What is shown
This page shows live earthquake data, displaying the last 24 hours of seismic data from one of MVO’s stations around the volcano. The image is automatically updated every 5 minutes with the latest data. This type of plot is known as a helicorder – see below (How to read/interpret) for more details.
Also plotted are the MVO classifications for any triggered events or earthquakes that occurred during this period, shown by the different coloured triangle markers. These may include Regional (i.e. in the Caribbean) or Teleseismic (more distant) earthquakes, as well as local volcanic earthquakes near to the volcano. Volcanic earthquakes are classified into different sub-types: Volcano-Tectonic (VT), Rockfalls and Pyroclastic Flows, Low-frequency or Long-Period (LP), and Hybrid.
See this page for more details.
Note that as event classifications are first reviewed by an MVO seismologist or seismic technician, they are not updated in real-time and there may be a delay before they are shown.
How to read
The helicorder image above shows the ground motion recorded by a seismometer at one of MVO’s seismic stations around the volcano (identified by the 4 letter code at the top, e.g. MBWH = Windy Hill). A traditional helicorder records the seismic signal on a piece of paper which is wrapped around a rotating drum, while a pen draws the signal on this paper. Digital or electronic helicorders plots such as the one shown here display seismic data on computer screens rather than on paper.
The plot itself is read like a book, from left to right and from top to bottom (this is the direction that time increases). Just like a line of text in a book, the right end of any horizontal line joins up with the left end of the line below it. In this plot each horizontal line represents 30 minutes of data, with 2 lines per hour giving a total of 48 lines making up the 24 hours. The alternating colours of the horizontal lines have no significance other than to make it easier to distinguish lines from one another, particularly if there is a large signal and lines overlap. The vertical lines are added to indicate equal intervals of time, in this case 1 minute apart. The time or hour is indicated on the left in Local Time (UTC-4), and on the right end in Universal UTC (same as GMT) time.
How to interpret
The MVO online helicorder shows seismic data for the last 24 hour period. Each line of data on the record is a 30 minute recording of the movement of the ground at one particular station. Typically, the traces are fairly flat, with only a small amount of background waviness due to natural seismic noise, mostly from the ocean and microseisms (very tiny earthquakes).
When an earthquake occurs and is recorded by the seismometer, the helicorder plot will show a signal or wiggle. The amplitude (height) of the signal depends on the size or magnitude of the earthquake, its distance from the seismometer and the sensitivity of the instrument.
On this plot you may see a variety of earthquake signals. These may be volcanic earthquakes (see here(link) for examples and more information on the different types), regional earthquakes occurring in the Caribbean or large earthquakes from elsewhere in the world. Almost any earthquake occurring anywhere in the world that has a magnitude greater than around M6.0 will be visible on this plot.
However, not all the signals or wiggles seen on this plot will be due to earthquakes. Anything that shakes or moves the ground close to the seismometer will be recorded. For example noise produced by windy conditions or ocean waves is very common on Montserrat. Storms or heavy rainfall can also produce noise, and on stations close to valleys there may also be signals due to lahars or mudflows during such times.
As the instruments are very sensitive, even animals, people or vehicles passing close by will produce a signal. Other electrical noise may also appear on the records, as well as other isolated spikes or data gaps. These are usually caused by transmission problems or other system problems.
If the amplitude of the signal is larger than a set threshold value, the signal may be clipped and coloured in red because the full amplitude would either cover too much of the plot or amount of the ground motion was too large to be recorded by the seismometer.