An earthquake results from a sudden slip on a geological fault caused by release of stress that’s built up as the sides of the fault are pushed together and one side of the fault moves with respect to the other. The slip releases energy as waves that travel through the Earth’s crust. It is these waves that cause the shaking associated with an earthquake. In some parts of the world, the stresses are released regularly as lots of tiny shocks and small earthquakes. Where stresses build up over many or thousands of years, the result of release may be one or more major earthquakes. The focal point of earthquakes can lie anything up to 800 kilometres beneath the Earth’s surface, and not all earthquakes result in a fracture at the Earth’s surface.

An earthquake may be preceded by what are known as foreshocks, which are usually centred in much the same place as the main earthquake. Similarly, aftershocks are smaller earthquakes that take place in the same general area anything up to several years after the main earthquake event.

Earthquake Magnitude and the Richter Scale

The Richter scale measures the magnitude of an earthquake source. The scale is related directly to the logarithm of the maximum amplitude of the seismographic waves recorded during an earthquake.

In practice, the scale goes from 3 to about 9 — the largest recorded earthquake, in Alaska in 1964, peaked at around 9.2. The energy released clearly shows the logarithmic scale. Each successive whole number in the scale indicates a roughly 30-fold increase in energy release.

On average, each year there is one earthquake of magnitude 8 or above, and many thousands of lower magnitude earthquakes, mainly of 3 and below.

Even minor earthquakes of magnitude around 3 cause noticeable shaking of the ground for perhaps only a few seconds. An earthquake of around magnitude 7 may produce shaking for around 10—15 seconds, and a giant magnitude 9 earthquake has caused the ground to shake for several minutes. The shaking can result in the collapse of buildings and other structures, and even fluidisation of the land itself.

A Scientific Approach to Earthquake Prediction

Long-range forecasts of earthquake probabilities are based on scientifically sound measurements of ground movement along faults, and historically verifiable records and evidence of earthquakes.

Short-term earthquake predictions are much harder to make. Scientists need to be able to detect warning signals, known as precursors, which will allow them to determine the likely size of an earthquake and which are reliable enough to act as genuine predictors.

The big problem is identifying the precursors. Over recent years, scientists have investigated geological movements, patterns of foreshocks and small earthquakes, long wave electromagnetic radiation, ground water levels, and levels of the radioactive gas radon in underground water supplies. None of these has yet led to a reliable predictor of earthquake activity.

Another approach is to carry out long-term studies at a single earthquake active site. This is what has been happening in central California, where instruments of many types have been installed in an area that is struck by magnitude 6 earthquakes roughly every 20 years. But so far, the next earthquake is yet to arrive, and there have been no precursors to detect. And it is an expensive project to run — around $1 million per year from the US Geological Survey’s annual budget for earthquake research.

Japan has spent billions of yen on earthquake prediction, with no success at all. The Kobe earthquake in 1995 took everyone by surprise and many lives were lost.

It is becoming ever more apparent that to predict earthquakes, scientists need a much better understanding of the processes involved in the lead-up to the stress release that we know as an earthquake. The big problem for scientists is that actual earthquakes are hard to study, and because they cannot be predicted, scientists may not be prepared for them when they happen. Added to that, the data is indirect and filtered through anything up to 800 kilometres of the Earth’s crust. And, because relatively little hard data is available, they are also hard to model. And even if better data is available, it may take a century or more for enough earthquakes to be monitored to decide which precursors make the best predictors.

But scientists certainly are not giving up. Earthquakes are perhaps the last big natural phenomenon to be adequately explained. And if earthquake prediction does eventually become a reality, many lives in future generations may be saved, even though many lives may be lost in the meantime, as earthquakes continue to happen across the world.