Author: Lauren Shea
How are earthquakes measured?
Seismologists often report earthquakes using the moment magnitude scale. The Richter scale may sound more familiar, but it is no longer in use. The moment magnitude scale has replaced the Richter scale because it includes more variables that are now able to be measured with modern-day instrumentation, making earthquake measurements more precise.
The moment magnitude scale is a function the seismic moment, which is a measure of the energy released during an earthquake.
Where is this release of energy coming from?
Generally speaking, sometimes along fault zones, the surfaces get “stuck”, thus building up stress; a natural earthquake occurs when there is a release of stored energy due to the locked fault rupturing. One way to measure the seismic moment is by measuring the area of the fault rupture, the displacement that the fault slipped, and the force that was required to get the fault to rupture. Another way to measure the seismic moment is by measuring the energy radiated from seismograms. An illustration of a slipping fault plane can be seen here:
Image source: IRIS
Is there really much of a difference between Boston getting hit with a magnitude 3.0 earthquake versus a magnitude 2.0 earthquake?
Yes! For each whole number increase on the moment magnitude scale, the amount of ground shaking increases ten times and the amount of energy increases 30 times. Here’s what that difference looks like:
The difference in energy released between the two is exponential. A magnitude 2.0 earthquake has an energy release equivalent to 56 kilograms of explosive, whereas a 3.0 earthquake has an energy release equivalent to 1,800 kilograms of explosive, about the same as a lightning bolt. As you move up the moment magnitude scale, this difference in energy release only increases, as can be shown here:
Image Source: IRIS
However, neither a magnitude 2.0 nor a magnitude 3.0 earthquake is likely to be felt by a Bostonian because both are considered to be “micro-earthquakes” that only do minor damage.
If Boston usually experiences earthquakes on the lower end of the magnitude scale, does that mean we’re safe from a major (large magnitude) earthquake?
Not necessarily! While it’s true that the New England area isn’t prone to large earthquakes (most are <3.0), they are still possible. The reason why many more small earthquakes are measured is because the higher up the moment magnitude scale you go, the less frequent the earthquakes.
While uncommon, Boston has seen large earthquakes before! In 1755, New England was hit by a major earthquake near Cape Ann, which is estimated to have been around a magnitude 5.9-6.3 earthquake. The energy released would have been equivalent to approximately 56,000,000 kilograms of explosive, similar to the Hiroshima nuclear bomb.
The moment magnitude scale measures the amount of energy released during an earthquake. As earthquake magnitude increases, the ground shakes more, thus increasing the risk for potential destruction, such as property damage and loss of life. In an area as highly populated and urbanized as Boston, even one large magnitude earthquake could have devastating effects.
Christine Regalla is a geology professor in the Earth and Environment Department at Boston University. See what she and her students do to study earthquakes by visiting their website or by following them on Twitter and Instagram (@Rengellia).