more large earthquakes in the next two years. At this rate, we have about a 1 in 8 chance of another magnitude 8 somewhere in the region in the next two years and a 1 in 3 chance of a magnitude 7.5 in the same time.

Birdie here:

Ok - one way to do this is to take a coconut or a Big Rock. put it on the sand. That's our 9M. If you have the fault map, draw the main faults in the sand...but that will be to explain "Where", which is a different, but related subject. Let's start with time then, move on to space :-)

For the concept of understanding this is about *When*, make one long pathway, starting with The Big Rock. Have everyone help you get a lot of rocks. lots and lots and lots and lots of smaller ones. A few medium ones. Find one that is the 8.7M. Find another that could be a 8 or 7.5. Make your time line - aftershock sequence two years long. The distance between Dec 26th & March 28th is 3 months. Move the 8M or the 7.5 around down the line, as if to say...maybe here? maybe there? maybe like this (8M) or maybe like this (7.5) make sure the bigger they are - the most little and medium rocks they have clustered right along them...and then finish with a trail of a few 6M rocks and all kinds of smaller ones....til they trail off...make them smaller and farther apart...

If you want to draw the whole fault line and draw where the islands are, in the sand, then, pick up all the rocks and put them where they would be on the actual aftershock map...fine, but I don't think anyone has the time!! Best to wait for summer for that...

Any rock the size of a 6M or bigger - you can use to show that a 6M > could damage a weak building or other unsound stucture. Otherwise, they are not that big a deal.

This will be best done after supplies have been unloaded, and a meal has been had by all.

There you go.

Should work for beach landings.

I'm running down to the beach today 4/13 to make a model of this in the sand with stones - will post pictures. I am using various data from this post and the USGS.

Birdie


Click on the title to find the current up-to-date eq map and list


5 Comments:

Blogger Birdie said...

Ok, I contacted Dr. Lucy Jones, the aftershock expert at the USGS, to see if she could contribute. She did. Thank you Dr. Lucy!

I've asked for a simple user friendly explanation of what an aftershock sequence is and what *might* be expected in this particular sequence from the 9.0M over the next weeks, months, years.

I still need to get it translated into Indonesian, and the local Nias and Simeulue dialects.

If anyone can help with this, give me a holler.

In the meantime....while I was waiting for the expert....I wrote this...

Be aware that earthquakes and aftershocks in the 7M range do not normally create tsunami's on their own. 7M's often last 10 seconds to 30sec, and longer if uplift is involved (60 sec). A local tsunami of maybe 1 foot could be observed locally, if uplift is involved for a 7M.

A good ex. 1992 Cape Mendocino 7.1M created 4 feet of uplift in 30 seconds. The eq was 60 seconds long, but the second 30 seconds was the coast settling after the reefs lifted. A small 1 foot tsunami was observed 20 minutes later, locally.

Fishermen, who were standing on a big rock, thought the water was draining away - for the first 30 seconds. They had no idea what was going on, but when the water stopped draining - the rock started to shake - for the remaining 30 seconds. They had no idea that the rock they were on was actually LIFTING for those first 30 secs. It's a very smooth ride up! It's when it stops lifting...and starts to settle, the shaking starts - When you are at Ground Zero.

For people inland, away from the uplift...they felt horizontal thrusting for the first 30 secs, and the last 30 secs were up and down vibrating shock waves.

I like to explain this in a simple way - you know how when you drop a stone in a pool of water and where the stone hits - it's just the stone - but all around it, it makes waves go out? It's like that, except the stone is coming UP and not down...and whoever is on the stone, may look around and go "the water is going down" as they may not know they could be going UP! Who would have guessed it??? The fishermen in NorCal no more guessed it than the people on the islands did.....If you don't know what's possible...you can't imagine this scenerio. That's all. The more people learn what has happened - and what can happen - They will be better able to cope.

If you are at the coast, and see the water drop around the reefs, pay close attention to any shaking before or after it recedes. If the water recedes *and then* it starts shaking, you are almost surely standing on an area that is lifting.

Correct, the water is not draining - the land you are on is raising up.

