ONE OF the main goals of measuring earthquakes is to assess seismic hazard. Often presented in the form of maps, seismic hazard indicates the amount of horizontal ground shaking one would expect in a given area over a given period of time.
These maps are used by engineers to guide them in designing anti-seismic buildings and civil structures. Older versions of seismic hazard maps were given in units of peak ground acceleration which is the static measurement of maximum ground amplitude expected in any 50 year period with 90 per cent probability that it will not be exceeded.
Increasingly, it was recognised that the values reflected in these maps were regularly exceeded by even moderate earthquakes. Nowadays the International Building Code recommends that seismic hazard must be expressed in terms of spectral accelerations at periods of 0.2 and 1.0 second, representing the exposure of short and tall buildings with a 98 per cent probability of non-exceedance. These more conservative frequency dependant maps are expected to represent seismic hazard more faithfully and are being developed by many countries, Jamaica being no exception.
