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Volcano-Tectonic Event
Pressure from a pool of magma has just cracked solid rock, creating a volcano-tectonic (VT) event. This type of quake produces relatively high-frequency shaking, usually between one and five cycles per second.
An increase in VT activity is often an early sign that a volcano is becoming active. This type of restlessness, however, can last anywhere from days to years, so it's not a reliable way to predict when a volcano might erupt.
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A VT event occurs when magma under pressure or cooling rock causes rock to crack or slip. The abrupt motion of the rock causes its seismic signal to appear abruptly on a seismogram. Even though the way they are produced is different, seismograms produced by volcano-tectonic earthquakes look like those produced by typical earthquakes (those caused by the motion of tectonic plates at plate boundaries, such as the San Andreas fault and the Mid-Atlantic Ridge).
VT events cycle as many as five times a second, particularly if the earthquake is two kilometers (1.2 miles) or more below the surface. The frequency of the VT signal shown here is five cycles per second.
Volcano-Tectonic Event |
Long Period Event |
Tremor |
Hybrid
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