MODULE 2:
TYPES, MAGNITUDES, AND FREQUENCY OF HAZARDOUS GEOLOGICAL PROCESSES

SUB-MODULE 2.1 SEISMIC HAZARD

LECTURES# 5-8
DEFINITIONS:
Tectonics refers to processes and landforms resulting from deformation of Earth's crust.
Active tectonics refers to those processes that produce deformation of Earth's crust on a time scale that is significant to humans. It is most concerned with catastrophic events such as earthquakes that cause severe damage and loss of life.
Forces deep within the Earth drive tectonic processes. Collectively these are known as tectonic cycle. The theory of plate tectonics explains how the Earth works.
EARTHQUAKES are a natural consequence of dynamic processes that have shaped the landforms –continents, mountain ranges, and ocean basins.
Stress is produced and strain builds up in rocks as new lithosphere is produced at divergent plate boundaries, older lithosphere is consumed at subduction zone boundaries, or as plates slide past each other at transform plate boundaries.
WHEN THE STRESS EXCEEDS THE STRENGTH OF THE ROCKS, THE ROCKS FAIL (RUPTURE, FAULT) AND ENERGY IS RELEASED IN THE FORM OF AN EARTHQUAKE.

As a result, faults are considered seismic or earthquake sources.
The very first step, therefore, in evaluating the earthquake risk in an area is the identification of seismic sources, that is faults.
Faults may be seismically active or inactive.

Fig. 1 is a map of global seismicity showing locations of earthquake epicentres, defining major plate boundaries shown on Fig. 2.
Examine these figures and comment on the seismicity of the Caribbean region. Comment on the type of plate of plate boundaries that define the Caribbean plate and the type of earthquakes expected. (Focal plane mechanism).
It must, however, be noted that damaging earthquakes have also occurred within the plates- INTRAPLATE EARTHQUAKES.