So I’ll wear pajamas, and give up pajamas
24 October 2006
As befits an earthquake whose aftermath was less than the 24-hour news channels made it out to be, this post will be less than initially promised. First of all, though, the HVO Current Eqs Map is up and running, though it is a little slow due to heavy traffic. Also, they have the seismograph readings from the Hualalai instrument station (closest to the quake).
I noticed that many media figures, including Hawai`i County Civil Defense officials, referred repeatedly to the magnitude of the quake on the Richter scale. While Richter’s 1935 system (based upon the much-abused scale for measuring star brightnesses, also called magnitudes; that’s a completely different rant) was designed for a specific purpose, it worked pretty well and had a catchy name, so it got pressed into greater service. Also, he was at CalTech when he developed it, and therefore had a lot of reputation to throw around. The Richter scale measures the amount of energy released during an earthquake, and was originally designed for use with a specific instrument at a specific location, but later rejigged for use pretty much wherever you wanted.
A less specific scale of similar utility, a sort of metric system of magnitude if you will, called moment magnitude, was developed in 1979. This new scale was designed to agree closely with the Richter scale, and it is logarithmic in the same way. In both scales, two earthquakes with an increase of one magnitude (4 to 5, say) means the amplitude of shaking is ten times as great at the same distance from both quakes. Likewise, a quake of 5 releases about 32 times as much energy compared with a quake of 4 on both the Richter and moment magnitude scales. Thus, the differences between the scales are pretty much mathematical (as shown here) but as a non-representative sample, I know more seismologists who use moment magnitude than Richter magnitude. These same seismologists do confess that they often use Richter when dealing with the lay public or media, however, because it is much better known. If you really want to get tricky, you can look at centroid moment tensors, which look cool on maps. Unfortunately, the Hawai`i one isn’t big enough to show up on the map they have on the web, but here is the raw centroid moment tensor stuff, for the truly geeked-out. So, use whichever scale you like, unless of course you’re talking about deep earthquakes, in which case, the body wave is a better measurement. Figure it out yourself. Here’s a pretty good description of the various kinds of scales for measuring the amount of energy released, courtesy of the HVO again.
My second point in all of this is that I can’t figure out why hypocenter never caught on the way epicenter has. Google ‘political epicenter,’ and you get over a million hits. Try it for ‘political hypocenter,’ and you get one percent of that. People often use epicenter when they actually mean ‘right where the dang thing happened,’ though. To clarify (and anyone who knows more Greek than I should feel free to display that at this time) the hypocenter is the point way down deep in the Earth where the rupture actually happens (cf hypodermic needle). The epicenter is the point on the surface of the Earth directly above that point (cf epidermis). For the topologically challenged, if you drew a line from the center of the Earth to the hypocenter and kept going until you start blinking really hard, you’d be at the epicenter. That spot doesn’t necessarily feel the greatest effects of the quake, or suffer the greatest damage, either. Those can vary greatly depending on the kind of soil or rock in the immediate neighborhood. Cracks, sinkholes, building collapses and so on do not reliably indicate the epicenter, and are almost never indicative of the hypocenter. Common depths for earthquakes are either 5-10km or about 30km, depending on location and type. All of which is to say that there can be a big difference between hypocenter and epicenter, and the word we use less usually designates the more important point, seismically.
All of this is by way of saying that scientific terms certainly get appropriated, and not always in the way they were intended. Without getting all literal and inflexible and stuff, these two examples might be a case where some clarification from the science-types would be useful in the fray of general discourse.
P.S. Hypocenter apparently is also used to designate the spot right underneath a nuclear bomb detonated in the air. An even less fun place to be than the previous definition.