Above is a seismogram which is what is drawn to show the strength of earthquakes
The eruption of Mount St. Helens was expected and prepared for, but in spite of the monitoring that had taken place, the exact date and time of the eruption took people by surprise.
There are some signs to look out for:
Earthquakes are a frequent sign of an impending eruption and their frequency and strength can be recorded
Bulging on one side of the volcano – the swelling is obvious and a clear sign of magma moving
Tiltmeters can identify small, subtle changes in the landscape
Global Positioning Systems (GPS) use satellites to detect movement of as little as 1mm
Satellites can detect changes in surface temperature
Digital cameras can be used to check for changes in volcanic activity
Gases being emitted from the vent change before an eruption
Robots called ‘spiders’ are often deployed to look inside the rim of the volcano
The past frequency of eruption is often investigated: the gap between eruptions and the pattern of lava flows, ash movement and lahars can tell people about how the volcano is likely to behave
This technology allows people to prepare for an eruption by organising the evacuation of people and arranging supplies.
Above is a seismograph which draws the seismogram in the first picture