earthquakes near and beneath active volcanoes alert scientists that magma and volcanic
gases may be on the move. This animation represents generalized patterns of
seismic and geologic activity at some, but not all long-dormant stratovolcanoes
worldwide during the months and hours leading up to an eruption. We will use an
example from Central America to illustrate how volcanic earthquakes are
interpreted. These types of volcanoes form where mantle rock, located above the
subducting plate, not the subducting plate itself, melts to form magma.
Melting of the mantle is facilitated by water released from the oceanic plate.
Let's zoom in closer to show how moving magma and volcanic gas and fluids trigger
earthquakes as they forced their way up through the crust into shallow
underground fractures causing faults to move and cracks to vibrate. Different
seismic signatures will reflect varying geologic processes. Sample seismograms
include deep low frequency and/or tremor, that occur as magma that formed in the
lower crust rises into conduits. These deep events can only be detected by sensitive
seismometers. An aseismic zone separates the deep and shallow
earthquakes. At shallow levels in the upper crust, common seismic events
include high-frequency volcano tectonic, or VT earthquakes, shallow low-frequency
earthquakes and hybrid earthquakes just below the summit. Before we focus
on the shallower processes, we notice the layers and forces that
affect how and where different earthquakes occur. To reorient you we
again cut away the volcano surface to reveal a more detailed view. The crust is
separated into an upper brittle seismic zone under hydrostatic pressure that has
a complex system of fractures and aquifers and a lower aseismic plastic zone
that is subject to lithostatic pore fluid pressure and which shallows
dramatically under volcanoes.
Along volcanic arcs, magma from previous eruptive episodes cools to 700-1000°C
and slowly crystallizes it roughly 6 to 12 kilometres depth.
Heat from this magma conducts through the surrounding rock
enlarging the plastic Zone. The two zones are separated by a transition of
contrasting permeability that prevents concentrated magmatic fluids from
escaping upward out of the plastic zone. As the magma reservoir is filled with
new magma from below it becomes pressurized, pushing the crust upward and
outward but may not be enough to rupture the brittle rocks above.
This stress change causes distal volcano- tectonic earthquakes to occur in the brittle
crust at the base of faults near the volcano.Magma tends to rise and pulses
As subsequent intrusions occur, the overlying rock is pushed upward.
The increased hydrostatic pressure is transferred long distances and causes
earthquakes on distal tectonic faults. A large enough intrusion will cause the
transition zone to rupture expelling stored magmatic fluids. Because pressure
above the transition is 2 1/2 times times lower than below, the sudden rupture
causes depressurization of deep fluids and resulting boiling and generation of
low density magnetic vapor. This process creates low-frequency earthquakes. The
rising vapor, mostly steam, make cool as it rises and may not be seen at the
surface. After the rupture, mineral precipitation reactions may quickly
reseal the transition zone.
During a pause between intrusion pulses, the vapors zone can migrate higher into
the uppermost part of the volcano. The next intrusion will again be preceded by
VT earthquakes and easily ruptures the newly resealed ductile/brittle transition,
causing the low-frequency earthquakes within the vapor zone beneath the summit.
The additional gas pressure may rupture rocks near the surface causing phreatic explosions
Occasionally we observe artesian springs issuing high on the edifice, which may form volcanic mudflows or lahars.
The phreatic explosions are usually accompanied and followed by low-frequency tremor as gases escape.
Because this relieves pressure in the aquifers, it results in a sudden drop-off of VT
earthquakes on t he adjacent fault scarps. After phreatic explosions have board a pathway
nearly to the surface and magma moves further upward to a depth of 1–2 km,
“Hybrid” earthquakes begin to occur. This type of earthquake starts with
both high and low frequency components, but then quickly become
entirely low-frequency. The earthquakes are initially fairly irregular in size and spacing.
a day or two before the magma reaches the surface and afterward the hybrid
earthquake may become identical in size, shape, and spacing although they still occur at the same
depth of 1–2 kilometres. The events can occur at rates of many per minute and
sometimes many per second (so-called "hybrid tremor") immediately prior to
and during magmatic extrusion and eruption.
Again this animation represents a generalized progression of seismic and geologic activity
at some long-dormant stratovolcanoes.