Saturday, February 27, 2010















Image source: USGS ShakeMap for Chile EQ


Contrast the Chile map with that of Haiti:

Image source: USGS ShakeMap for Haiti EQ


Other sources of information for classes:

IRIS's teachable moments collection


Elizabeth 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).

Jazinator 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.

Raúl 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.

Jazinator 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.

Elizabeth 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.)