Civil Engineering Reference
In-Depth Information
Fault trace
Fault plane
Dip (d
)
Foot wall
Hanging wall
Fault plane
F
Fault plane
F
F
F
Normal Fault
Reverse Fault
Fault plane
F
F
F
F
Fault plane
Left-Lateral Strike Slip Fault
Right-Lateral Strike Slip Fault
Figure 1.7
Fundamental fault mechanisms
has both vertical and horizontal components; the latter can be of normal or reverse type according to
the algebraic sign of the angle
λ
.
The 'focus' or 'hypocentre' of an earthquake is the point under the surface where the rupture is said
to have originated. The projection of the focus on the surface is termed ' epicentre ' . The reduction of
the focus to a point is the point-source approximation (Mallet, 1862). This approximation is used to
defi ne the hypocentral parameters. However, the parameters that defi ne the focus are similar to those
that describe the fault fracture and motion. Foci are located by geographical coordinates, namely latitude
and longitude, the focal depth and the origin or occurrence time. Figure 1.8 provides a pictorial depic-
tion of the source parameters, namely epicentral distance, hypocentral or focal distance, and focal depth.
Earthquakes are generated by sudden fault slips of brittle rocky blocks, starting at the focus depth and
observed at a site located at the epicentral distance.
Most earthquakes have focal depths in the range of 5-15 km, while intermediate events have foci at
about 20-50 km and deep earthquakes occur at 300-700 km underground. The three types are also
referred to as shallow, intermediate and deep focus, respectively. Crustal earthquakes normally have
depths of about 30 km or less. For example, in Central California the majority of earthquakes have focal
