Due to the recent earthquake in Chile, we thought the campus should be enlightened about the nature and cause of earthquakes and tsunamis. Matt Powell, assistant professor of geology, offered some earth-shaking explanations of natural disasters.
Why are there so many earthquakes in Chile?
There are so many because it is a convergent plate boundary, which is a point where two tectonic plates push against and force one below the other. In this case, it is the Nazca and South American plates. The Nazca is being pushed beneath the South American at 7 centimeters per year. To put that in perspective, the fastest moving plate rate is 15 centimeters per year- that’s about as fast as your fingernails grow. The strongest earthquake ever recorded was in Chile reaching a 9.1 on the Richter scale and this past quake was the seventh strongest.
How are tsunamis formed and why did this one not hit anything it was predicted to?
Actually, it did cross the ocean and hit where it was said to. Hawaii reported that there were erratic tides due to the earthquake. It was just not as big of an effect as predicted. Earthquakes generate tsunamis because they are the displacements of rock. If the plates slip vertically, then there is an earthquake that causes a tsunami, because the water is displaced and violently ripples outward. But if the rocks are displaced vertically and they are sliding and pushing past each other, then the rocks won’t necessarily generate tsunami. You can have a big earthquake without a tsunami.
What happens to the ocean right before it hits the land?
All waves have peaks and troughs – high and low points, respectively. If the trough is going to hit first, then the water will withdraw and leave a lot of shore and create a huge wave. But if the peak drops first, there is no warning on the shore and the wave will flood without withdrawing. That’s what happened in this instance as well as in the tsunami that affected Indonesia. In both kinds of impacts, there are these huge wavelengths- from one top of the ripple it is about 100 miles until the next. Once it hits the shore, it just keeps rolling in for hours, and then it takes hours to recede. Everything is covered in a torrential, giant river that behaves as a wave but it’s much more like an instantaneous rise of sea level. The initial crash of a tsunami can be really damaging, but the aftermath of the wave invading the shores is where most of the devastation occurs.
How often do tsunami occur how many causalities are generally brought about?
Earthquakes happen every day. It’s random and depends upon the type and magnitude of the earthquake. Basically, big tsunamis happen one or two times per decade. To put things in perspective, over the span of the 20th Century, there were 21 damaging tsunamis, 12 of which were major and overall 70,000 people died. But the tsunami that impacted Indonesia in 2004 killed 245,000 people. It was 9.1 on the Richter scale, which is logarithmic; hence if an earthquake were 7.1 on the scale, the quake in 2004 would be 100 times greater than it.
Natural disasters are man-made, in the sense that if the area of impact is not prepared for destruction. For example, Haiti doesn’t have the sound-structured buildings, warning systems, and training that Chile does which is why it hasn’t been as great of a disaster.
-Sam Stroup ’12, Juniata Online Journalist