A Megathrust Earthquake And Tsunami off Chile!

Thursday, September 17, 2015
1:24 p.m.

The port town of Coquimbo, Chile after the earthquake and tsunami. Credit: Wikimedia User Sfs90

Chile is hands-down the most active place in the world when it comes to megathrust earthquakes. Three of the past six megathrust (subduction zone earthquakes over moment magnitude 8) earthquakes have been in Chile, and one of those three that was not in Chile was in Peru (the others were the 2004 Indian Ocean Earthquake/Tsunami and the 2011 Japan Earthquake/Tsunami). Heck, they had an 8.2 earthquake just last year, and a 8.8 earthquake back in 2011. Here in Cascadia, we are frightened (and rightfully so) about a mega-quake that occurs once every 200-700 years. But Chile’s gotten three magnitude 8+ of those quakes since 2001! We haven’t had one of those quakes since 1700.
Their most recent quake struck 29 miles west of the Chilean city of Illapel. This is a very seismologically active region even for Chile; 15 7+ magnitude earthquakes had struck with 400km of this area within the last century prior to yesterday’s earthquake. It was an 8.3 earthquake, so while it was powerful, it was not the same magnitude as the aforementioned Japan or Indian Ocean earthquakes. The moment magnitude scale is a logarithmic scale, meaning that the energy released by an earthquake increases exponentially with linearly increasing magnitude. For example, a magnitude 9 earthquake is not three times as powerful as a magnitude 3 earthquake… it is a billion times more powerful! If you ever want to compare earthquake magnitudes, the ratio between the energy released due to earthquake 1 and earthquake 2 = 103/2(Magnitude of Earthquake 1 – Magnitude of Earthquake 2). Nothing wrong with a little math to keep your brain nice and fresh! Using that calculation, it was about 11 times less powerful than the 9.0 2011 earthquake of the Pacific coast of Japan, so while it wasn’t as powerful as some of the powerful quakes of years past, it was still pretty darn powerful… nearly 6 times more powerful than the catastrophic Nepal earthquake we saw earlier this year.
Source: Environmental Physical Geography

Just like all of the other Chilean megathrust earthquakes, this earthquake was formed by the subduction of the Nazca Plate under the South American Plate. As this subduction occurs, certain places become “locked” due to friction and can no longer undergo the subduction process. Below this locked zone, however, warmer temperatures make the plates less rigid, allowing them to continue to slide past each other. As this occurs, stress builds up along the locked zone, and when the stress exceeds a certain threshold, all of the energy is released and a massive earthquake ensues, often accompanied by a tsunami.

Source: Cascadia Region Earthquake Workgroup

The reason why Chile gets so many of these violent earthquakes is because the rate at which the South American Plate is subducting the Nazca Plate. The faster the rate of subduction, the more stress builds up in a certain amount of time, and the more frequent the earthquakes. The South American Plate is moving westward at 10 cm/year, and the Nazca Plate is moving eastward at 15-17 cm/yr, giving an approximate subduction rate of 26 cm/yr (Monroe & Wicander, 2006). In comparison, the Cascadia fault has an approximate subduction rate of 4 cm/yr (Cascadia Region Earthquake Workgroup, 2013). 

Now, let’s move on to the actual earthquake at hand!

Source: USGS Earthquake Hazards Program

The USGS map above shows the Mercalli intensity Scale, which is a qualitative measurement and measures the intensity of the earthquake, unlike the moment magnitude scale which is a quantitative measurement and measures the total energy released. The intensity of an earthquake is dependent on many factors and can vary substantially from one region to another, but one of the biggest things that it is dependent on is the depth of the earthquake. If you have a 8.0 earthquake 600 miles deep, there will be no damage. However, if you have an 8.0 earthquake right at the surface, the damage will be catastrophic. With a depth of 15 miles, this earthquake was comparable to depth to other recent megathrust quakes.

As you can see, strong to severe shaking was reported over a very large area. Even in Buenos Aires, some 690 miles away from the epicenter, buildings swayed and car alarms were spontaneously set off. People even reported shaking in São Paulo 2,100 miles away. That’s the same distance as New Orleans is from Seattle!

And let’s not forget about the tsunami.

Boats stranded in Coquimbo after the tsunami. Credit: Wikimedia User Sfs90

Due to this “only” being a magnitude 8.3 earthquake, it did not have the massive tsunamis that the Japan and Indian Ocean earthquakes had. However, it still sent a 15-foot wave to Coquimbo, stranding boats and flooding parts of port city. Over a million people have been ordered to leave their homes in Chile due to the tsunami and earthquake, and hundreds of thousands around the Pacific have been ordered to evacuate. A tsunami advisory was actually posted by the National Weather Service for Southern California for waves less than one foot in height, but it was later cancelled.

The National Tsunami Warning Center issues these awesome “propagation forecasts” after these earthquakes occur. Look at how the energy is not distributed evenly; there are little filaments of higher wave energy scattered within the overall wave dispersion. You may see the low wave height values and say to yourself: “I thought this wave was one foot when it hit L.A. Why does the map show it as less than five centimeters?” Well, the answer is that this map shows the height of the tsunami as it travels through the open water at 500 mph. As the waves approach the shore, they slow down considerably and increase dramatically in height.

Source: National Tsunami Warning Center
The physics of tsunami propagation. Credit: Régis Lachaume

One of my favorite things to do after these events is look at tide gauges. Even though it is very unlikely that anybody in southern California observed the tsunami, the tide gauges definitely reported it. Let’s take a look at some! By the way, all these charts (and many more) can be found at NOAA’s Tides & Currents website.

Santa Monica reported a 0.7 wave. But how about some other locations around the U.S.?

Hilo Bay, much more exposed to the tsunami, saw a 5’11” drop in sea level today as the waves raced 455 mph across the ocean. Impressive. 

Even the Washington coast saw some action! Take a look at La Push. You can clearly see fluctuations in water level from the tsunami occurring this afternoon.

Of course all of this pales in comparison to Coquimbo. 

One thing to take away from all these gauges is that the first wave often isn’t the strongest, you don’t always experience the ocean receding before the first wave, and tsunamis can last for hours. If you live in a tsunami-prone zone, as soon as you feel shaking, evacuate to high ground, and stay there! Even if you don’t feel shaking, it’s always important to be aware of any tsunami advisories or warnings, as earthquakes thousands of miles away can still send devastating tsunamis into certain locales where the sea floor amplifies incoming waves. Crescent City, a coastal city situated in Northern California, was devastated by a deadly 20 foot tsunami due to the Good Friday Earthquake of 1964. Even the 2011 Japan earthquake sent an 8 foot tsunami to the harbor, killing one person.

At some point, we will get a megathrust earthquake off our coast. It might be five seconds from now, and it might be 500 years from now, when the world is 900 degrees and we all have the same color hair. But it’s crucial that we prepare by rebuilding our infrastructure and having better disaster management/evacuation strategies, because the longer we put off dealing with those things, the bigger a hole we are digging ourselves in the future.

On that note, have a nice evening!
Charlie

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