0
120 240
(s)
COL
HON
GDH
GAC
ANMO
Strike-slip faulting, west of Oregon, March 13, 1985
Location: 43.5°N, 127.6°W. Depth: 10 km
Strike: 302°, Dip: 90°, Slip: 186°
ANTO
SLR
CHTO
GUMO
CTAO
Normal faulting, mid-Indian rise, May 16, 1985
Location: 29.1°S, 77.7°E. Depth: 10 km
Strike: 8°, Dip: 70°, Slip: 270°
BCAO
COL
NWAO
CHTO
HON
SNZO
Thrust faulting, Vanuatu Islands, July 3, 1985
Location: 17.2°S, 167.8°E. Depth: 30 km
Strike: 352°, Dip: 26°, Slip: 97°
CTAO
4.2 Focal mechanisms 227
Fig. 4.2-17 Focal mechanisms and some seismograms for three different
earthquakes. Compressional quadrants are shown shaded.
of the stress axes, as noted in Fig. 2.3-9. If the P axis is vertical,
the fault plane dips at 45°, and normal faulting occurs. If,
instead, the T axis is vertical, the fault geometry is the same, but
reverse faulting occurs. When the null axis is vertical, strikeslip motion occurs on a fault plane 45° from the maximum
principal stresses, which are in the plane of the surface.
Figure 4.2-17 shows the focal mechanisms and a few of the
seismograms for three earthquakes. Note that in some cases the
first arrival is small and difficult to identify. This is especially
likely when the station is near a nodal plane, where the amplitude is small. It is also worth noting that often many stations
plot near the center of the focal sphere, because they are at large
distances from the source, so rays to them have small angles of
incidence. As a result, it is sometimes hard to constrain nodal
P
P
T
T
Compressional
quadrant
Dilatational
quadrant
Faults
45° Dipping thrust
P
P
P
P
T
T
T
Side
view
45° Dipping normal
T
T
T
T
P
P
Side
view
P
To obtain P and T axes:
P
T
On the meridian connecting
the poles, the points
half-way between the
nodal planes are the
P and T axes
Fig. 4.2-16 Cartoon illustrating the relation between fault planes and the
maximum compressive principal stress (P) and the minimum compressive
stress (T) axes. The P and T axes can be found by bisecting the dilatational
and compressional quadrants, respectively. On a stereonet, this is done by
using the great circle (meridian) connecting the poles for the two nodal
planes and finding the point halfway between them.
and the normal vectors, and is the intersection of the two nodal
planes.
To bisect the angle between the two nodal planes on the
stereonet, we find the poles for the two planes (each of which is
in the other plane), draw the great circle (meridian) connecting
them, and mark the point on it halfway between the poles (Fig.
4.2-16). We can thus infer stress directions from a focal mechanism. Different fault types correspond to different orientations
120 240
(s)
COL
HON
GDH
GAC
ANMO
Strike-slip faulting, west of Oregon, March 13, 1985
Location: 43.5°N, 127.6°W. Depth: 10 km
Strike: 302°, Dip: 90°, Slip: 186°
ANTO
SLR
CHTO
GUMO
CTAO
Normal faulting, mid-Indian rise, May 16, 1985
Location: 29.1°S, 77.7°E. Depth: 10 km
Strike: 8°, Dip: 70°, Slip: 270°
BCAO
COL
NWAO
CHTO
HON
SNZO
Thrust faulting, Vanuatu Islands, July 3, 1985
Location: 17.2°S, 167.8°E. Depth: 30 km
Strike: 352°, Dip: 26°, Slip: 97°
CTAO
4.2 Focal mechanisms 227
Fig. 4.2-17 Focal mechanisms and some seismograms for three different
earthquakes. Compressional quadrants are shown shaded.
of the stress axes, as noted in Fig. 2.3-9. If the P axis is vertical,
the fault plane dips at 45°, and normal faulting occurs. If,
instead, the T axis is vertical, the fault geometry is the same, but
reverse faulting occurs. When the null axis is vertical, strikeslip motion occurs on a fault plane 45° from the maximum
principal stresses, which are in the plane of the surface.
Figure 4.2-17 shows the focal mechanisms and a few of the
seismograms for three earthquakes. Note that in some cases the
first arrival is small and difficult to identify. This is especially
likely when the station is near a nodal plane, where the amplitude is small. It is also worth noting that often many stations
plot near the center of the focal sphere, because they are at large
distances from the source, so rays to them have small angles of
incidence. As a result, it is sometimes hard to constrain nodal
P
P
T
T
Compressional
quadrant
Dilatational
quadrant
Faults
45° Dipping thrust
P
P
P
P
T
T
T
Side
view
45° Dipping normal
T
T
T
T
P
P
Side
view
P
To obtain P and T axes:
P
T
On the meridian connecting
the poles, the points
half-way between the
nodal planes are the
P and T axes
Fig. 4.2-16 Cartoon illustrating the relation between fault planes and the
maximum compressive principal stress (P) and the minimum compressive
stress (T) axes. The P and T axes can be found by bisecting the dilatational
and compressional quadrants, respectively. On a stereonet, this is done by
using the great circle (meridian) connecting the poles for the two nodal
planes and finding the point halfway between them.
and the normal vectors, and is the intersection of the two nodal
planes.
To bisect the angle between the two nodal planes on the
stereonet, we find the poles for the two planes (each of which is
in the other plane), draw the great circle (meridian) connecting
them, and mark the point on it halfway between the poles (Fig.
4.2-16). We can thus infer stress directions from a focal mechanism. Different fault types correspond to different orientations