Epicenter of the December 26, 2004 earthquake
Image courtesy USGS
|
On December 26, 2004, an earthquake-generated tsunami devastated many countries surrounding the Indian Ocean. It caused more casualties than any other tsunami in recorded history. In the following months, many more moderate to strong earthquakes have taken place in the region. The surviving residents have understandably been very frightened each time the ground shakes. Fearing another tsunami, they flee to higher ground. Yet, for the most part, the tsunamis did not materialize. Those that did were small and insignificant, resulting in very little damage.
The country of Indonesia was especially hard hit by the December earthquake, which measured 9.0 on the Richter scale. The resulting tsunami created further devastation. The combined death toll was over 200,000 people. Just three months later, on March 28, 2005, a quake with a magnitude of 8.7 struck the region again, killing more than a thousand people and destroying several hundred buildings. A tsunami alert went out, and residents fled inland. This time, however, any resulting tsunami-driven surges of water were hardly noticeable. A three-meter (nine feet) tsunami did some damage to a nearby port, and surges as high as one meter (three feet) were observed in some places along the coast of Sumatra. In contrast, the December tsunamis measured as high as 10.5 m (34 feet) when they reached shore.
At the beginning of April 2005, another series of earthquakes ranging from magnitude 6.0 to 6.9 struck in the same region. Once again, no significant tsunamis were generated.
|
Epicenters and magnitudes of major Indian Ocean earthquakes, December 2004 to April 2005
Image courtesy USGS
|
|
 |
Why is it that some earthquakes result in significant tsunamis and some don’t? All three quakes were centered off the west coast of Northern Sumatra. The epicenters of the December and March earthquakes were relatively close to each other. Why was there such a difference in the size of the tsunamis they generated?
Several factors determine the size of a tsunami following an earthquake. One is the magnitude (strength) of the earthquake itself. The magnitude of a quake indicates the energy it releases. On the Richter scale, the difference in one unit of measure means that about 30 times the energy is released. Thus it would take about 30 earthquakes of magnitude 8.0 to equal the amount of energy released in one magnitude 9.0 quake! In the case of the December quake, the slippage along the fault line extended for more the 850 km (528 miles). The shift that took place in the March quake was about 250 km (155 miles) long. As a result, the December earthquake released about three times as much energy as the one in March. So, even though the magnitude numbers seem close together, the March earthquake was considerably smaller than the one that took place in December.
|
The direction and type of slippage also play a role. Earthquake motion has both horizontal and vertical components. The December quake was more vertical than horizontal, but the March quake was more horizontal than vertical. Thrust (vertical) earthquakes are much more likely to bring about a tsunami. Occasionally a slip-slide (horizontal) movement may result in a tsunami, perhaps causing a change in the sea level near the epicenter. For tsunami destruction and sea level changes to take place in a broader area, the magnitude needs to be at least 7.9.
Another factor is the depth of water at which the slippage occurs. The ocean around the fault area of the December quake is as deep as 2000 meters (6561 feet). Even though its epicenter was only about 200 km (125 miles) away, the March quake took place on the continental shelf where the water is only several hundred meters deep. Thus far less seawater was displaced when the quake caused the ocean floor to rise.
On the other hand, a slip deep beneath the ocean floor can lead to a smaller rise in the seabed than one that takes place closer to the surface. The rupture of the March earthquake was at a deeper level of rock than that of the December quake.
The location of the fault is also significant. Rather than being beneath open ocean, the March earthquake took place on a fault extending beneath Nias Island, where most of the damage took place. This, too, may have affected the height of the resulting tsunami.
When an earthquake occurs, seismologists move quickly to determine if a tsunami will follow, as well as its potential size. This is difficult to do until they know the details of the earthquake. It takes more than merely having tsunami detection instruments in place or measuring events as they happen.
|
Things To Do
Science
Center
Schools
SEED People
