In this answer I will explain how the occurrence of earthquakes and volcanic eruptions can be monitored and predicted.

At present seismologists can predict where earthquakes are likely to occur; however, it is very difficult to predict when earthquakes might occur.  Seismologists use a variety of measures to monitor and predict the occurrence of earthquakes. Seismologists examine the patterns of earthquake activity. By examining previous earthquakes seismologists can predict the likelihood of another earthquake occurring. The longer the time since the last earthquake, the more imminent the next earthquake will occur in the region. Areas along a faultline that have not experienced seismic activity for a long period are also monitored. These locations are referred to as seismic gaps and are likely locations for earthquakes. Instruments called tiltmeters are also used to monitor and predict the occurrence of earthquakes. Tiltmeters record changes in the level of the land. When pressure is exerted on the crustal plates an increase in the height of the land may occur. Early warning systems are being established using a global network of seismographs. These detect and signal immediate seismic waves. Animal behaviour may also predict earthquake activity. There have been reports of animals refusing to enter buildings prior to an earthquake.

Geologists use a number of methods to monitor and predict the occurrence of volcanoes. By studying the material deposited on the sides of volcanic cones they can establish the nature of previous eruptions. This helps them to identify the frequency of previous volcanic eruptions. A study of this material can also help geologists to predict possible future eruptions. Equipment, such as a tiltmeter, also helps geologists monitor and predict the occurrence of volcanoes. Tiltmeters detect a bulge in the side of a volcano, which indicates that an eruption is likely to occur. Before an eruption, magma builds up in the magma chamber causing the surface to bulge. Geologists use laser beams and satellite images to accurately measure any changes that might occur.  Mount St Helens swelled by 1.5 metres per day in the month prior to its eruption in 1980. An increase in volcanic gases also suggests that an eruption is likely to occur. As magma rises closer to the surface, its pressure decreases and gases begin to escape. An increase in gases means that the magma is getting closer to the surface. The movement of magma produces small earthquakes and vibrations in the earth’s crust. Seismographs are placed around the sides of a volcano to detect any vibrations. These vibrations indicate that a volcanic eruption is imminent.