Surprise!

Make an effort to focus and tell the difference. Shaking *will* occur as the land stops lifting and starts to settle. Don't be fooled! Know the difference. And, get to high(er) ground. Uplift can cause tsunamis.

On the otherhand, if you feel very strong shaking first, and then see the water recede, get to high
ground. Deformation or slides underwater may have been caused by the earthquake.

Same goes if you feel no shaking at all, whatsoever (before, during and after) and see the water recede. Chances are it is a tsunami from an offshore slide or distant location. No local shaking required at all. Run to higher ground as fast as you can.

I do know that education needs to continue to happen within all of Indonesia, and people need help understanding what has happened, and, after strong aftershocks, what to look for!

The single best thing to teach villagers about aftershocks, at this point, is they are normal, and to start to count how long they last, each time.

If they last less than 10 seconds - and are not very strong - they can try to simply enjoy them and should go on with their business. If they are very strong, no matter how long they last (stronger eq's generally last over 20 seconds up to 6 minutes) they need to get to higher ground (just in case).

Teach them to count how long they last! Practice makes perfect, tell them they will all be experts before this is all over. They will be!

A great way to teach children, their mothers, fathers and everyone in the villages the big simple picture of what is going on - is that there are two ways islands and land are created.

One is through volcanoes, and the other, through earthquakes.

The earthquakes are making the islands grow. The uplifted reefs are like new teeth. They get pushed up. It's painful. It takes awhile, they grow in spurts, and once they are all the way up - you have beautiful new teeth or in this case, beaches... and eventually, trees will grow closer and closer to the sea again, and the uplifted reefs will no longer be bare. The islands are very young, and still growing. That is the big picture. They are beautiful islands and the villagers should be very proud.

8:41 AM  
Anonymous Nature said...

News
Published online: 16 March 2005

Quake threat rises after tsunami slip

Michael Hopkin

Nearby faults are under increased strain after December's catastrophe.

The huge earthquake that caused the Indian Ocean tsunami has increased the stress on neighbouring fault lines, experts have calculated. The knock-on effect could be another large earthquake in Sumatra or even another tsunami, they warn.

Seismologists have studied the fault lines around the Burma microplate, whose border with the Indian plate was the scene of the huge jolt in December 2004.

Using a mathematical model, they conclude that the increased strain on the neighbouring Sumatra fault, which runs through the island itself, makes Sumatra the most likely place for a subsequent earthquake.

The December earthquake shunted almost 250,000 square kilometres of the Indian plate under the Burma microplate's western edge, explain John McCloskey and his colleagues at the University of Ulster in Coleraine, UK.

This has increased the pressure on the Sumatra fault on the plate's eastern edge, which runs close to the already ravaged city of Banda Aceh.

The most immediate threat is probably an earthquake of magnitude 7-7.5 on the island, McCloskey's team predicts. This region has produced such earthquakes in the past, although not for more than a century, meaning that the fault may now be close to breaking point.


Also under increased strain is the Sunda trench, to the southeast of the region where December's earthquake struck. Like its neighbour, it is also an undersea 'subduction zone' with a history of destruction: in 1833 and 1861, earthquakes on the Sunda fault triggered fatal tsunamis.

Under pressure

The model used by McCloskey's team shows how the stresses have moved through the region in the wake of December's slip. They report in this week's Nature1 that pressure on a 50-kilometre stretch of the Sunda trench has increased by up to 5 bars, and a 300-kilometre segment of the Sumatra fault is now under an extra 9 bars of strain.

It's hard to tell whether this will trigger an earthquake at either location, McCloskey says. "The problem is that we don't know the failure stress for any of these structures."

And the calculations are based on a crude estimate of the amount of slip that occurred on 26 December, which does not take account of any subsequent shifts in the region, points out Kerry Sieh, a geologist at the California Institute of Technology in Pasadena. "This is just a first crack at the problem," he says.

But seismologists have used similar information to 'telegraph' earthquakes before, McCloskey says.

