FAULTS
Fig.3 and Fig. 4 show how faults are mapped including movement on faults. Fault zones, lengths of faults, fault segments.
How are faults shown on geological maps?
Examine the geological map of Jamaica and comment on the fault pattern.
Fig. 4 shows a case where fault rupture has not propagated to the surface- these are classified as buried faults and are usually associated with folded rock strata.
Active faults and scale of activity along faults.
It is difficult in most cases to prove precise activity of a fault and the rate of slip along it.
Active fault zones are those for which it can be demonstrated that a fault has moved during the past 10 k.y. (Holocene Epoch).
Most of the Caribbean landscape has been produced during the Quaternary Period (the past 1.65 my before present).
A potentially active fault is the one where it can be demonstrated that movements on the fault have occurred during Quaternary.
In order to determine this, trenching has to be done along the fault zones and, where ever possible radiometric dates (C14) are obtained.
Paleoseismology. Faults that have not moved during the past 1.65 m.y. are generally classified as inactive.
A fault is sometimes designated as "capable" if it has moved at least once in the past 50 k.y. or more than once in the past 500 k.y. See Table ?.
NOTE: Seismic zoning often employs active faults in an area.
Slip rate on a fault is defined as the ratio of slip (displacement) to the time interval over which that slip occurred. What will be the slip rate on a fault where 0.5 m movement has been recorded during a time interval of 500 years?
The average recurrence interval on a particular fault is defined as the average time interval between earthquakes on that fault. This may be determined or estimated by:
Paleoseismic data: by averaging the time intervals between earthquakes
recorded in the geologic record.
Slip rate: by assuming a given displacement per event and dividing that number by slip rate.
Historic seismicity: using historical earthquakes and averaging the time intervals between events.
All these approaches must be used with caution as the underlying concepts are far from simple.