Earth tremors, and particularly changes in patterns of seismicity, can be important indicators of volcanic activity. Monitoring involves setting up a network of permanent and portable seismometers around the volcano. Some or all of these may transmit their records by radio to a central recording station. With such a network, the position and depth of each epicentre can be precisely located.
The use of accurate surveying techniques such as levelling can determine very small changes in ground elevation relative to a fixed point (benchmark). Precision may be good enough to detect changes as small as 1 mm over a distance of 1 km. Regular repetition of the survey is necessary to detect changes.
Tiltmeters can detect extremely small changes in slope, down to a few microradians (1 microradian is the angle turned by a 1 km long board if one end is raised or lowered by 1 mm). A network of tiltmeters is needed, arranged both radially and tangentially to the volcano, to determine the overall pattern of slope changes on the volcano.
Line lengths between known points (benchmarks) are most accurately measured using EDM (electronic distance measurement). The instrument is set up at one benchmark, and a laser beam is transmitted to a reflective target at the other benchmark. The beam is reflected back to the EDM, and the phase shift between the outgoing and incoming beams gives an accurate calculation of the distance between the benchmarks. Measurements are repeated regularly to determine any changes in line length. Changes as small as a few mm in 1 km can be detected under normal field conditions, and greater precision is possible. As with other instruments, a network of benchmarks is necessary to make sensible interpretations of overall changes on the volcano.
The best results for prediction of volcanic eruptions will be obtained by using all the available information, not just one of the methods outlined above. For example, tiltmeter, levelling and EDM data all enable the detection of inflation and deflation of the volcano. Volcanoes commonly inflate before an eruption, and deflate after an eruption or an intrusive episode.
is essential to predicting its future behaviour. Techniques include geological mapping of volcanoes and dating of their deposits, as well as examination of historical records. Using all this information, hazards maps may be constructed, showing the likely areas to be affected by the various volcanic hazards. A good hazards map will answer the question where, but not when. Important factors include: