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


liz VanBoskirk said...

WIth teaching and using these two earthquakes for comparison I feel that one should also discuss enforcing earthquake building engineering standards. A few years ago I attended a meeting of the Americas where South America and Central America city planners, political figures, others from universities and the UN discussed how they implemented disaster mitigation in their city.

Kim said...

Elizabeth - definitely! In this case, I had planned to talk about EQ damage in class on Wednesday. By then, I hope there will be reports out of Chile that show that building codes made a difference, and that the damage was much less than it could have been. Until them, I am keeping the people of the area in my thoughts (and sending money to the Red Cross).

Jim L. said...

I always found the Mercalli scale as not particularly useful since it is subjective in so many instances.
1. Different types of rock/sediment will give different results for the same magnitude
2. If no one is in the area then there will nothing on the Mercalli
3. A lot of the Mercalli information relies on people's interpretation of the events that happened.
It makes it very difficult to impossible to compare earthquakes because they are usually in complete different circumstances. Even in the same city 100 years apart, the same magnitude earthquake will result in different Mercalli results.

Kim said...

Jazinator: I think the Mercalli scale is useful for some things. The USGS's PAGER population exposure information, for instance, says what kind of damage is likely in different areas (and how many people live in those places) - that's got to be very useful information for planning rescues! I don't know how the maps are generated - they must use the seismograms (and/or other remote information) to estimate ground acceleration and then Mercalli intensity, or something. But they put the likely damage into words that are clear to non-scientists: broken dishes vs collapsed buildings. So yes, the scale is subjective, but it's related to quantifiable things like ground acceleration and ground velocity. And the subjectivity may be a strength for describing what's important about the damage.

Unknown said...

I think another important factor is the difference between focal depths of both earthquakes. Haiti EQ was about 13 Km depth while Chile EQ was 35 Km depth. This factor together with the rheology difference in both cases could cause that with a less amount of energy you could have higher damages (beside the building codes, poverty, etc.). I think the key factor for the intensity (not the magnitude as you explained here) is the capability of the quake to generate surface waves. What do you think about it?

Kim said...

Raul - I agree, the depth is probably a big part of the reason why the maximum intensity of the Chile earthquake. (I didn't mention it because I wasn't sure if the depth had been calculated, or whether it had been set by the location program. Though depth is distance in another dimension, I guess.)

I don't know all the things that control the ground acceleration. (I'm not sure if it's mostly the magnitude of the surface waves, or if there's more to it than that.)

Brian Garrett said...

I think the subjectivity of the Mercalli scale is precisely the point. Empirical measurements such as seismograms and the moment magnitude derived from them give a good description of the geophysics involved, but convey very little on the effects at the human level--building damage, emotional reactions, phenomena witnessed. Also, while an earthquake will have only one magnitude (at least, only one per any given magnitude scale used), MMI intensities will vary widely depending on local circumstances. These enable scientists and others to investigate the human dimension.

Anonymous said...

This is an excellent explanation and much more useful than the hyperbolic statements in the media. Just from the photographs of Haiti and Chile, the devastation in Haiti looked worse after taking account of the differences in construction. This piece explains why that is right. For example, CBS Morning News reported that the Chile earthquake was "500 times" stronger than Haiti, yet the destruction was clearly not 500 times greater in Chile. Thanks for this, clear, common sense science.

Jim L. said...

Your points are valid and I can understand the usefulness of the Mercali scale. I guess I will try not to bash it as much in the future (probably not, but hey I can try).

mcdevo said...

Is there a way to know what the ground acceleration was during the Chilean earthquake? Is this known from the Richter measurements, or are auxillary measurements needed? And ... is the acceleration mapped for a surface area in the region, e.g., the peak acceleration near epi-center was XX m/s^2, and in center of Santiago, the peak accleration was yy m/s^2 ? Thanks if you can help me to understand this.

liz VanBoskirk said...

Seismic PBO network data link:
All sites are in N America, the west coast and Yellowstone.

Kim said...

mcdevo -

I don't know how the ground acceleration was determined for the Chile earthquake. I know there are instruments that can measure ground acceleration, but I don't know if the initial estimates came from instruments, or whether they can from modeling the relationship between the record of many seismic stations and ground movement. (I wonder if there's modeling involved, because there were similar maps made right after the Haiti earthquake, and I don't think Haiti had the necessary instruments.)

Elizabeth - thank you for that link!

Kim said...

(And this is where I should say that I'm a structural geologist, not a seismologist. If you want details of how seismologists study earthquakes, you're better off talking to Ian at Hypo-theses or Julian at Harmonic Tremors or the authors of the Shaking Earth blog.)