Sunday, May 13, 2012

GeoNet and the Art of Earthquake Location - Part 1

Using the recordings of earthquake waves at GeoNet stations and some simple mathematics we can easily calculate an earthquake’s location. Yeah Right! (non-New Zealanders should check here and Figure 1 to understand the above statements). Earthquakes are complicated ruptures of the rock within the Earth. We imagine them as simple fault breaks deep underground, usually showing as nice straight lines where they reach the Earth’s surface. This simple picture is far from what actually happens - most earthquakes do not break the Earth’s surface, and larger earthquakes usually rupture more than one fault. This is why asking “what fault was that earthquake on?” is usually the wrong question unless you are talking about a large earthquake. For example, only the Darfield (September 2010) earthquake in the Canterbury earthquake sequence caused an identifiable surface rupture. Using various kinds of land surveying (very accurate GPS and satellite radar mapping) and many recordings from ground shaking sensors we can build up a picture of the faults which ruptured in the major earthquakes in the sequence. What we have found is that each earthquake is actually made up of several fault breaks within the Earth.

Figure 1: Yeah right! Tui beer is promoted through a humorous advertising campaign which uses stereotypes, heavy irony and the phrase Yeah Right. This phrase has become a part of New Zealand culture.

Let’s look at the earthquake location process in a bit of detail, including an “Earthquakes 101”. When we talk about the location and depth of an earthquake we are actually referring to the place where the fault rupture starts and begins sending out earthquake waves. A very big earthquake can break a fault (or faults) 100s of kilometres long, but its location will be given as the point where it starts. Technically we refer to the point on the Earth’s surface above where the rupture starts (referred to as the focus or hypocentre) as the epicentre (or just the location). The earthquake’s focus will be some depth below the Earth’s surface directly below the epicentre (Figure 2).

Figure 2: Earthquake location terms. Image from “Earthquakes and Plates

The location process involves measuring the arrival time of the earthquake waves (referred to as phase arrivals or just phases) at our ground shaking sensors. There are two main types of earthquake waves, imaginatively called primary (P; see Figure 3) and secondary (S; see Figure 4) waves. P-waves are like sound waves which travel through the Earth and are much faster than S-waves, which could also be referred to as shaking waves as they cause a side to side motion. It is the S-waves that cause most earthquake damage. In the upper 10 km of the Earth’s crust P-waves travel at about 4.5 to 6.5 km per second and S-waves at 3 to 4 km per second. There are other kinds of earthquake waves which are a combination of the main wave types. The difference in arrival time between these two wave types indicates the distance from the earthquake to the recording station (a bit like counting the seconds between the lightning flash and the sound of thunder gives an estimate of how far you are from a storm).

Figure 3: A representation of how P-waves, which are compressional waves (like sound waves) travel through the Earth. Copyright 2004-10.  L. Braile.  Permission granted for reproduction and use of animations for non-commercial uses.

Figure 4: A representation of how S-waves, which are transverse waves travel through the Earth. Copyright 2004-10.  L. Braile.  Permission granted for reproduction and use of animations for non-commercial uses.

Calculating the location, depth and size of an earthquake would be much easier if the earth beneath our feet was uniform and composed of just one kind of rock. But the rocks are layered, made of a variety of rock types, are full of fractures and far from uniform. In fact because of the alignment of some rock crystals and cracks earthquake waves may travel at different speeds in different directions! So that simple mathematics I mentioned above (tongue in cheek) gets complicated very quickly. Usually we ignore all these complications and just assume the speed of earthquake waves only varies with depth within the Earth. This works reasonably well if the wave speeds only change a small amount from place to place, but New Zealand’s location on a tectonic plate boundary means that using the simple approach can introduce large errors. The earthquake location process uses the phase arrival times to calculate the position of the earthquake source in relation to all the stations which recorded the earthquake waves using the travel times and distances involved (the simple mathematics I talked of above, see here for a more detailed description). In general the more stations recording an earthquake the better the estimation of location and depth will be.  But also the most accurate locations are calculated when the recording stations surround the earthquake, and the poorest locations are when the earthquake occurs outside the sensor network (such as offshore). The long thin nature of New Zealand means recording stations often do not surround an earthquake’s location.

In my next blog I will talk about how we identify the P and S waves that are crucial to getting an earthquake location.

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