Haiti

I don’t have much to say about the human side of the Haitian earthquake except that it’s terrible, as media reports will tell you.  I saw the waves coming in on our seismograph drum, which gave some idea of the distance and size of the earthquake but not the location. The location in itself is about as bad as possible — a large, shallow earthquake very close to a major city, in a country too poor to afford earthquake-resistant construction. All I can do is suggest that you donate to relief organizations and press the Canadian and other governments to get aid in place as quickly as possible.

I can, at least, say something meaningful about the seismology. The best source for information on recent earthquakes is usually the USGS page. The basic parameters of an earthquake are time, location, magnitude, and source mechanism. Time is generally given in universal time code (UTC) rather than in any particular time zone. The location is given in three dimensions: latitude, longitude, and depth. Depth is the hardest of the three to measure accurately, but the value given (10 km) is relatively shallow — well within the crust. Shallower earthquakes are more likely to be damaging than deeper ones due to the shorter distance to the surface. This map puts the event in context with respect to other earthquakes in the region: Haiti’s actually west of the main subduction-zone seismicity — earthquakes are much more frequent elsewhere in the Caribbean. The fault yesterday’s earthquake was on is a strike-slip (horizontal motion, like the San Andreas) fault with left-lateral motion (if you stand on one side of the fault, the other side’s moving to the left).

The magnitude of the earthquake was, based on current estimates, a 7.0. That’s big, comparable to the 1989 Loma Prieta earthquake that did serious damage to San Francisco and environs. It’s not big enough to be all that rare on a global scale, however — a quick search of the ANSS catalog shows 145 events of magnitude 7.0 or greater since the beginning of 2000 (i.e. a bit over one per month). The earthquake magnitude scale is logarithmic, meaning that the Sumatran earthquake of 2004 (a 9.0) was roughly 100 times the size of this one. An earthquake’s effect on humans depends on a lot more than the magnitude, though — where it is in relation to populated areas is of critical importance, as are the response of local soils (soft ground can make matters worse) and the types of construction used.

Finally, there’s the source mechanism. It’s possible to work out, by looking at the waves radiated out in different directions, what the “radiation pattern” of energy transmitted by the earthquake was. There are different characteristic patterns for different types of fault motions. This earthquake’s pattern is characteristic of a slightly oblique strike-slip fault; based on the pattern, two different fault orientations are possible, but one (WSW-ESE) lines up with known fault traces and gives the expected left-lateral motion.

As I mentioned before, the earthquake showed up very strongly on the old analog seismograph we have as part of a museum display. The fancier digital instrument chose this opportune moment to play dead, but I obtained seismograms from an instrument operated by the Canadian National Seismograph Network that is located fairly close by.

The instrument in question records ground motion along three axes (N-S, E-W, and vertical); in the above plot I’ve rotated the horizontal components into radial (parallel to the path to the source) and transverse (perpendicular to the path to the source) components to show how different the result is. At this distance, four major wave types are observed: P and S waves, which pass through the Earth’s interior, and Love and Rayleigh waves, which travel along the surface. The surface waves are much lower in frequency and higher in amplitude than the body waves. P and S waves are also different in frequency and polarization, as we can see by zooming in:

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1 comment so far

  1. Kathleen on

    so, how long (in minutes) did this quake last? Nobody at USGS is talking about that
    thanks


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