by Dave Hennen
CNN Senior Meteorologist
In 2004 a 9.1 magnitude quake occurred in a similar region to today’s large quake generating a tsunami that killed more than 200,000 people. Again last year a similar magnitude 9.0 quake also generated a killer tsunami killing over 15,000. Today’s quake was strong enough to generate a killer tsunami, but some key differences in the quakes probably saved thousands of lives.
Likely the biggest difference today, was the type of quake that occurred. Both the 9.1 quake in 2004 and the Japan quake were considered “thrust” quakes. In both cases the sea floor was violently thrust upward. This pushed a large amount of water towards the surface that produced the widespread tsunami. Think of dropping a large rock into a pond. A wave spreads outward from where the rock is thrown. The same happens at the epicenter of the quake as a tsunami is generated.
Today’s quake was a different type of quake all together. Instead of the plates being thrust upwards, the quake today was categorized as a “strike/slip”. In this case the plates move horizontally not vertically so not as much water was displaced. These types of faults can be deadly. The Haiti quake in 2010 killed thousands. The 7.0 magnitude quake was centered over a highly populated area near Port-Au-Prince, so it was collapsing buildings, not a tsunami that caused most of the fatalities.
The other main factor was the size of the quake. A 9.1 magnitude quake may seem very similar to today’s 8.6 magnitude but the scale is logarithmic which means the devastating 9.1 quake in 2004 was 3.2 times larger, or more than 5 times stronger than today.
Still 8.0 or higher magnitude quakes are rare. According to the USGS nearly 3 million quakes occur annually across the globe, but since 1900 only 89 quakes have had a magnitude of greater than 8.0, and the fact that we had the initial shock today of 8.6, followed by a 8.2 aftershock makes today even more unprecedented.