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Volcano's Deadly Warning

Seismic Signals


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Long period event image  

Long Period Event

Sudden changes in pressure within magma-filled cracks and channels cause long-period (LP) events. LP events are volcano-related earthquakes that are lower in frequency than volcano-tectonic (VT) events. The frequency of LP events is one half of a cycle to three cycles per second.

Unlike VT events, LP events can reveal magma flow and the buildup of pressure within a volcano. This knowledge can help seismologists predict eruptions.


Long period event seismograph image

The shaking that causes LP events is similar to the "water hammer" that happens in household water pipes. When water is moving quickly through a pipe and the faucet is turned off, the water is forced to stop. But instead of coming to an abrupt stop, it bounces against the closed valve, creating a wave of pressure that moves back and forth within the pipe. The rate at which the wave bounces is determined by the pipe's resonant frequency, a natural frequency of vibration that is, in turn, determined by several factors, including its length and shape. This bounce causes the pipe to clang loudly.

The same thing happens within a volcano's magma channel, except that the channel's end is already closed, and the abrupt change is caused by variations in the magma's pressure. Also, the frequency of the bounce is much slower within the channel.

Volcano-Tectonic Event | Long Period Event | Tremor | Hybrid


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