April 26 – All shook up!

Today’s Factismal: Today is Richter Day, in honor of the birth of Charles Richter, who developed the first objective earthquake scale.

Earthquakes are scary things. They make buildings shake and collapse, and rivers run backwards, and can change the course of history. But the scariest thing about earthquakes to a scientist is that until recently they were very hard to measure.

A model of the first seismometer (Image courtesy Shizhao)

A model of the first seismometer
(Image courtesy Shizhao)

Though the Chinese had developed a seismometer in 132 CE, it was a purely descriptive device. It consisted of dragons with pebbles in their mouths; when the shaking of an earthwuake happened, the dragons would drop their pebbles. By triangulating the dropped pebbles from several seismometers across the kingdom, the Chinese could locate the affected area and speed relief to them.

The next step in measuring earthquakes happened nearly two thousand years later in 1873, when two Italian geophysicists created a scale based on the amount of damage done by the earthquake. Their scale was modified a short time later by Giuseppe Mercalli, creating the well-known Mercalli Intensity Scale. Though this did help geophysicists by providing a common scale to use as they drank beer and argued which earthquake was worse, it still didn’t tell us much about how much energy each earthquake released. That’s because a weak earthquake that happens in an area with soft soil and old masonry buildings could do more damage and have a higher rating than a strong earthquake that happens in an area with strong soil and well-constructed buildings. We needed a scale that would tell us how much energy was released.

And in 1935, we got one. Charles Richter had been studying earthquakes in California using the newly-invented Wood-Anderson torsion seismograph (a seismometer is any instrument that records an earthquake, whereas a seismograph actually records the motion of the Earth itself during the event). Because he correctly surmised that the amount of energy might go up very rapidly with larger events, he designed the scale to be logarithmic; that is, a magnitude 2 event would have 10 times the ground motion and 31 times the energy of a magnitude 1 event.

Due to the limitations of his equipment, Richter’s scale (known cleverly enough as the Richter scale) had some problems of its own. Though it did well at distinguishing the amount of energy in small, local earthquakes, it didn’t do well when ground motion was recorded for events that were either far away or very strong. As a result, geophysicists now use the Body Wave Magnitude (and reporters regularly get it mixed up with the actual Richter Magnitude). But Richter’s accomplishment still stands as an example of how careful observation and thought can turn an observational science like seismology into an experimental science like seismology.

If you’d like to help with modern seismology, then wait for an earthquake and then head on over to the USGS’s Did You Feel It? page, where you can help seismologists as they try to turn old Mercalli records into estimated magnitudes and improve our understanding of earthquake risks.

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