Sunday, May 27, 2012

GeoNet and the Art of Earthquake Location Part 2

In my previous blog I discussing the principles of earthquake location, but we also have some reasonably difficult practical issues. The most important of these is how to identify the arrival of the earthquake waves when there are many sources of ground shaking. These include the background actions of the oceans on the shores, weather noise (such as wind, rain, thunder, etc.) and humans and other animals (see Figure 1). In fact it is what we call “cultural noise” which causes us the most difficulty. This is the noise us humans make going about our everyday lives (vehicles, factories, and just people walking around). This is obviously worse in cities where there are many of us causing ground noise. To avoid this many of our recording sites are as far away from people as possible! Another GeoNet blog (see GeoNet – Shaken not stirred) gives a very good example of seismic noise made by a large group of people. For all these reasons considerable skill is required to “pick” the first arriving earthquake waves which may be buried in ground shaking noise. Moving this to an automated process is difficult, but good progress has been made.  Machines now do the job more consistently than humans, but can still more easily be fooled by noise.

Figure 1: The GeoNet seismograph station near Denniston on the west coast of the South Island. The image shows two earthquakes near the centre, but also a lot of "cultural" noise. This site is prone to disturbance by nearby mining operations, which show as small, similarly-sized blobs during usual working hours.

An additional practical problem is making sure the correct earthquake arrivals are associated with the correct earthquake. In New Zealand where more than 20,000 earthquakes are located each year there are often earthquakes happening at the same time in different parts of the country. If the automatic processing mixes the arrivals from one earthquake with another event the calculated location will be inaccurate. To avoid this the computer is actually making 100s of estimates every second seeing if a “picked” phase arrival will fit any earthquake location. In this process an earthquake location needs to have a good level of accuracy before it is accepted. But some bad events do get through when there is a large amount of ground noise or signals from distant earthquakes are mixed with nearby events.

Our new earthquake analysis system, GeoNetRapid (currently in Beta) is based on the SeisComP3 system developed by GFZ in Potsdam, Germany which is made freely available and has a large and active user community (for details see my colleague’s blog). This system automatically identifies earthquake wave arrival times (phases), associates the phase into earthquake events and then provides a location and depth with error estimates (and magnitude estimates). Additionally, within GeoNet Rapid we are using many decades of earthquake and tectonic research in New Zealand in the form of a three dimensional model of how earthquake wave speeds vary around New Zealand. This allows for the more accurate estimation of the true location and depth of earthquakes. But even with all this new technology the machines will sometimes get it wrong. For larger felt earthquakes recorded on many stations this is now rare and will continue to improve as we refine GeoNet Rapid. For more details on how to use GeoNet Rapid see GeoNet Rapid - Why is it different?

How do seismologists locate an earthquake?
Foo Fighters rocked Auckland!
GeoNet Rapid (the Beta website)
The SeisComP3 earthquake Analysis System (the heart of GeoNet Rapid)
GeoNet Rapid - Being Faster
GeoNet Rapid - Why is it different?

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