SEISMIC ACTIVITY IN THE SNOWY MOUNTAINS REGION AND ITS RELATIONSHIP TO GEOLOGICAL STRUCTURES
IV. FAULT PLANE ANALYSIS
Information about the nature and directions of the fault motions was obtained using the technique of fault-plane analysis developed by Byerly (cf. Hodgson and Milne, 1951) and modified for near earthquakes within the crust by Sutton and Berg (1958). The method assumes that movement is produced by a single couple acting in a fixed direction, and depends upon the recognition of surface zones of compression and dilatation as evidenced by the directions of first-motion recorded at seismic stations. A compression is registered as an upward movement, and a dilatation as a downward movement of the earth at the station.
- The “critical circle” is defined as the circle on the earth’s surface, with the epicentre as centre, which separates first arrivals of crustal waves P1 from waves Pn which have been refracted at the Mohorovicic discontinuity. Zones of compression and dilatation are then separated as follows:
- (i) Within the critical circle, by the traces of the fault plane and the auxiliary plane, which is defined as the plane through the focus perpendicular to the direction of movement in the fault plane.
- (ii) Outside the critical circle, along azimuths where the cone of down- going critically refracted Pn rays intersects the fault plane and/or the auxiliary plane. Of course no intersection can occur when the dip angle of the plane is less than the angle to the horizontal at which Pn. rays are critically refracted.
- (iii) By sectors of the critical circle connecting the traces of the planes to the azimuthal lines of (ii) ; or by the sector connecting one end of the trace to the other end, in the case where no azimuthal lines intervene.
Note that the fault and auxiliary planes are orthogonal, and that both pass through the focus. With these limitations upon the mutual positions of the planes, a satisfactory solution can often he obtained even when few data are available.
Within the confines of the method it is impossible to distinguish between the fault plane and the auxiliary plane, so that a variety of possibilities must be explored and correlated with other information–or example, the geology of the area. The situation and method may be seen by reference to Fig. 4 for the Berridale earthquake. Here E is the epicentre with focus at a depth of 17 km., and QTW is the critical circle. Since the fault and auxiliary planes must define the limits of a zone which includes the epicentre, this suggests that a region of compression exists between the dilatational regions of Wambrook-Jindabyne and Canberra-Cabramurra. (Other possibilities exist, but these may be eliminated by trial solutions.)
