 |  | | Seismic Activity
Before an eruption, magma moves into the area beneath the volcano and collects in a magma chamber, or reservoir. The movement of magma produces small earthquakes and vibrations. This is called seismic activity. |  | | | | | | | | |  | |  | |  | | | | Magma rises into a chamber beneath the volcano | | Rising magma and volcanic gases cause pressure | | High pressure cause rocks to break, triggering earthquakes | | |
| A seismometer is an instrument that measures ground vibrations caused by earthquakes. To keep track of earthquake activity scientists in Montserrat installed 14 seismometers close to and on the volcano. The ground movements are converted to radio signals and then transmitted to the MVO where computers record the earthquakes 24 hours a day | | | The photograph shown below shows the data from a seismometer close to the Soufrière Hills Volcano. The results at 6 am on 25 June 1997 show hybrid earthquakes. Hybrid earthquakes occur at shallow depths when magma is rising. The earthquakes were so frequent that they merged into a continuous tremor. |  |
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