Today earthquakes feature significantly in popular culture because of a few spectacular events in the recent past – the Chile event of 1960, the Kobe event of 1995, the Christchurch events of 2010 and 2016 are just a few examples. Even the famous Lisbon quake of 1755 still resonates with many people.
But, of course, this fascination with these events and what causes them has been around for many thousands of years. The ancient Greeks tended to attribute them to air vapours in the cavities of the earth, Norse mythology attributed them to the violent struggles of the god Loki. In Japanese mythology, Namazu is a giant catfish who causes earthquakes. Today we know they are caused anywhere in the earth where there is sufficient stored elastic strain energy to drive fracture propagation along a fault plane. The longer the fault line, the bigger the quake.
You must admit though, some of the old myths make for more interesting stories than the scientific one. The old adage – do you want a good story or the truth?
The Richter Scale
In the age of science we of course need to measure these things. So, in 1935 Charles Richter devised a scale for measuring, on the basis of seismograph oscillations, the magnitude (size) of an earthquake. The scale is logarithmic – so a magnitude 2 quake is 10 times more powerful than a magnitude 1 event and a magnitude 3 event is 100 times more powerful.
Contrary to popular belief, this scale does not go from 1 to 10 or between any limits at all – it is open ended. Magnitude 0 earthquakes happen all the time. Some quakes are so small that they are expressed in negative numbers. It is estimated that about 500,000 earthquakes, detectable with current instrumentation, occur each year. Of these, about 100,000 can be felt. The smallest quake that can be felt is about 1.5 on the Richter Scale while the biggest quake ever recorded was the 1960 event in Chile which registered 9.5.
If a magnitude of 1.5 is the smallest quake that can be felt, at 4.5 the quake causes minor damage and events 8.5 and above are devastating. Remember, the Richter Scale measures magnitude – it is not a measure of damage. The damage caused depends upon many other issues as well as the magnitude.
Since 1900, there have been an average of 18 major earthquakes (magnitude 7.0 to 7.9) and one great earthquake (magnitude 8.0 and above) per year. This average has been relatively stable over this period.
Interestingly, while a magnitude 10 earthquake is theoretically possible, there is no fault line long enough to generate such a quake. It is estimated that a fault line of this size would extend around most of the earth. The Chile fault line was about 1,000 km long.
Over the years we have often been asked about the relationship between the Richter Scale and the vibration numbers generated by environmental vibration monitoring. On the surface, it is a very reasonable question.
After all, your ETM or GTM will monitor an earthquake event and will generate a velocity reading for that event. The various seismic bodies around the world will also monitor the same earthquake event, but they will generate a number on the Richter Scale. So what is the relationship between these two numbers? Can I do a calculation on one of these numbers to generate the other? In other words, what is the relationship between these two technologies?
Let’s start with the very simple answer to both questions:
- There is no direct relationship between a velocity reading and the Richter Scale.
- Consequently, there is no way of calculating one of these values from the other.
Why is this so?
The reason is very basic – these two parameters are measuring different things. The most concise definition for each measurement is:
- The Richter Scale measures the total magnitude of an earthquake, and
- A vibration monitor measures the intensity of ground motion at a given location.
Another way of saying this is that the magnitude of an earthquake (or a blast for that matter) is the same no matter where you choose to monitor it. That is, the energy released by the event is fixed.
If you are using a vibration monitor, you are measuring the intensity of the event at your location. If you move the monitor to another location, this intensity will change – even though the magnitude of the event remains the same.
So there is no relationship between these two measurements – they are measuring different things.
This answer to the original question generates at least one more question – can blasting activities (or other manmade events) create earthquakes?
Unfortunately, the answer is yes. And sometimes, reasonably sized ones too. For example, the largest underground explosions conducted by the US were at the Nevada test site and these generated seismic magnitudes of 5.6 to 5.7. The equivalent tests by the Soviet Union in Kazakhstan in 1976 generated magnitudes of around 6.1.
It has been estimated that for a blasting activity to generate a magnitude 4 quake, it would have to involve the simultaneous detonation of something like 90,000,000 kg of explosives – a very unlikely event, to say the least.
There are other manmade activities that may generate earthquakes. For example, it is likely that the quick filling of the Zipingpu dam in China in 2008 was at least partly responsible for the subsequent quake which had a magnitude of 7.9. In Europe and the US wastewater injection, commonly associated with the geothermal industry, has also been known to generate quakes of magnitude 3 to 3.5. Even we had at least one associated with geothermal energy – a magnitude 3.7 event in the Cooper Basin in 2003.
Another possible generator of quakes in the future may be climate change. As large glaciers and ice-sheets disappear the change in the balance of mass may be enough to generate quakes.
Now to confuse you further. The seismology industry has actually moved away from the Richter Scale as a measure of magnitude. The current measure is the Moment Magnitude Scale (MW). This is another logarithmic scale. The two scales are essentially the same for earthquakes up to about magnitude 5, but above that, the Moment Magnitude is the more accurate.
The on-going challenge for us is acquiring the ability to forecast earthquake events. Despite the work of many people over many years, we still have no way of reliably predicting the timing of earthquake events – much less the magnitude of them. Having said that, with the multiplicity of factors involved in creating an earthquake, we may never become very good at forecasting them. They are almost the ultimate example of a complex system.