He points out that the earthquake that devastated Izmit, Turkey, in 1999 was preceded by a 2-bar increase in pressure that seismologists spotted some 18 months before the quake.

A tsunami triggered by the Sunda trench would mostly dissipate in the Southern Ocean, McCloskey told news@nature.com.

But Indonesia, particularly Sumatra, would almost certainly be struck again, and the wave would probably wash up on southern African shores too.

By showing how one earthquake can trigger another nearby, the
researchers have underlined yet again the case for installing a tsunami warning system in the Indian Ocean, McCloskey argues.

"It is urgent," he says. "People think 'well you've had your bad luck', but earthquakes cluster in time and in space."


A warning system will be most useful to people on distant shores, rather than the Sumatran population, Sieh adds. "Sumatrans will most likely have ample warning of an impending tsunami given by the severe shaking of the earthquake," he reasons.


1. McCloskey J., Nalbant S. S. & Steacy S. Nature, 434. 291 (2005 ). | Article | PubMed |

2:06 PM  
Anonymous Nature said...

Published online: 30 March 2005

Indonesia still in jeopardy

Michael Hopkin

News@nature.com talks to the expert who predicted where second earthquake would strike.


The latest earthquake devastated the Indonesian island of Nias.

On 17 March, a team of seismologists published a paper in Nature that attempted to evaluate the knock-on effects of the 26 December earthquake.

Eleven days later, another horrific earthquake struck, almost exactly where they said it would.

The magnitude-8.7 tremor, which has left some 2,000 feared dead, occurred in the Sunda trench.

This runs south from the site of the previous Sumatra rupture and was one of two fault lines identified by the researchers as being under increased stress.

When the researchers predicted that the Sunda fault was likely to give way as a result (see "Quake threat rises after tsunami slip"), they didn't know how quickly they would be proved right.

John McCloskey's team analysed the seismic after-effects of the first giant quake.

"We got the location and the size bang on, but not the timing," says team leader John McCloskey of the University of Ulster, UK. He notes that seismologists are notoriously wary of the term 'prediction': "It conjures up all that's worst about the science."

Experts prefer to talk about 'earthquake risk'. Although modelling can tell us where an earthquake might strike next, it is still almost impossible to say when, because we don't know enough about the breaking strains of individual fault lines.

However, given that McCloskey's team were so accurate this time around, where does that leave the region's other at-risk fault line: the Sumatra fault that runs directly underneath Banda Aceh. Has the latest earthquake increased the danger still further?

Wider stress

"It doesn't look as if the size of the stress has increased, but the extent has," McCloskey says. The first earthquake affected some 300 kilometres of the Sumatra fault; the latest event could have elongated the stressed section to some 600 kilometres. "The extent is important," he adds. "It increases the chance of there being a weak patch."

This is based on crude estimates of how much slip might have occurred in the latest tremor, McCloskey admits.

Researchers at the California Institute of Technology in Pasadena are "burning the midnight oil to calculate these things", he says, adding that a detailed profile of the event should be available within days.

We got the location and size bang on, but not the timing.

What is clear is that this was another fearsomely big earthquake.

"I wouldn't be surprised if it's the biggest in the world this year," McCloskey says. Anything with a magnitude above 7.5 is generally classed as a giant earthquake.

Log scales

So why was there no tsunami this time? The answer may be that this earthquake, although huge by everyday standards, is still small compared with that of 26 December.

Researchers in this week's Nature confirm that the earlier shake had a magnitude of 9.3.


"Superficially, 8.7 and 9.3 seem similar. But because the scale is logarithmic, Monday's event released a fraction of the energy," McCloskey explains. "We're stunned with superlatives for the Boxing Day event: it was a true giant. It seems that damaging tsunamis are thankfully very rare."

Nonetheless, he was impressed by the speed at which Indonesian villagers fled their homes for higher ground.

"The response was exactly right. I would recommend that people err on the safe side: children in Japan are taught to just get away from the sea if the ground is shaking. Those closest to the epicentre are not helped by high-tech warning systems.

2:23 PM  
Anonymous Nature said...

