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VOLCANO
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VOLCANO

Our Beautiful World

Volcanic Tremors

(It has taken quite a bit of time to get so far as to understand what the difference and meaning
of all the quakes, which are reported from different places, really mean.
It is hoped this page will help
you to understand a little bit more.)

Seismic monitoring at HVO (Hawaiian Volcano Observatory) has helped to clarify the distinction between the two main classes of earthquakes, volcanic and tectonic. Although all earthquakes associated with active volcanoes are ultimately related to volcanic processes, volcanic earthquakes are directly associated with magma movement, while tectonic earthquakes occur in zones separated from the principal areas of magma movement. Tectonic earthquakes on Hawaii share characteristics with seismic events elsewhere that are not associated with volcanic systems, such as the earthquakes generated by the San Andreas fault zone in California.
During inflation the rocks surrounding the reservoir become stressed, and this stress is partly relieved by increasing numbers of earthquakes, too small to be felt, but easily recorded by seismometers.These earthquakes, called short-period or tectonic, are recorded as high-frequency features on a seismograph.

During deflation the stress is completely relieved. The short-period earthquakes stop, but their place is taken by low-frequency earthquakes, called long-period or volcanic, which reflect adjustments related to the exit of magma from the summit reservoir to feed the eruption.

The long-period earthquakes are related to harmonic tremor, the continuous seismic record of underground magma movement. Normally these kind only are registered by the seismometers, but at special violent eruptions, it may be felt as far as up to 8 kilometers from the center of the eruption.
Volcanic tremor, a type of continuous, rhythmic ground shaking different from the discrete sharp jolts characteristic of earthquakes. Such continuous ground vibrations, commonly associated with eruptions at volcanoes in Hawaii, Iceland, Japan, and elsewhere, are interpreted to reflect subsurface movement of fluids, either gas or magma.

VA = Deep volcanic tremors, VB = Shallow volcanic tremors

 


Researcher at the Geophysical Institute, University of Alaska Fairbanks, Volcano Seismologist
Steve McNutt, and Geophysical Institute graduate student John Benoit recently detailed the habits of
volcanoes that erupted from 1979 to 1989.
They found the sleeping giants often go through the same rituals before waking with a bang to spew ash,
hot gases, and molten rock.

After sifting through a decade of information, McNutt and Benoit noticed a pattern. Volcanoes often go
through the following steps before erupting:

  • Step 1: High-frequency earthquakes (during which the ground shakes very fast) increase within a
    volcano during the earliest stages of activity. This swarm of earthquakes is caused as molten rock,
    also known as magma, forces itself upward.


  • Step 2: As the volcano progresses toward eruption, different kinds of earthquakes shake the volcano. These low-frequency earthquakes, which vibrate slower than the first type, happen when molten rock invades spaces between rocks and form magma-filled cavities. These chambers slowly resonate in response to underground impulses, much like a bell rings when struck by a clapper.
     
  • Step 3: The number of earthquakes often decreases after seismometers scratch out the slew of low-frequency earthquakes, but seismologists don't relax. Volcanoes typically quiet down right before they erupt.

  • Step 4: After a volcano seems to snooze, the buildup of gases and the reaction of hot magma with cold ground water often cause volcanic tremor, slight earthquakes that signal an eruption can happen at any time.
     
  • Step 5: The pattern completes itself when the magma spews forth from a volcano (often quite explosively), and gases and ash are belched from the vent.

 Links:
 Earthquakes and Seismicity, when you wish to understand more. USGS

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