Seismic monitoring

Recording the number and type of earthquakes being produced underneath the volcano during the eruption is the foundation of the Montserrat Volcano Observatory monitoring operation. During the eruption over 30,000 earthquakes have been recorded, although the majority of these cannot be felt.

Earthquakes are recorded in the field by seismometers (right) sited at over 14 places around the volcano. The stations are powered by batteries augmented by solar energy.

The ground movements are converted to radio signals and then transmitted to the Montserrat Volcano Observatory (right) where computers record the earthquakes 24 hours a day.

The signals from four stations are also recorded on paper (seismographs) on rotating drums (right); this gives the scientists an immediate view of the current seismic activity.

The earthquakes are classified and, if possible, their locations are calculated by skilled seismologists at MVO. These are some of the common types of earthquakes recorded on Montserrat.

Volcano-tectonic

Earthquakes are interpreted as being caused by rock breaking. They were predominant in the phreatic phase of the eruption. Click on the seismograph on the right to see an example of a volcano-tectonic reading.

Long period

These events are probably related to gas movement in and beneath the dome and sometimes precede rockfalls and pyroclastic flows. Click on the seismograph on the right to see an example of a long period reading.

Hybrids

Hybrids commonly occur in swarms at shallow levels under the dome due to pressurisation. Click on the seismograph on the right to see an example of a hybrid reading.

Rockfall signals

These are caused by material collapsing from the dome. Pyroclastic flows have a similar but longer duration signal. Click on the seismograph on the right to see an example of rockfall signals.

Explosions

An explosion signal is made of several distinctparts. The initial part of the seismic signal is caused by the explosion itself, as material is driven vertically upwards from the volcano. This is followed after a few tens of seconds by the pyroclastic flow signal, lasting for a few minutes, as material collapses out of the eruption column. A lower level tremor continues for several tens of minutes after the explosion as the volcano continues to vent ash and gas. The amplitude of this volcanic tremor decays gradually with time as the venting gets weaker.