News

Published online: 29 March 2005

Second giant quake rocks Indonesia

Helen Pearson

Seismologists puzzle over absence of tsunamis.


Seismographs at the Meteorology and Geophysics office in Jakarta, Indonesia, show the strength of the March 28 quake.

A huge earthquake struck the coast
of Indonesia on Monday, just three months after last year's devastating tsunami. But researchers are scratching their heads over why this one did not trigger giant waves.

The magnitude-8.7 earthquake struck just after 11:00 pm local time on 28 March, around 200 kilometres west of northern Sumatra. It is classified as a 'great earthquake' and occurred south of the earthquake that hit the region on 26 December 2004.

There are reports of significant damage and around 1,000 deaths on the nearby Indonesian island of Nias, although the final figures are not yet clear. "I'm sure the death toll will go up," says Waverly Person, a geophysicist at the United States Geological Survey in Golden, Colorado.

Initial reports from the region suggest however that this earthquake did not generate vast waves. By contrast, last year's earthquake, now thought to be magnitude 9.3, triggered tsunamis that spread as far as Africa and killed an estimated 300,000 people.


Deep tremor

Seismologists are not yet clear why the ocean remained calmer this time.

The earthquake released only a quarter of the energy of its predecessor (the scale on which magnitude is measured is logarithmic). Nonetheless, it is one of the eight most powerful earthquakes measured since 1900.

Lesser earthquakes in 1861 and 1833 in the same region did trigger tsunamis. "It's very puzzling," says geophysicist Rob McCaffrey, who studies seismic activity at Rensselaer Polytechnic Institute in Troy, New York. "It's probably one of the biggest earthquakes in history."

The latest quake occurred just south of the disastrous December event.

One possible explanation for the absence of tsunamis could be that the latest tremor occurred much deeper in the fault line that slices through the Indian Ocean, McCaffrey says. This might have avoided shifts in the sea bed that can displace water and prompt a tsunami.

Seismologists will be studying the position and depth of the earthquake to try and answer these questions.

Yesterday's tremor is not classified as an aftershock of the December event, because it did not occur in exactly the same area. But Waverly says that it was probably triggered by last year's earthquake.

Researchers had predicted that the earlier judder, which increased stress on neighbouring fault lines, would prompt more earthquakes (see 'Quake threat rises after tsunami slip').

Monday's event seems to have occurred in the Sunda trench, one of the places pegged as likely candidates for a quake as a result of stress changes caused by December's monster.

"This quake may be a vindication [of our understanding] of the stress transfer process," says Bill McGuire, an expert on earthquake hazards at University College London.

Last year's disaster prompted governments to commit resources to a tsunami warning system in the Indian Ocean, but it is not yet up and running.

Even so, some coastal areas did issue alerts and evacuated coastal areas more promptly because of lessons learned from the earlier catastrophe.

Additional reporting by Michael Hopkin.

2:34 PM  
Anonymous USGS said...

FAQ - Everything Else You Want to Know About this Earthquake & Tsunami

Magnitude 9.0 Sumatra-Andaman Islands Earthquake FAQ



Question: Where can I find more information on tsunamis?

Answer: Please see our Tsunami Information Links.
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Is the magnitude of the December 26th, 2004 Sumatra-Andaman Islands earthquake greater than 9.0? 04/05/05

Many detailed studies on the magnitude of the December 26th, 2004 Sumatra-Andaman Islands earthquake are being 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 are used and new techniques being developed. The magnitudes used by the USGS are either those that come from validated, routinely used techniques or from a consensus of the seismological community based upon such in-depth investigations. Since determining the magnitude of the Sumatra-Andaman Islands earthquake is still an active area of research, there is currently no firm consensus on the "correct" magnitude of this earthquake. The USGS magnitude is likely to change when a consensus is reached. This will be done in consultation the groups that provide routine information to USGS earthquake catalogs.

