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Skip Navigation LinksHome : Earthquakes : Frequently Asked Questions
EARTHQUAKES
Frequently Asked Questions

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.