FAQ on Volcanic & Seismic Hazards in Dominica (PDF updated 30/11/09)
Science
What is an earthquake?
Earthquakes are caused by the movement of plates (huge slabs of rock) making up the surface of the Earth. The region where two or more plates meet is called a plate boundary. The plates are constantly moving but this plate movement is neither smooth nor continuous, rather the plates often lock together at plate boundaries causing a build-up of energy. When the plates eventually move out of this locked position the energy that is released may be felt as an earthquake.
Where do most earthquakes
occur?
Much of the
world’s earthquake and volcanic activity takes place along plate boundaries,
the area where plates meet. Countries located along plate boundaries, such as
the Eastern Caribbean islands, Japan, Chile and the USA (California) are likely
to experience earthquakes. At these
plate boundaries the plates interact with each other in different ways; some of
them slide past each other, others spread apart and others move toward each
other with crumpling or with one dipping beneath the other. This last type of
plate boundary is called a subduction zone which is the main type of plate
interaction occurring in the Eastern Caribbean.
What is a fault?
A fault is a plane along which rocks are
displaced. Faults can vary in size from
the smallest that can be recognized at about 2 cm to large plate boundaries
hundreds of kilometers long.
What is meant by an
‘earthquake sequence’?
A
significant magnitude earthquake with which is associated, in time and space,
smaller magnitude earthquakes (the lower the magnitudes, the higher the numbers
of events), possibly preceding, but certainly following, is described as an
earthquake sequence.
What is the difference
between volcanic earthquakes and tectonic earthquakes?
As magma
makes its way through the crust to the surface of the earth, it breaks apart
surrounding rock thereby generating volcanic earthquakes. Volcanic earthquakes are one of the main
signs that a volcano is restless.
Tectonic
earthquakes are caused by the movement of plates when energy accumulated within
plate boundary zones is released. Tectonic earthquakes are usually larger than
volcanic earthquakes.
What is a foreshock/aftershock?
The terms foreshock and aftershock have no
strict scientific definition. They are
used to describe the events within an earthquake sequence to distinguish those
events that preceded the mainshock from those that followed it.
Can an aftershock have a
larger magnitude than the main earthquake?
The term mainshock refers to the largest event
in a sequence. However, as energy
release within a seismogenic system proceeds, it may be found that an event
that was thought to be the largest was not the mainshock and a larger
event occurs. The category into which
events within a sequence would fall can only be definitely set after the
high level activity of the sequence is complete. Research is ongoing to try to distinguish
between large magnitude precursory events and mainshocks as the events are
occurring (i.e. in real time).
Can climate change or hot
weather cause earthquakes?
No,
climate change or hot weather does not cause earthquakes. Earthquakes are caused by processes deep
within the Earth while hot weather and climate change are related to the
atmosphere.
Can earthquakes be
predicted?
Scientists
are unable to predict the location, time and date of when an earthquake will
occur, however, forecasts can be made based on past patterns of activity in a
region. The Eastern
Caribbean is a seismically active area, which has generated very
large earthquakes in the past. Therefore,
we will continue to have earthquakes of varying magnitudes.
Are there earthquakes on planets other than Earth?
The easy answer is "no". They would not be called "earthquakes" if they don't occur on Earth. The real answer is that we would expect earthquakes to occur on any rocky planet which has Plate Tectonics. In our Solar System, only the Earth has active Plate Tectonics. But other solar systems will have formed in the same way as ours and it would be very surprising if there were no planets with Plate Tectonics.
We detect earthquakes on Earth using sensitive instruments called seismometers. Four of the Apollo missions to the Moon deployed seismometers. You can find a photo and some more information on
NASA's web site. There is no evidence of Plate Tectonics on the Moon and so scientists were a bit surprised at the number of Moonquakes they recorded. Most of these are generated by the strong gravitational force of the Earth. NASA are now planning to return to the moon and might have to plan for
moonquakes.
One of the early unmanned missions to Mars (Viking 2) included a seismometer. In the short time it was operating, it recorded no conclusive evidence of Marsquakes. None of the recent Mars missions included seismometers.
Amazingly, there is some evidence for earthquakes on the Sun, or
Sunquakes. These Sunquakes are very different from Earthquakes and Moonquakes and maybe should be called something else.
Monitoring
How do scientists measure
the size of an earthquake?
The size of the earthquake recording on
seismogrammes is related to the size of the event at its source. Scientists, therefore, use various
characteristics of the signature of the recording in mathematical equations to
calculate the earthquake size or magnitude. As a result, there are different types of magnitude depending on the
characteristics used in its determination e.g. body wave, surface wave and
duration. The values so obtained may
differ, but those using the same type of waves usually agree within a few
decimal units.
What is the difference
between magnitude and intensity?
Magnitude is a value related to
the energy generated by an earthquake. It is a fixed number that does not vary
regardless of which island you are located. For example, the duration magnitude of the Martinique Earthquake (2007/11/29)
which was widely felt throughout the Eastern Caribbean
was 7.3.
Intensity scales categorise the
severity of an earthquake at a given location by describing the effects on
people, structures and geological formations (e.g. whether or not buildings
were damaged, whether items fell from shelves etc.) Each degree of intensity is
described by a Roman numeral, (I, II, III etc.) the largest being XII, and the
effects of the earthquake roughly double in severity for each one-division increase
in intensity. The intensity of an
earthquake varies depending on where you are in relation to the earthquake’s
epicenter. For example, the intensity of
the Martinique Earthquake (2007/11/29) was roughly V in Trinidad and VI and
higher in Martinique.