One problem with prematurely revising the magnitude to 9.3 is that of consistency problems with our heavily-accessed list of largest earthquakes. This list is periodically updated, but there is the problem of comparing "apples-to-oranges". It is quite clear from almost all geophysical measures of earthquake size that the 1964 Alaska earthquake, with a magnitude of 9.2, was larger than the 26 December Sumatra-Andaman Islands earthquake. Unfortunately, it is not possible to use the same method that led to the 9.3 magnitude estimate for the 2004 Sumatra-Andaman Islands earthquake on the 1964 Alaska earthquake because modern seismometers and the broadband data they produce did not exist in 1964. At this point, we do not know if the slow-slip component of the 2004 Sumatra-Andaman Islands earthquake was unique, or a feature common to all, or most, magnitude 9+ earthquakes.

So changing the magnitude of the 2004 Sumatra-Andaman Islands earthquake to 9.3 at this time would not only be premature, but could place it in the wrong position on the list of largest earthquakes.
--------------------------------------

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.
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Question: Is there a possibility of another tsunami in the area?

Answer: Of major concern following the 26 December 2004 magnitude 9.0 earthquake is the potential of another damaging tsunami being generated by an aftershock. The only way to know for certain if a tsunami has been generated is to directly measure the height and propagation of the ensuing wave using ocean pressure sensors and tide gauges. However, a system of such instruments does not exist in the Indian Ocean. Absent of a system of instruments, we must defer to historical earthquake-tsunami records to calibrate our thinking. For reasons summarized below, we conclude that the chance of a tsunami resulting from an aftershock is small, but finite. Nevertheless, we emphasize that the great tsunami of 26 December is extremely unlikely to reoccur in the near future.

Although magnitude is one factor that does affect tsunami generation, there are other important factors to consider. The earthquake must be a shallow marine event that displaces the seafloor. Thrust earthquakes (as opposed to strike slip) are far more likely to generate tsunamis, but small tsunamis have occurred in a few cases from large (i.e., > M8) strike-slip earthquakes. Although a number of the Sumatra-Andaman Islands aftershocks are thrust events, many are not. Similarly, the depth of these aftershocks is quite variable. This variability (which is common in aftershock sequences) is an important consideration when evaluating the potential of any given aftershock triggering a tsunami. With these caveats, we offer the following general guidelines based on historical observations and in accordance with procedures of the NOAA Richard H. Hagemeyer Pacific Tsunami Warning Center.

Magnitudes below 6.5
Earthquakes of this magnitude are very unlikely to trigger a tsunami.

Magnitudes between 6.5 and 7.5
Earthquakes of this size do not usually produce destructive tsunamis. However, small sea level changes may be observed in the vicinity of the epicenter. Tsunamis capable of producing damage or casualties are rare in this magnitude range but have occurred due to secondary effects such as landslides or submarine slumps.

Magnitudes between 7.6 and 7.8
Earthquakes of this size may produce destructive tsunamis especially near the epicenter; at greater distances small sea level changes may be observed. Tsunamis capable of producing damage at great distances are rare in the magnitude range.

Magnitude 7.9 and greater
Destructive local tsunamis are possible near the epicenter, and significant sea level changes and damage may occur in a broader region.

Note that with a magnitude 9.0 earthquake, the probability of an aftershock with a magnitude exceeding 7.5 is not negligible. To date, the largest aftershock recorded has been magnitude 7.1 that did not produce a damaging tsunami.
------------------------------------------------------------------------

Question: What was the background seismicity in the region before the M9.0 earthquake?

Answer: This table represents the number of earthquakes in the aftershock zone of the magnitude 9.0 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 9.0. 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 9.0) 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 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: 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.

Also see:
NASA Details Earthquake Effects on the Earth
How the Earthquake Affected the Earth - NASA
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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.
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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.
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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).
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Question: What other significant tsunamis have occurred in the region?

Answer:

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
K. Sieh
R. Bilham
Oritz and Billham, 2003 JGR, VOL. 108, NO. B4, pp 2215
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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 recent magnitude 9.0 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.
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Question: How have tsunamis affected the United States?

Answer: Please see Can It Happen Here?
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Question: Could a tsunami such as the one that affected the Indian Ocean on December, 26, 2004 happen in the United States?

Answer: Please see Can It Happen Here?
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