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Tuesday, 9 October 2007

When the Earth moves

Ted Nield* has identified a present science journalists can give geologists in UN International Year of Planet Earth 2008. Stop saying “on the Richter Scale”.

Ever since the first landslide victory, journalistic language has been full of geological metaphor. Landslides, triggered close to the epicentres of political earthquakes and their aftershocks, or eruptions of popular sentiment, may tell of seismic shifts as tectonic plates of fixed belief move along fault lines in the bedrock of voter support, sending tsunamis that may even raze the high ground of our moral topography and bring overdue extinction to the dinosaurs stalking the political landscape. But geology itself is also frequently in the news, particularly when Mother Earth moves to prove yet again what an unsuitable parent she is.

The first of the problems affecting earthquake copy is usually how to describe severity. You should always write it thus: “An earthquake of Magnitude 7 has struck the island of Sumatra”. Do not on any account write: “An earthquake measuring 7 on the Richter Scale…”.

Earthquakes are no longer measured on the Richter Scale, defined in 1935 by the Californian nudist and geophysicist Charles F Richter (1900-1985) and his more frequently clothed colleague Beno Gutenberg (1889-1960). Although Richter first developed the idea of using a logarithmic scale (which he reputed to have described as "the work of the devil"), his was designed to measure quakes in southern California; and like many things from around there, it possessed certain magic characteristics.

Because of its limitations (especially a tendency to become saturated at high magnitudes, so that very large events could not be distinguished) a more uniformly applicable scale known as “Moment Magnitude”, Mw, was developed in 1979. This is the measure used today and whose numbers are nearly always verbosely and redundantly misreported as “on the Richter Scale”.

Each point on the Magnitude scale betokens an earthquake releasing about 30 times more energy than one on the previous point. For this reason, the higher the fewer. Earthquakes are happening all the time, but Magnitude 8 earthquakes occur about once every 10 years - Magnitude 9s about once a century. They don’t get very much bigger than 9 simply because the strength of rock is finite - such huge amounts of energy cannot be stored up without being released.

Sometimes, in stories about ancient earthquakes - like the one that may have brought down the Walls of Jericho - you may hear the term “Earthquake Intensity”. Intensity is measured on the Modified Mercalli Scale, whose categories go from I to VIII (or higher). These should not be confused with Moment Magnitude, which uses arabic numerals. Whereas Magnitude measures the energy released by an earthquake and is calculated from seismograph measurements, “intensity” measures the shaking produced at a certain location. Intensity measurements are more subjective, and are determined from effects on people, structures, and the environment – hence their use in analysing historical reports.

One of the thing affecting intensity is depth. The point inside the Earth where an earthquake occurs is known as its “focus”. If the focus is 150km deep, surface effects will be less noticeable than if the same amount of energy were released at only 2km. Think of a vibrator next to your skin, as opposed to under the mattress.

A line drawn from the centre of the Earth through the focus intersects the surface at the “epicentre”. This often misunderstood word is the point on the ground directly above the focus. Because this is where seismic waves have the shortest distance to travel to surface, it is often (though not always) where an earthquake’s intensity is greatest, and where it is first felt.

Earthquake magnitudes are often revised as measurements from more distant seismographs are added in to the equation. So, when you find later reports coming across your desk conflict, do not correct against previous copy. Finally, “earthquake” and “quake” are permissible alternatives. American writers often use the word “temblor” for variety. It is from Spanish, but largely unknown among English speakers on Eurasian Plate, and should be avoided here.

Finally, “earthquake” and “quake” are permissible alternatives. “Tremor” can mean any seismic disturbance, but has no particular scientific meaning that distinguishes it from “earthquake” or words nobody outside America uses, like “seism” or “temblor”. It is best reserved for minor quakes, especially weaker aftershocks in the context of “dying away”.

Useful sources

You can receive daily earthquake notifications from the United States Geological Survey’s National Earthquake Information Center, which sends out emails when an earthquake anywhere in the world tops Mw 5.5 – or any “felt” earthquake within US borders.

In the UK, the British Geological Survey is the point of first call for earthquake information and comment, especially about earthquakes in the UK.

Dr Ted Nield NUJ FGS is the Editor of Geoscientist, the monthly colour magazine of the Geological Society of London. He is Chair of the Association of British Science Writers (ABSW). His book Supercontinent – 10 billion years in the life of our planet is published by Granta in October.