For Your Consideration

For Your Consideration

Richter or moment magnitude? Tomato, tomato

However you report it, an 8.2 magnitude earthquake can be a dangerous one

A magnitude 8.2 earthquake is a massive one.

But just how massive kind of gets lost in the math. Most people are kind of aware that the scale used to report quakes is logarithmic, but wrapping your head around what it really means is problematic for humans who are notoriously bad at grasping big numbers.

Logarithmic scales are really handy to deal with huge numbers. So, for Charles F. Richter, who developed the original scale in 1935, it made sense to log the relative intensity of ground shaking from earthquakes as ordinals between one and 10.

Otherwise, the 8.2 quake that occurred just south of Alaska last week would have to be reported as something like 10 million times stronger than a 1.2, each integer being 10 times the strength of the previous one.

LAST WEEK: It is an interesting language we speak

While convenient, the Richter Scale didn’t really work for scientists, for a number of reasons, including the fact it was based on conditions in southern California so didn’t really hold for other locales where the conditions were different.

Richter was also comparative, measuring the intensity of California quakes relative to each other.

What seismologists really want to measure is the amount of energy released by a seismic event. Since the 1970s the Richter magnitude has been replaced by the seismic moment, a complicated calculation that boils down basically to the area of rupture along a fault multiplied by the average displacement of the fault multiplied by the rigidity of the earth.

To complicate things even further, when applied to the Richter Scale each integer represents roughly a 32-fold jump in energy released.

MORE FOR YOUR CONSIDERATION: Grocery bag dilemma, behaviour versus composition

For context (for what it’s worth), the largest ever recorded, the 9.6 (or 9.5 depending on who you ask) magnitude 1960 quake off the coast of Chile released 2.5 x 1023 joules of energy, a stupidly big number, which is virtually meaningless to all but the greatest mathematical geniuses on our shaking planet.

Just for fun, though, it is about the equivalent of 20,000 Hiroshima atomic bombs each of which pack the power of 15,000 tonnes of TNT, also virtually meaningless to the average person except to say, “holy [expletive deleted] that’s a lot.”

This gets us back to the problem of reporting earthquake magnitudes. Although authorities have now been using the seismic moment rather than Richter magnitudes for half a century, we’re back to the logarithmic approach converting the seismic moment data to moment magnitudes on a scale of zero to 10, which produces roughly the same number as Richter.

In practical terms, of course, what people want to know in, say, Kitamaat Village, which was evacuated when a tsumami warning was issued after the July 28 Alaska quake was recorded, is will a giant wave come and engulf our homes.

While that potential threat was rescinded within hours, an 8.2 on either the Richter or moment magnitude scales could definitely produce such a wave if, in addition to releasing an unfathomable amount of energy, it also created a significant displacement of the seafloor.

This one, thankfully, did not.

My own fascination with tectonics, earthquakes and tsunamis was perhaps forged in childhood. My favourite book when I was a kid was The Burning Rice Fields by Sara Cone Bryant.

This is the story of an old man and a grandson who save the people of their coastal Japanese Village from a tsunami by setting fire to the rice fields on top of the mountain.

Fortunately, these days we have more sophisticated ways of early warning than burning our food supply.

Like us on Facebook and follow us on Twitter