Where can I find a Richter
Scale?
A Richter Scale is not a physical scale. Charles F. Richter was the first person to
devise a mathematical formula to calculate the sizes of the earthquakes he was
studying. The equation he used may be
found in any seismology text.
Effects
Is it possible for the
earth to open up and “swallow” a person during an earthquake?
Yes. In
general, ground shaking is the more familiar hazard. However, along the fault trace, for very
large earthquakes, rupture can extend to the surface of the Earth and the
ground on either side of the fault can move apart and then close. Anything smaller than the opening in that
immediate vicinity can be shaken into the opening and sealed in when the ground
closes.
Can geothermal exploration cause
earthquakes?
Yes, geothermal exploration does
have the capacity to trigger earthquakes. Deep earthquakes occurring below geothermal
exploration depths are unlikely to have been caused by geothermal exploration.
Can an earthquake trigger a
volcanic eruption?
Yes. Research suggests that large earthquakes can both trigger a volcano to
begin erupting or shut down a volcano that is already erupting.
Does drilling for oil cause
earthquakes?
In some parts of the World, earthquakes have
been found to be associated with oil exploration.
Can buildings collapse
during aftershocks?
Yes, if a building has been sufficiently
weakened during the main earthquake it could collapse during an aftershock.
Vulnerability
Why is it that earthquakes
in some parts of the world like India,
China, or Turkey seem to be more devastating than
earthquakes in the Caribbean?
The rate at which faults accumulate strain
energy and release it in earthquakes is directly proportional to how fast plates
move. The convergence rate of the plates
in the Eastern Caribbean is about 2 cm/year, while the rate in Asia is much higher. As a result, their largest earthquakes occur more often than those in
the Eastern Caribbean. This means that one day an earthquake as
large as those seen in Asia will occur in the Eastern
Caribbean.
Do we get a lot of
earthquakes in the Eastern Caribbean?
Each year, over 1200 earthquakes are recorded
in the Eastern Caribbean. Not all of these
events are felt but they serve as a reminder that the region is seismically
active.
Which islands in the Eastern Caribbean are most susceptible to earthquakes?
The answer to this question depends on what is
meant by “susceptible”. Every island in
the Eastern Caribbean is within 200 km of the
largest events to have occurred in the past and as such experienced damage
during those earthquakes. In this context all of the islands in the Eastern Caribbean are susceptible to earthquakes. If
“susceptible” means feels earthquakes most often, then those areas are near
Trinidad and near Martinique.
What was the largest
earthquake ever recorded in the world?
The largest recorded earthquake
occurred on 22nd May, 1960 off the coast of Chile, South America.
The earthquake was of magnitude 9.5, resulted in over half billion dollars in
damage and generated tsunamis which reached the shores of Hawaii,
Phillipines and Japan.
What was the largest
earthquake in the Caribbean?
The largest recorded
earthquake to have occurred in the Caribbean is believed to have been the El
Cibao earthquake in the Dominican
Republic in 1946 with aftershocks extending
into 1947-48. The earthquake was of
magnitude 8.1 and generated a tsunami which caused 75 deaths and rendered 20,000
homeless.
The largest earthquake to have
occurred in the Eastern Caribbean (St.
Kitts-Nevis to Trinidad & Tobago region) since continuous instrumental
monitoring began in the region was the earthquake near Antigua
on 8th October, 1974. The earthquake was of magnitude 7.5.
On 8th
Feb 1843, the biggest earthquake known to have affected the Eastern Caribbean
occurred. Damaging intensities were experienced from St. Maarten to Dominica. In
Antigua, the English Harbour sank and in Point-a-Pitre, Guadeloupe,
all masonry was destroyed in the earthquake, with an associated fire consuming
wooden structures. One third of the
population, estimated at 4,000-6,000 persons, perished. The event was felt as
far south as Caracas and British Guiana and was
even felt 2,000 km away in Washington, Vermont and Charlestown,
U.S.A. This earthquake was not instrumentally
recorded. The magnitude is estimated to have been in the range 8.0-8.5.
How do scientists know the
size of earthquakes which occurred before recording instruments were used e.g.
1843, 1690 earthquakes?
The level of shaking from an earthquake
diminishes with distance. Mathematical
relationships between the area experiencing a specified level of shaking and events
with instrumental magnitudes have been derived. These mathematical equations can then be used to estimate the magnitude
of pre-instrumental earthquakes, for which the area with the specified level of
shaking is known.
Safety
I read that during an
earthquake it is better to move to the side of the desk not under it. Is that
true?
This recommendation known as the “Triangle of
Life” is a widely circulated suggestion which states that during an earthquake
it is safer to curl up next to a desk or bed rather than to go under it. This practice is not recommended
for the following reasons:
·
The “Triangle of Life” is not scientifically proven
and tested.
·
It is unknown if during the earthquake
these “triangles of life” – triangular spaces next to desks, beds etc.- are
impacted in any way which may make them unsafe areas.
·
In the Eastern
Caribbean, ‘pancaking’ or crumbling of buildings which would crush
occupants as described in the “Triangle of Life”, is not expected.
How can I protect myself
during an earthquake?
The
most important thing to do during an earthquake is to stay calm. If inside,
stay inside away from windows and mirrors. Do not run outside as you may be
injured by falling objects. Protect your
head and face and duck, cover and hold
on to a strong desk or doorway. After the earthquake, check for structural damage or broken gas
lines.
Where can I get more
information on earthquake safety?
Visit the Earthquake Safety pages on this
website or contact your local disaster management office for earthquake safety
tips appropriate to your island.