page thirty-four

The Seismograph

Thirty-three rooms, and the ground under all of them has kept still. Not today. The day's number sets off a small earthquake somewhere under an invented country and lets a handful of stations record it. Two kinds of wave leave the fault together and arrive apart, and the gap between them is a distance. Three distances make a place.

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The records

The P wave is the first thing to arrive: a small, quick flicker out of the noise. The S wave comes later and much bigger. Choose what you are picking, then click on the record. The loupe magnifies five seconds around the pointer (and triples the height for P and S), and the buttons under each record move a pick by tenths of a second.

Picking

The map

Each station's circle is drawn at eight kilometres for every second of S − P. Click where you think they meet.

The nomogram

Richter's chart. A line from the S − P interval to the amplitude crosses the middle scale at the local magnitude.

Event
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The arrow keys walk to other days. Every day is a different country, a different network and a different earthquake.

What is real here and what is not

The method is the one seismologists used for most of the twentieth century, and still use to check a computer's answer. The country, the stations and the service are the day's number.

  1. 01

    Two waves, one gap

    An earthquake sends out compressional P waves and slower shear S waves at the same moment. In the upper crust P travels at about 6 km/s and S at about 3.5, so the gap between them grows by roughly a second for every eight kilometres. That rule of thumb is real, and it is exact on this page.

  2. 02

    Circles meet

    One station only knows how far away the earthquake was, so it could be anywhere on a circle. Two circles cross in two places. A third settles it. Real networks use dozens of stations and solve for depth and origin time too, but the idea is this one.

  3. 03

    Richter's chart

    Charles Richter defined local magnitude in 1935 for southern California: the logarithm of the largest swing, in millimetres, on a standard Wood-Anderson seismograph, corrected for distance. The nomogram on this page uses the formula from his textbook, ML = log A + 3 log(8 Δt) − 2.92. Each step of one is ten times the swing.

  4. 04

    The rest is simplified

    The crust here is one uniform layer, the focus is shallow enough to ignore, every station is a vertical instrument already corrected to Wood-Anderson millimetres, and there is no station-by-station correction beyond a little random scatter. Real records come with refracted phases, aftershocks, lorries and wind.

Thirty-four rooms, and one number doing all the work. Today it breaks a fault under a country nobody lives in and hands you the paper from the drums.