By Sara Israelsen
A magnitude 8.7 earthquake that shook Sumatra, Indonesia, Monday was not an aftershock of the deadly December quake there, but an independent event, said BYU geology professor Ron Harris.
The quake hit 125 miles west, northwest of Sibolga, Sumatra, and as of press time, had a death toll of almost 2,000, according to The Associated Press.
This is the second natural disaster in the area in the last four months, after the island nation was slammed by a magnitude 9.0 quake on Dec. 26 that killed more than 166,000 in Indonesia and left thousands more homeless.
Relu Burlacu, a seismologist with the University of Utah Seismograph Stations, said it?s not unheard of to have two earthquakes in such a short time frame.
?No one was prepared for this event to come,? he said. ?However, it?s not usual to have this situation. Big earthquakes are ? almost always followed. In general, earthquakes occur in clusters.?
And it was the cluster effect that might lead some to think this quake was an aftershock of the December event. However, Harris said it doesn?t fill the description of an aftershock for two reasons: it didn?t happen in the same area as the first December quake and the magnitude was too large to be a by-product of a previous event.
?The area that ruptured in December has been producing aftershocks ever since,? Harris said. ?But the rate of them has been tapering off, the magnitude tapering off. Now there?s only one every couple days.?
Right after the December quake and tsunami, aftershocks rippled through the area, but mostly in the magnitude-5 range. Harris said there is no way the same area would have had enough power left to produce Monday?s magnitude 8.7 quake, as building up that much power takes hundreds of years.
December?s earthquake was a 9.0, Monday?s was an 8.7. Despite the seemingly close magnitude ranking, the 9.0 earthquake actually contains three times the energy, Burlacu said. Because of that difference, the 8.7 wasn?t enough to cause a tsunami. However, four aftershocks had already been felt, at magnitudes 5.4, 5.5, 5.8 and 6.0 ? events Burlacu said are likely to continue.
An earthquake that earns an 8.7 ranking has enough power to destroy frame structures, bend railroad ties and throw objects into the air, according to the United States Geological Survey Web site.
Lower on the scale, a 5-point magnitude quake, although felt by most people, only causes negligible damage to well-constructed buildings. However, previously damaged buildings may be more heavily damaged.
An earthquake that scores in the 6-point magnitude range can cause partial collapsing in mediocre construction and great damage in poorly built structures. On the higher end of the 6-point scale, buildings can even shift off their foundations.
Although the quake is not an aftershock, there is still a definite connection between the two quakes.
The quakes occurred because of movement between the Burma and Indian/Australian plates. The two giant plates are colliding and the Indian/Australian plate is being forced underneath the Burma plate.
During a time span of 100 to 150 years, the plates got pushed to the north, then, in a second, when the Burma plate snapped back to its original southwestern position, the huge influx of energy caused an earthquake.
Because of the sudden release of stress in December, the lower areas of the fault line became ?loaded? and more prone to earthquake eruption because of a situation called stress contagion.
Monday?s event means new research for Harris, who has studied earthquakes and tsunamis in Indonesia and forecasted the first December quake. His new forecast is that this earthquake will bring along its own string of problems.
?Many people said that this big earthquake was the end ? but some of us think it?s the beginning,? he said. ?It?s the trigger that may set off a whole series along that southern coast of Sumatra. When the first one broke, we actually warned everybody that this might result in an earthquake storm.?
An earthquake storm is a collection of earthquakes, which while not aftershocks of each other, are all connected through the loaded stress contagion levels.
Harris said as soon as the first Sumatra earthquake happened, he and others began warning the Indonesians about future earthquakes.
?The surprising thing is, it hasn?t taken 10 years, it?s only taken three months,? he said.
And more tremblings may not be far off. But forecasting earthquakes is a difficult business, Harris said, like predicting when a bent twig will snap.
?We?re doing a lot,? Harris said. ?We?re working our hardest, using all the information we can get. We know it?s eminent ? we don?t know the exact moment.?