Deep Earthquakes and Magnitudes

Let’s just have one more look at magnitude before moving on to other topics. Some people have noticed that the magnitudes being given for deep earthquakes under the North Island by GeoNet Rapid (Beta) are much lower than the official Local Magnitudes being given on www.geonet.org.nz. This is related to GeoNet Rapid (Beta) moving to magnitudes based on estimates of Moment Magnitude, as discussed in my last blog. It highlights why understanding earthquake magnitude can be complicated – particularly in New Zealand where we have deep earthquakes. The magnitude estimate used by GeoNet Rapid (Beta) is removing the bias in the Local Magnitude caused by the way the earthquake waves lose or do not lose energy as they travel through the complicated earth structure beneath the North Island. To understand this let’s look in a little detail at what lies below our feet (assuming you live in the North Island as I do).

Under the North Island of New Zealand the Pacific and Australian tectonic plates are colliding, and the Pacific plate is being pushed down (subducted) under the Australian plate (for more details see article in Te Ara). It is a slow collision compared to a car crash at only around 5 cm a year, but reasonably fast in geological terms. This can be seen in the image of earthquakes under the North Island of New Zealand (see diagram) – shallower earthquakes (orange) near the east coast give way to deeper earthquakes (green, blues to purple) as we travel west outlining the Pacific plate getting deeper beneath the Australian plate. By the Taranaki area the earthquakes are hundreds of kilometres deep and by Auckland you have moved out of the region where there is a subducting Pacific slab at depth. Above the Pacific plate under much of the central North Island, the material has been disturbed by this collision and subduction process forming a region of volcanic and geothermal activity.

When an earthquake happens deep under the North Island the earthquake waves travel up and along the colder rock of the Pacific plate without losing much shaking energy, but the waves travelling up through the hotter volcanic zone lose most of their shaking energy. This explains why these earthquakes are often strongly felt on the East Coast of the island but are sometimes not even felt directly above where they occur! Putting all this together we see why measuring the magnitude of a deep North Island earthquake is difficult. Our instruments record high levels of shaking along the East Coast of the North Island and even felt levels of shaking along the same coast in the South Island, but low levels of shaking directly above the earthquake and to the west (depending on the location).

When the New Zealand Local Magnitude scale was devised in the 1970s, these complications were not taken into account fully and so deep earthquakes under the North Island are assigned higher magnitudes than newer techniques like Moment Magnitude give. This explains why GeoNet Rapid (Beta) which uses a magnitude estimate based on Moment Magnitude gives values lower than Local Magnitude for deeper North Island earthquakes by around 0.5 units or more. Similar effects happen for deep earthquakes in the Fiordland region of the South Island.

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.