Geoscience Reference
In-Depth Information
Figure 4.28.
(a) A fault-plane solution for a predominantly normal-faulting
earthquake that occurred on the East African Rift. (b) A fault-plane solution for a
predominantly thrust-faulting earthquake that occurred on the Macquarie Ridge.
Closed circles, first motion is compressional (positive); open circles, first motion is
dilatational (negative); crosses, no clear first motion. (After Sykes (1967).)
Figure 4.29.
Fault-plane
solutions for earthquakes
(•) occurring along a plate
boundary (solid line).
Arrows indicate the
azimuth of the possible
horizontal components of
the slip vector. The
consistent arrows are the
east-west set. The region
is in extension, and the
area is characterized by a
mixture of strike-slip and
normal faulting.
How to use earthquake first-motion directions to determine a
fault-plane solution
To use earthquake first-motion directions you need to have tracing paper, pencil,
eraser, pin and an equal-area projection net (also known as a Lambert or Schmidt
equal-area projection net) (Figs. 4.28(a) and 4.54(c)). On that projection, great
circles are the north-south lines of longitude; azimuth, or strike, is measured by
counting lines of latitude clockwise around the edge of the projection.
