There are two main types of volcanic earthquakes: 1) volcano-tectonic earthquakes, which occur due to stress changes in rock from magma movement and can cause land subsidence and cracks, but do not necessarily indicate an impending eruption. 2) long-period earthquakes, which result from pressure changes due to sustained magma injection and indicate volcanic activity is underway or an eruption may be imminent. Volcanic tremors detected prior to eruptions at Mount St. Helens and Pinatubo have successfully warned of eruptions. Structures should be earthquake-resistant and built away from unstable slopes to prevent damage from volcanic quakes.

1. Introduction:
The lowest part of the earth surface is formed of large pieces, called "tectonic plates". Those are
floating over the boiling center of the earth, which is called "magma".
As magma boils, it tends to move tectonic plates, that collide against each other. Obviously, those
collisions take place at plates' edges. There, they generate "collision waves" that travel in all
directions, but tend to dampen over distance. Thus, eartquakes tend to occur in the neighborhood
of plates edges.
On the other hand, magma tends to escape the center of earth at whatever leak it finds; those leaks
are far more frequent along plates' edges rather than at the center of them. Hence, volcanoes
(magma escape holes) tend to occur also along the tectonic plates' edges.
http://answers.yahoo.com/question/index?qid=20071007090909AAVBfjH
An earthquake can be caused two different ways, there is a volcanic eaerthquake and a tectonic
earthquake. Tectonics earthquakes are much more common than volcanic earthquakes. Volcanic
earthquakes are when the volcano produces acidic lava, which drys quickly, when it drys quickly
it blocks the top of the volcano. So no more magma can escape. Presure starts to build up and
eventually the acidic lava can no longer stand the presure. So the volcano is free to explode, the
pressure is released so fast that an earthquake is caused. A volcanic earthquake is usually kept
within 10-20 miles of the volcano.
http://wiki.answers.com/Q/What_is_volcanic_earthquake
Volcanically triggered earthquakes have the potential to cause cracks, ground deformation, and
damage to manmade structures. They typically are much smaller than earthquakes caused by nonvolcanic sources. The largest felt volcanic earthquake in the Cascades was a magnitude 5.5 in
1981, under Mount St. Helens.
http://www.pnsn.org/outreach/earthquakesources/volcanic
Earthquakes Caused By Volcanoes
Volcanic earthquakes are defined as the earthquakes which occur or are linked up with the
volcanic activity.

2. First, often before an eruption minor seismic activity increases in the vicinity of the volcano.
Some kilometers below the volcanic vent, very hot viscous magma moves sluggishly under high
steam pressure through a network of veins and arteries from one storage chamber to another. Due
to this motion, various parts of the surrounding rock become hotter and more strained as the
magma pushes them. These forces fracture the neighboring rocks, and the strain is relieved by
small or moderate earthquakes.
Second, sometimes fault rupture precedes the motion of magma and eruption of lava. The
earthquake waves from the rupturing fault may shake up the molten material in the storage
reservoir beneath the volcano. In a way, similar to the violent shaking of a bottle of soda pop,
steam and gas that have previously dissolved in the magma begin to boil off and accelerate the
escape of lava and gaseous material. This, in turn, would disturb the unstable equilibrium of the
magma below the vent, and stimulate local volcanic earthquakes.
Volcanic earthquakes are far less common than tectonic plate related ones. They are triggered by
the explosive eruption of a volcano. When a volcano explodes the associated earthquake effects
are usually confined to an area 16 to 32 km around its base.
The volcanoes which are most likely to explode violently are those which produce acidic lava.
Acidic lava cools and sets very quickly when it contacts air. This chokes the volcano’s vent and
blocks the escape of pressure. The only way a blockage can be removed is by the pressure
building up until it literally explodes the blockage outward. The volcano will explode in the
direction of its weakest point, so it is not always upward. Extraordinary levels of pressure can
produce an earthquake of considerable magnitude. The shock waves have been known to produce
a series of tsunami in some instances.
http://www.universetoday.com/82050/how-earthquakes-happen/
General principles of volcano seismology:
 Seismic activity (earthquakes and tremors) always occurs as volcanoes awaken and
prepare to erupt and are a very important link to eruptions. Some volcanoes normally have
continuing low-level seismic activity, but an increase may signal a greater likelihood of an
eruption. The types of earthquakes that occur and where they start and end are also key signs.
Volcanic seismicity has three major forms: short-period earthquake, long-period

