Time for some geological news. On Friday morning, there was quite a large earthquake of the western coast of Kamchatka (in the Sea of Okhotsk), with a magnitude of 8.3. Luckily there have been no reports of damage or injuries. Anyway, we are all accustomed to hearing about earthquakes with a magnitude of x, but what does this actually mean? In essence, it can be considered as a measure of the energy released during an earthquake.
Modern earthquakes are not really measured by the Richter scale any more, as there are various issues it has with recording earthquakes with large magnitudes. Instead, a different scale is used, which is called the moment magnitude scale (which does not have these problems).
The magnitude of the seismic moment is what is often reported in the news, but where does this number come from, and what does it mean? Essentially it is defined by the logarithm of the “seismic moment” (what this is will be explained later), with a few constants thrown in (so it is similar to the Richter scale, as that is what people are used to). For those of you who are interested, the magnitude (Mw) is defined as being:
The seismic moment (M0) is defined by the following equation:
“A” represents the area of the fault that slipped in the earthquake, with “d” being the distance it moved. “μ” is the “rigidity modulus”, which, for an earthquake, describes how the rock changes shape when one of its faces is subjected to a shear force, while the other is subjected to frictional resistance. You can see this for yourself. Put your hand on a wooden table, and try to slide it forwards. You’ll be able to feel your skin resist the movement, and see it change shape slightly (look at the tips of your fingers). It is this resistance and shape changing that the rigidity modulus describes.
Plugging in the numbers for this particular event, the seismic moment comes out as being around 3 billion trillion Newton metres. (3×10^21 Nm). Put another way, the amount of energy this represents is enough to provide electricity for the whole of the United Kingdom for the next 250 years (or, enough energy to make about 30,000 trillion cups of tea). For comparison, the largest earthquake ever recorded on Earth had a magnitude of 9.5, which took place in Chile in 1960, and is equivalent to releasing 15 times as much energy as the magnitude 8.3 earthquake yesterday in Kamchatka.