**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.

How about listing what kind of Magnitude it is on the geonet rapid page? (With a link to what each Magnitude is)

ReplyDeleteThat would hopefully clear up some confusion!

(Especially re twitter and #eqnz..)

@Kyhwana GeoNet Rapid displays Summary Magnitude, giving a smooth transition from Local Magnitude for small earthquakes to an estimate of Moment Magnitude at larger magnitudes.

ReplyDeleteHmm, but it doesn't say that on the page, is what i'm saying. I dunno, would adding magnitude be confusing to people or make it better?

DeleteGeoNet Rapid is beta at present so we are looking at how to improve the pages. A mouse-over (as used for status, etc.) is an option, but the explanation would need to be brief.

DeleteThis comment has been removed by the author.

ReplyDeletethe reference number of the latest recording in your data is 3716032 so when was the 1st number started and are these 3 million plus recordings all refering to NZ earthquakes

ReplyDeleteThis number started in late 1986 and counts up from there, but it is just an identifier used for any possible earthquake. Most of the "counts" do not turn out to be real earthquakes. Now if we assume 20,000 earthquakes a year (including aftershock sequences) times about 25 years that gives a total of around 500,000 earthquakes since 1986, rather than the 3.7 million suggested by the identifier.

ReplyDeleteIn the new GeoNet Rapid system we use identifiers based on time so this sequence will soon come to an end.