3. earthquake, and harmonic tremor.
 Short-period earthquakes are like
normal fault-generated earthquakes.
They are caused by the fracturing of
brittle rock as magma forces its way
upward.
These
short-period
earthquakes signify the growth of a
magma body near the surface and are
known as 'A' waves. These type of
seismic events are often also referred
to as Volcano-Tectonic (or VT) events or earthquakes.
 Long-period earthquakes are believed to indicate increased gas pressure in a volcano's
plumbing system. They are similar to the clanging sometimes heard in a house's plumbing
system, which is known as "water hammer". These oscillations are the equivalent of acoustic
vibrations in a chamber, in the context of magma chambers within the volcanic dome and are
known as 'B' waves. These are also known as resonance waves and long period resonance
events.
http://en.wikipedia.org/wiki/Prediction_of_volcanic_activity
Types of Volcanic Earthquakes
Two types of volcanically generated earthquakes are volcano-tectonic (VTs) earthquakes and long
period earthquakes (LPs).
Earthquakes produced by stress changes in solid rock due to the injection or withdrawal of magma
(molton rock) are called volcano-tectonic earthquakes (Chouet, 1993). These earthquakes can
cause land to subside and can produce large ground cracks. These earthquakes can occur as rock is
moving to fill in spaces where magma is no longer present. Volcano-tectonic earthquakes don't
indicate that the volcano will be erupting but can occur at anytime.
Volcanic-tectonic earthquakes (VTs) are, simply put, caused by slip on a fault near a volcano.
Volcanoes are often found in areas of crustal weakness and the mass of the volcano its self adds to
the regional strain. Most VT earthquakes have nothing to do with the magmatic system of the
volcano but occur in response to regional strain exerted in an area of weak faults. VTs can also be
generated from changes of pressure under the volcano caused by the injection or removal of
magma (molten rock) from the volcanic system. After the withdrawal of magma from a system,
an empty space is left to be filled. The result is a collapse of surrounding rock to fill the void, also

4. creating earthquakes. VT earthquakes can result in land
deformation, collapse and/or ground failure but they are
usually small and leave no trace on the surface. While
these earthquakes may cause damage, they usually don't
due to their small size which produces weak shaking.
Occasional VTs or even in a significant swarm of dozens
of events do not in themselves indicate an impending
eruption. However, if a significant introduction of new
magma from depth occurs in the system, both VTs and long period earthquakes are likely to be
generated.
The second category of volcanic earthquakes are long period earthquakes which are produced by
the injection of magma into surrounding rock. These earthquakes are a result of pressure changes
during the unsteady transport of the magma. When magma injection is sustained a lot of
earthquakes are produced (Chouet, 1993). This type of activity indicates that a volcano is about to
erupt. Scientists use seismographs to record the signal from these earthquakes. This signal is
known as volcanic tremor.
Volcanically-caused long period earthquakes are produced by vibrations generated by the
movement of magma or other fluids within the volcano. Pressure within the system increases and
the surrounding rock fails, creating small earthquakes. In 2004, Mt. St. Helens began dome
building eruptions as magma was thrust upwards accompanied by long period earthquakes. These
earthquakes are an indication of magmatic activity and may be a precursor to an eruption. When
these earthquakes occur continuously the result is volcanic or harmonic tremor. Mount St. Helens
exhibit these long period earthquakes months in advance of 1980's eruption.
The volcanic tremor can be used to warn of an impending eruption allowing people living near the
volcano can to be evacuated to safer areas. This method of prediction has been used successfully

