THE KURRAJONG EARTHQUAKE. 103 



the Sydney Observatory was of course, much more unsuit- 

 able for measurement. It was studied by a low power 

 microscope fitted with a micrometer eyepiece. The mean 

 of a number of observations gave a value of 7*1 seconds for 

 the total duration of the first and second preliminary 

 tremors. The variations in measurement, however, indicate 

 that this value may be in error by plus or minus one second. 

 The time estimated would have little value by itself, but 

 is>useful as agreeing with the value obtained at Riverview. 



These values, when used in conjunction with Omori's 

 curves (4) for near earthquakes give the distances of the 

 earthquake centre as 57*5 and 59 miles from the Riverview 

 and Sydney Observatories respectively. Unfortunately 

 the initial impulses were not sufficiently well marked to 

 enable the direction to be ascertained. 



Geological Considerations. 



The Kurrajong fault is about 37 miles distant from the 

 Riverview Observatory. If the shock originated on the 

 Kurrajong Fault at a depth of fifteen miles, the actual 

 distance of the centre of the disturbance would be fifty- 

 two miles from the Riverview Observatory and fifty-five 

 miles from the Sydney Observatory. Hence the instru- 

 mental records confirm the distribution of the isoseismal 

 curves of the eastern limb which indicate the Kurrajong 

 fault as a seismic focus. The form of the isoseismals how- 

 ever, shows that the seismic focus is not a simple one. 

 The obvious interpretation of the isoseismals is that the 

 earthquake was due to block faulting, in which one corner 

 of a crustal block was simultaneously displaced along two 

 intersecting fault planes. 



Although the eastern limb of the isoseismals can be 

 definitely correlated with a known feature — the Kurrajong 

 fault, <2) no such correlation can be made for the western 

 limb of the L-shaped curves. The area over which the 



