Saturday, February 27, 2010

How big was that EQ? Magnitude vs intensity in Chile and Haiti

My intro class is covering earthquake size on Monday. I talked to them about the Haiti earthquake at the beginning of the semester, and now an even larger earthquake has struck the subduction zone off the west coast of Chile. I put together a bunch of powerpoint slides for my class using some images shared by geobloggers, journalist-bloggers, and the USGS. And although I'm currently on blogging hiatus, I decided to share them with the world, in case anyone else could use them.

There are two different ways to talk about the size of earthquakes: magnitude and intensity. If you listen to news reports about disasters, you've heard of earthquake magnitude. You may not have heard of intensity, but if you've experienced an earthquake, the Mercalli intensity scale should sound very familiar to you. The two ways to talk about size are complementary - they describe different things. Both are important, but in different ways.

An earthquake's magnitude is related to the amount of energy released by an earthquake. The Richter scale, which isn't actually what's used any more, is a kind of magnitude scale. Richter's scale used the amplitude of shaking on a seismograph to estimate the size of an earthquake - the bigger the squiggle, the bigger the quake. (The moment magnitude scale used today includes other information that gives a more complete picture of the energy released by the earthquake. That information is especially important for understanding large earthquakes, like the one that just occurred in Chile.)

All earthquake magnitude scales are logarithmic: a magnitude 8 earthquake is an order of magnitude larger than a magnitude 7 earthquake. To illustrate this concept for my class, I borrowed the seismograms recorded by Ian Stimpson and shared on his blog (Chile and Haiti), re-scaled the Haiti seismogram, and put the two seismograms on the same powerpoint slide:

The maximum amplitude of shaking at the Chile earthquake is around 35 times greater than that of the Haiti earthquake. The difference in energy is even more extreme - the Chile earthquake released something like 500 times the energy released by the Haiti earthquake. One of the reasons for that can be seen in the two seismograms: the Chile earthquake shook for longer than the Haiti earthquake. The Chile earthquake also broke over a larger area (around ten times larger). Edit: as Ian Stimpson pointed out on Twitter, there's another reason why the energy difference is even larger. His seismograph saturated for the Chile earthquake - at some point, the seismograph can't record increases in the amount of shaking. (That's one reason that moment magnitude is used for large earthquakes - and it's one reason why initial reports stated that the Chile EQ was M 8.3, not M 8.8.)

We won't know how much damage has happened in Chile for some time. It's likely to be significant, but not 500 times worse than Haiti (and the Haiti earthquake has probably killed more people). Those differences are partly due to differences in the buildings, but they are also reflected by the second way of looking at the size of an earthquake: seismic intensity.

The intensity of an earthquake describes its effect on the Earth's surface (including its effect on people and the things they build). Intensity is measured on a descriptive scale called the Modified Mercalli Intensity scale. The Mercalli intensity scale includes descriptions like:

I. Not felt except by a very few under especially favorable conditions.


XII. Damage total. Lines of sight and level are distorted. Objects thrown into the air.

In between, it describes earthquakes that wake up people who are sleeping (intensity V), move heavy furniture (intensity VI), and cause chimneys to fall over (intensity VIII). Intensity is related to the energy released by the earthquake, but it's also related to distance from the fault that slipped, to the way that the fault broke during the earthquake, and to rock and soil conditions that can increase shaking.

And those differences mean that Port-au-Prince was hit really hard - probably harder than any place in Chile.

Although reports of damage are just coming in, the USGS has maps that estimate the amount of shaking in the Chile earthquake:

Image source: USGS ShakeMap for Chile EQ

A huge area was affected by shaking of Mercalli intensity of VII or VIII - strong enough to destroy some buildings, but to leave many standing (especially if they were designed to withstand earthquakes).

Contrast the Chile map with that of Haiti:

Image source: USGS ShakeMap for Haiti EQ

The Haiti earthquake shook a small area very intensely. Unfortunately, that area included the city of Port-au-Prince - a city with a dense population and buildings that are not built to withstand earthquakes. The results were devastating.

Other sources of information for classes:

IRIS's teachable moments collection
Terremoto Chile, a blog (in Spanish) about the Chile earthquake