February 27, 2010 5 Comments
It seems that the Internet is as confused by all that’s happened as we are. I want to address a question that seems to be oozing out of every news story:
How does the Chile quake compare to Haiti’s?
Everyone is scrambling to frame the Chile earthquake in terms of the Haiti one. It’s a misguided but understandable impulse—at the moment I suppose it’s the most immediately available tool for comparison. It should go without saying that the comparison is basically misguided: Chile has a long experience of earthquakes, a developed infrastructure, and a comparatively favorable colonial and economic history—all things that Haiti lacks.
Still, attempts to make the comparison have been made, and
they’re confusing, even at the numbers level. So I think it’s worth spending some time setting the record straight about what these figures can and can’t tell us.
The Confusion: 50x as great? 100x? 500x?
Sometimes accounts of the Chile earthquake sound like a bidding war. According to the Miami Herald, “the energy released is between 500 and 900 times that of the magnitude 7.0 quake that hit Haiti on Jan. 12 …. On the complex scale that measures earthquakes, an 8.0 quake releases 30 times the energy of a 7.0, and a 9.0 would release 30 times that, meaning 900 times more energy. An 8.8 would be somewhat less, Dixon said.”
According to AP/CBS News: “The 8.8-magnitude earthquake that struck Chile early Saturday morning released 500 times the energy of the 7.0-magnitude quake that struck Haiti last month, a geophysicist told CBS’ “The Early Show.””
Other sites are claiming that the Chile earthquake was 50 or 100x more powerful than the Haiti quake.
So, how do we account for the discrepancy in these figures?
What the Moment magnitude scale (and the Richter) do and don’t mean
I spoke with Dale Grant at the U.S. Geological Survey to get some help. Grant explained that neither the Richter scale nor its more advanced cousin, the Moment magnitude scale (in which current earthquake ratings are reported) account for energy released during a quake—that has to do with several variables like depth, location and geology, and can at best be estimated, even using the Mms. The Richter scale, now out of use, measured amplitude alone, and was therefore only useful as a measure of the magnitude of difference.
The Moment magnitude scale, Richter’s better multitasking cousin, is better at tracking the relationship between amplitude and energy, but it’s far fro perfect. Like the Richter scale, the MMS is logarithmic, where an earthquake one whole number greater has 10 times the amplitude and has approximately 31.6 times more energy. For example, M7.0 has about 31.6 times the energy of M6.0, and 1000 times the energy of M5.0.
This is where those differences in the figures reported are coming from. Some are comparing amplitude; others are comparing energy released.
What these figures can tell us:
Again, the difference between integers (say, 7 to 8 ) represents a 10x difference in amplitude. Therefore the difference between the Haiti quake (for convenience, we’ll call it 7) and the Chile quake (for convenience we’ll call it 9) is 10 x 10=100. This means that the Chile quake was roughly 100 times more powerful than the quake in Haiti.
Again, according to the Moment magnitude scale (which is logarithmic) the difference between integers represents an increase of 31.6x energy released. This means that a difference of two integers (7 to 9) means 31.6 x 31.6=998.6, so the Chile quake probably released *approximately* a little less than 1000x as much energy than the Haiti quake.
What these figures can’t tell us:
Intensity of effect. See the Mercalli Intensity Scale below.
Anything exact. All of that depends on where the epicenter of the quake is, how deep, etc.—considerations that don’t make it into that estimate.
To understand what the above figures mean in less abstract terms, it might be helpful to take a look at charts that measure these energies according to a scale we can (sort of) understand. Below are two charts, one that measures the Moment magnitudes in terms of nuclear bombs, another that measures them in terms of explosives (TNT).
From the USGS:
According to this chart, the energy released by the 8.8 Chile earthquake correlates roughly to that released by 25,000 nuclear bombs. The Haiti quake released energy equivalent to roughly 25 nuclear bombs.
(Again, take this with a grain of salt—these estimates are for the amount of energy released in total, which in an earthquake would be spread out over a much larger area than, say, a bomb.)
From the University of Nevada, Reno Seismological Lab, a similar chart expressing the energy released in terms of tons of TNT [click to enlarge]:
According to this, the Chilean quake resulted in an energy release approximately equal to 5 billion tons of TNT, and the Haiti quake resulted in a release of 160 million tons of TNT.
A third way to measure earthquakes: the Mercalli Intensity Scale
What the above systems don’t measure are the effects of the earthquake. A scale has been developed for that too. The Mercalli Intensity Scale measures an earthquake’s effects based on descriptions of the experience (in other words, it relies to some extent on subjective accounts, on how the earthquake is narrated):
- I. People do not feel any Earth movement.
- II. A few people might notice movement if they are at rest and/or on the upper floors of tall buildings.
- III. Many people indoors feel movement. Hanging objects swing back and forth. People outdoors might not realize that an earthquake is occurring.
- IV. Most people indoors feel movement. Hanging objects swing. Dishes, windows, and doors rattle. The earthquake feels like a heavy truck hitting the walls. A few people outdoors may feel movement. Parked cars rock.
- V. Almost everyone feels movement. Sleeping people are awakened. Doors swing open or close. Dishes are broken. Pictures on the wall move. Small objects move or are turned over. Trees might shake. Liquids might spill out of open containers.
- VI. Everyone feels movement. People have trouble walking. Objects fall from shelves. Pictures fall off walls. Furniture moves. Plaster in walls might crack. Trees and bushes shake. Damage is slight in poorly built buildings. No structural damage.
- VII. People have difficulty standing. Drivers feel their cars shaking. Some furniture breaks. Loose bricks fall from buildings. Damage is slight to moderate in well-built buildings; considerable in poorly built buildings.
- VIII. Drivers have trouble steering. Houses that are not bolted down might shift on their foundations. Tall structures such as towers and chimneys might twist and fall. Well-built buildings suffer slight damage. Poorly built structures suffer severe damage. Tree branches break. Hillsides might crack if the ground is wet. Water levels in wells might change
- IX. Well-built buildings suffer considerable damage. Houses that are not bolted down move off their foundations. Some underground pipes are broken. The ground cracks. Reservoirs suffer serious damage.
- X. Most buildings and their foundations are destroyed. Some bridges are destroyed. Dams are seriously damaged. Large landslides occur. Water is thrown on the banks of canals, rivers, lakes. The ground cracks in large areas. Railroad tracks are bent slightly.
- XI. Most buildings collapse. Some bridges are destroyed. Large cracks appear in the ground. Underground pipelines are destroyed. Railroad tracks are badly bent.
- XII. Almost everything is destroyed. Objects are thrown into the air. The ground moves in waves or ripples. Large amounts of rock may move.
The Chile earthquake has received a Mercalli Intensity rating of VII. The Haiti quake, in contrast, received Mercalli Intensity rating of IX.
It goes without saying, I hope, that these numbers are the least interesting and least significant aspects of what a comparison between the Chile and Haiti quakes should be. These are (barely) mathematical relationships; even the Mercalli intensity scale captures next to nothing about what it would mean to experience an earthquake in Haiti and one in Chile. They don’t express how Chile and Haiti differ in earthquake preparedness, infrastructure, and wealth; neither do they account for differences in geography and political climate. They are what they are, and to make too much of them would be like evaluating two people based on their respective weights; the information you can usefully extract is awfully limited. Still, these numbers are there, floating in the internetiverse, and since they are, we might as well understand what they do and don’t mean.