Aftershocks to the Sumatran Earthquake
April 11, 2005
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.
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.
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