5. to predict the eruptions of Mount St. Helens in the 1980's and the 1991 eruption of Mt. Pinatubo in
the Philippines.
http://www.pnsn.org/outreach/earthquakesources/volcanic
Why does volcanoes and earthquake happen mostly in the same place?
Volcanoes usually form on plate boundaries. They form because the plates push upward against
each other forming a small hole where magma can come out from the mantle. (the small hole is
the where the magma comes out of on top of a volcano). Earthquakes usually are at faults lines
and fault lines are near the plate boundaries. Since volcanoes and earthquakes happen usually at
plate boundaries, they mostly happen in the same area.
Volcanic eruptions and earthquakes take place in the same place as those regions have tectonic
plates that are prone to movements. Sometimes, the plates move in different directions and at
different speeds. They also crash, sideswipe each other or even pull apart. When this happens
volcanic eruptions and earthquakes might occur.
http://wiki.answers.com/Q/Why_does_volcanoes_and_earthquake_happen_mostly_in_the_s
ame_place
What is the differences between volcanic and tectonic earthquake?
Volcanic earthquakes are caused by volcanic activities, or magma's activities to be precise.
Tremors can be felt or recorded even if the volcano has not erupted yet. Increasing intensity and
frequency of the tremors are usually used to predict when the volcano will erupt.
Tectonic earthquake are caused by tectonic plates' movement relative to each other. These plates
move in relation to one another at plate boundaries. Plate boundaries lock as the plates move past
each other, creating frictional stress. When the frictional stress exceeds a critical value, called
local strength, a sudden failure occurs, causing an earthquake.
http://answers.yahoo.com/question/index?qid=20080227031654AAbBDiW
Preventions from Volcanic Earthquakes:
Earthquakes related to volcanic activity may produce hazards which include ground cracks,

6. ground deformation, and damage to manmade structures. Earthquakes exhibiting volcanic tremor
warn of an impending eruption so that people can be evacuated to areas of safety. The volcanic
tremor signal has been used successfully to predict the 1980 eruptions Mount St. Helens and the
1991 eruption of Pinatubo. Volcano-tectonic earthquakes can cause damage to manmade structures
and landsliding. To prevent damage from being done, structures should be built according to
earthquake standards, building foundations should be constructed on firm ground and not
unconsolidated material which may amplify earthquake intensity, and buildings should be
constructed on stable slopes in areas of low hazard potential.
http://www.geo.mtu.edu/volcanoes/hazards/primer/eq.html
Conclusion:
Earthquakes related to volcanic activity may produce hazards which include ground cracks,
ground deformation, and damage to manmade structures. These earthquakes can cause land to
subside and can produce large ground cracks. These earthquakes can occur as rock is moving to
fill in spaces where magma is no longer present. Volcano-tectonic earthquakes don't indicate that
the volcano will be erupting but can occur at anytime.

7. ground deformation, and damage to manmade structures. Earthquakes exhibiting volcanic tremor
warn of an impending eruption so that people can be evacuated to areas of safety. The volcanic
tremor signal has been used successfully to predict the 1980 eruptions Mount St. Helens and the
1991 eruption of Pinatubo. Volcano-tectonic earthquakes can cause damage to manmade structures
and landsliding. To prevent damage from being done, structures should be built according to
earthquake standards, building foundations should be constructed on firm ground and not
unconsolidated material which may amplify earthquake intensity, and buildings should be
constructed on stable slopes in areas of low hazard potential.
http://www.geo.mtu.edu/volcanoes/hazards/primer/eq.html
Conclusion:
Earthquakes related to volcanic activity may produce hazards which include ground cracks,
ground deformation, and damage to manmade structures. These earthquakes can cause land to
subside and can produce large ground cracks. These earthquakes can occur as rock is moving to
fill in spaces where magma is no longer present. Volcano-tectonic earthquakes don't indicate that
the volcano will be erupting but can occur at anytime.