The Earth is made up of four layers. At the center, deep below the surface, lies the inner core, over it the outer core, followed by the mantle, and the outer most layer is called the crust. The crust and the top of the mantle aren't one big sheet, but are many separate pieces placed together.
These pieces called tectonic plates keep moving around, sliding past one another and sometimes bumping into each another. The edges of the plates (plate boundaries), are rough and get stuck together while the rest of the plate continues to move.
The plate boundaries are made up of many faults, and most of the earthquakes around the world occur on these faults. The energy that makes the plates move gets stored up when the edges stick together.
When the force of the moving blocks finally overcomes the friction of the jagged edges, the plate boundaries separate on one of the faults, and subsequently, all the stored up energy is released.
The energy released moves outward from the fault in all directions in the form of seismic waves, similar to ripples in a pond. The waves shake the Earth as they move through it, and when the waves reach the Earth's surface, they cause the ground to shake.
When two blocks of the Earth suddenly slip past one another, an earthquake takes place. The point below the surface of the Earth where the quake starts is called the hypocenter.
Very often, an earthquake starts with smaller tremors called foreshocks. These are followed by the bigger mainshocks. The foreshock and mainshock is experienced in the same place. The mainshock is always followed by aftershocks, which are smaller, and can occur for weeks, months, or even years after the mainshock!
Though forecasting is a science that is still at a developing stage, scientists have mastered it enough to give official warnings. This will go a long way in enabling people to prepare for a big earthquake and help the government and relevant organizations to conduct rescue and disaster recovery efforts.
Currently, it is similar to weather forecasting. Scientists can predict that an earthquake has a certain probability of occurring within a given time frame, but not one that will definitely take place.
Quakes usually occur in clusters that strike the same area within a specific period of time, and it is this characteristic that enables scientists to predict them. In a cluster, most quakes are aftershocks. There may be one or two powerful aftershocks that follow a mainshock, which can be predicted.
Knowledge of past patterns, magnitude and the seismic history of the fault on which it occurred, are among the important factors that go into forecasting an earthquake. Equipped with this information, some large quakes can be predicted by foreshocks.
As the time period between a foreshock and a mainshock may be short, warnings must be communicated instantly. Sometimes, an earthquake may take place after the predicted time period. However, the warning is still useful, as people and organizations are already made aware
The US Geological Survey (USGS) began forecasting aftershocks after the 1989 - magnitude 7.1 catastrophic Loma Prieta earthquake in California.
From a study of previous earthquakes, scientists were able to detect patterns in the way aftershocks decrease in number and magnitude with time. Using this knowledge, scientists were able to estimate the daily odds of the occurrence of damaging aftershocks following large quakes in California.
In the field of earthquake forecast, new research is constantly being carried out. Ways and means to better present methods are also coming to light. Accurate forecasting may enable us to make the transition from thinking of this as a terrifying natural disaster to a manageable natural disaster.