Earthquakes

Definition and Measurement

Earthquake. An earthquake is a sudden motion or trembling of the ground produced by the abrupt displacement of rock masses. Most earthquakes result from the movement of one rock mass past another in response to tectonic forces.

The focus is the point where the earthquake's motion starts, and the epicenter is the point on the earth's surface that is directly above the focus. Figures 2.1 and 2.2 show maps of Eastern Caribbean earthquake epicenters for the periods January to June and July to December 1989 respectively.

Figure 2.1

Eastern Caribbean Earthquake Epicenters, January - June 1989

Figure 2.2

Eastern Caribbean Earthquake Epicenters, July - December 1989

Earthquake Magnitude. Earthquake magnitude is a measure of the strength of an earthquake as calculated from records of the event made on a calibrated seismograph.

In 1935, Charles Richter first defined local magnitude, and the Richter scale is commonly used today to describe an earthquake's magnitude.

Earthquake Intensity. In contrast, earthquake intensity is a measure of the effects of an earthquake at a particular place.

It is determined from observations of the earthquake's effects on people, structures and the earth's surface. Among the many existing scales, the Modified Mercalli Intensity Scale of 12 degrees, symbolized as MM, is frequently used (see Annex 1).

Table 2.1 lists some of the strongest earthquakes that have affected the Larger and Lesser Antilles. The given degree of MM intensity is representative of the most affected inhabited area. The name of the island is stated even if the intensity only occurred in specific sites of that island.

Based on the known effects of past events, mean return periods of the expected MM intensity are given in Table 2.2. The values given must be properly interpreted. When it is said that in Barbados the mean return period of intensity grade VII lies between 50 and 70 years, it means that according to available data, the expected number of years between occurrences of that intensity ranges from 50 to 70 years, although it can be shorter or longer.

Figures 2.3 and 2.4 show maps of two of the reported earthquakes. The impact of these events has shown that the local effects of earthquakes can have an enormous range in both space and time. This was clearly demonstrated by the 1766 earthquake (Figure 2.3), centered in northeast Venezuela, which generated aftershocks for 14 months and caused severe damage in West Trinidad and in areas as far as Encaramada on the Orinoco River. Figure 2.4 shows the wide area affected by the 1843 earthquake in the Caribbean. Centered in the northeastern Caribbean, heavy damage was done in Antigua (40 killed and the sinking of English Harbour), Montserrat (16 killed), Guadeloupe, Dominica, St. Kitts; landslides, liquefaction and permanent settlements occurred.

Table 2.1 List of Destructive Earthquakes in the Larger and Lesser Antilles (Caribbean)

Table 2.2 Mean Return Periods of Modified Mercalli Intensity in Years

* Not observed in historical times

Figure 2.3

Figure 2.4

Earthquake Hazards

Earthquake hazards can be categorized as either direct hazards or indirect hazards.

Direct Hazards

· Ground shaking;
· Differential ground settlement;
· Soil liquefaction;
· Immediate landslides or mud slides, ground lurching and avalanches;
· Permanent ground displacement along faults;
· Floods from tsunamis or seiches.

Indirect Hazards

· Dam failures;
· Pollution from damage to industrial plants;
· Delayed landslides.

Most of the damage due to earthquakes is the result of strong ground shaking. For large magnitude events, trembling has been felt over more than 5 million sq. km (1.93 sq.miles). As a consequence, engineering decisions are normally taken on the basis of ground shaking evaluations, expressed in terms of expected maximum ground accelerations.

Site Risks

Some common site risks are:

(I) Slope Risks - Slope instability, triggered by strong shaking may cause landslides. Rocks or boulders can roll considerable distances.

(ii) Natural Dams - Landslides in irregular topographic areas may create natural dams which may collapse when they are filled. Field inspections are therefore necessary to avoid potentially catastrophic avalanches after strong seismic shaking.

(iii) Volcanic Activity - Earthquakes may be associated with potential volcanic activity and may occasionally be considered as precursory phenomena. Precautions should therefore be taken against explosive eruptions which are normally followed by ash falls and/or pyroclastic flows, volcanic lava or mud flows, and volcanic gases.