Visit Your Local PBS Station PBS Home PBS Home Programs A-Z TV Schedules Watch Video Donate Shop PBS Search PBS
NOVA Home Find out what's coming up on air Listing of previous NOVA Web sites NOVA's history Subscribe to the NOVA bulletin Lesson plans and more for teachers NOVA RSS feeds Tell us what you think Program transcripts Buy NOVA videos or DVDs Watch NOVA programs online Answers to frequently asked questions
Volcano's Deadly Warning

Seismic Signals


Volcano's Deadly Warning homepage

Volcano-tectonic image  

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.


Volcano-tectonic seismograph image

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


Send feedback Image credits
   
NOVA Home Find out what's coming up on air Listing of previous NOVA Web sites NOVA's history Subscribe to the NOVA bulletin Lesson plans and more for teachers NOVA RSS feeds Tell us what you think Program transcripts Buy NOVA videos or DVDs Watch NOVA programs online Answers to frequently asked questions

Support provided by

For new content
visit the redesigned
NOVA site