principles and applications of microearthquake networks

146 6. Methods of Dutcr Analysk
important axes: the axes normal to the two nodal planes (A and Ci, the P
axis, and the Taxis. In the general case, the Taxis is 45" from the A and C
axes and lies in the compressional quadrant, and the P axis is 90" from the
T axis. The P axis is commonly assumed to represent the direction of
maximum compressive stress, whereas the T axis is commonly assumed
to represent the direction of maximum tensile stress. If we select the first
nodal plane (arc WBCE) as the fault plane, then point C (which represents
the axis normal to the auxiliary plane) is the slip vector and is commonly
assumed to be parallel to the resolved shearing stress in the fault plane.
This assumption is physically reasonable if earthquakes occur in homogeneous
rocks. Many if not most earthquakes occur on preexisting faults in
heterogeneous rocks, and thus the assumption is invalid on theoretical
grounds. Readers are referred to McKenzie (1969) and Raleigh et al.
( 1972) for discussions of the relations between fault-plane solutions and
directions of principal stresses. It must also be emphasized that we cannot
distinguish from P-wave first motion data alone which nodal plane represents
the fault plane. However, geological information on existing faults in
the region of study or distribution of aftershocks will usually help in
selecting the proper fault plane.
(3d) Let us now summarize our results from analyzing the P-wave first
motion plot using an equal-area net. As shown in Fig. 30 we have
(a) Fault plane (chosen in this case with the aid of local geology): strike
N 90" E, dip 44" N.
(b) Auxiliary plane: strike N 54" W, dip 52" SW.
(c) Slip vector C : strike N 36" E, plunge 38".
(d) P axis: strike N 161" W, plunge 4".
(e) Taxis: strike N 96" E, plunge 70".
(0 The diagram represents reverse faulting with a minor left-lateral
component.
6.2.2.
Pitfalls in Using P-Wave First Motion Data
Unlike arrival times, which have a range of values for their errors, a
P-wave first motion reading is either correct or wrong. One of the most
difficult tasks in operating a microearthquake network is to ensure that the
direction of motion recorded on the seismograms corresponds to the true
direction of ground motion. Care must be taken to check the station
polarities. Large nuclear explosions or large teleseismic events often are
used to ensure accuracy of station polarities and to correct the polarities if
necessary (Houck et al., 1976).
Because the P-wave first motion plot depends on the earthquake location
and the take-off angles of seismic rays, one must have a reasonably