What’s the Magnitude?

And whose Magnitude is it anyway?

With the introduction of GeoNet Rapid (our new automated earthquake analysis system - http://beta.geonet.org.nz) we have began the move to a unified magnitude estimate (Summary Magnitude, or just M) based on Moment Magnitude. Moment Magnitude is more closely based on the full earthquake characteristics, and will align better with the magnitudes given by international institutions such as the United States Geological Survey (USGS).

Was that a magnitude 4.8 or 5.3? That earthquake felt much larger than the one last week but GeoNet says it was ONLY a magnitude 4.8! And why has the magnitude now gone up? These are the questions we are asked all the time. And once we move to GeoNet Rapid I am sure even more questions will come our way.

For example, on the last day of December 2011 a (GeoNet) magnitude 4.8 earthquake occurred at 1:44 in the afternoon about 10 km east of Christchurch. The USGS calculated it was magnitude 5.3. Who was right? The media suggested the USGS was (see http://www.stuff.co.nz/the-press/news/christchurch-earthquake-2011/6206867/Aftershock-may-be-one-of-biggest), and several people in Christchurch agreed saying that it felt much bigger than 4.8. As stated in the article the USGS usually gives lower magnitudes than GeoNet, but in this case they did not because they used a different magnitude method than usual. Independent estimates using Moment Magnitude gave a value a little lower than GeoNet as expected.

I repeat this story here to show the trap of using magnitude as a measure of earthquake size without knowing the detail. Currently GeoNet publishes Local Magnitude (sometimes called Richter Magnitude after its inventor, Charles Richter) for most earthquakes but uses Moment Magnitude for large (usually greater than around magnitude 6) earthquakes because local magnitude is unreliable for larger events.

The magnitudes of earthquakes cause much confusion, with different organisations publishing different values for the same earthquake. And often more data results in the magnitude being revised up or down.

It does not help that there are more different magnitude methods than I have fingers! Or that it is not possible to sum up the size of a complicated natural phenomenon like an earthquake with a single number.

We describe an earthquake as happening at a place (the epicentre), at a distance below the Earth’s surface (depth), and having a size (magnitude). Real earthquakes start breaking the rock somewhere “down there”, and continuing to rupture for a time in a particular direction (or in more than one direction). To fully describe an earthquake requires many numbers rather than the three listed above. So why do we use magnitude?

Magnitude is an estimate of the size of an earthquake independent of the location of the person experiencing it (l'll talk some more about felt intensity in a later blog). Originally magnitude was based on the size of the traces on a particular type of earthquake drum (the Wood-Anderson seismograph). And this is still how Local Magnitude is calculated (although now computers transform modern seismograph signals into “pretend” Wood-Anderson instruments before the measurement is made). The values we give are an average of many measurements on many drums and are accurate to about one decimal place (for example 4.1) although the average usually has many decimal places. Most countries have developed their own Local Magnitude estimations.

Over the years scientists have developed other magnitude methods for particular uses. Probably the most useful of these is Moment Magnitude which is based on the actual earthquake source dimensions and properties. To fully characterise the Moment of an earthquake takes many numbers, but these are then reduced to the one number – the Moment Magnitude. There are always downsides, and in the case of Moment Magnitude it takes longer to calculate (because you have to wait for more data to arrive), and it cannot be calculated for smaller earthquakes (much below about magnitude 4).

The way around this is to use Local Magnitude as the preferred estimate for magnitude for smaller earthquakes and a quickly calculated estimate of Moment Magnitude for larger ones. This is what GeoNet Rapid will provide using Summary Magnitudes (or just M) based on this idea. It is not as simple as that because the scales need to mesh together, be consistent with previous methods, so over time we will be making refinements as the new system develops. And we will be working with USGS to get consistency with them, but be warned - magnitude is an estimate and it is rare for two institutions to give exactly the same value for an earthquake. Within 0.1 is the best we can expect.