SEISMIC ACTIVITY IN THE SNOWY MOUNTAINS REGION AND ITS RELATIONSHIP TO GEOLOGICAL STRUCTURES
The Wambrook station is equipped with three-component variable-reluctance Benioff seismometers, with short-period galvanometers; the other three Snowy Mountains stations have short-period Benioff verticals. All stations are situated in underground vaults, at sites selected for good pier foundation, with a minimum of weathering, along with practical considerations such as access, power supply, distance from traffic, etc. Film recorders are used which, at the single-component stations, will run for three days without requiring record change. Radio time signals are automatically recorded on Elm several times per day.
This paper will be concerned with the analysis of 44 tremors which have been recorded and accurately located in this area over a period of 3 ½ years. The largest of these were of magnitude 5 north of Berridale on 18.5.59, and of magnitude 4 near Rock Flat on 1.9.58.
In Section II, the locations of the tremors will be discussed. In Section III, the isoseismals for the Berridale and Rock Flat earthquakes will be described. In Section IV, fault-plane solutions based on the available first-motion data will be presented. The pattern of strain release will be discussed in Section V and finally, in Section VI, an attempt will be made to interpret the results in terms of the local geology.
II. LOCATIONS
Extensive use has been made of a programme for near earthquake location designed for the IBM650 computer by E. A. Flinn (1960). As this programme operates on a minimum of six readings of arrival times of direct waves, it is necessary with a network of this size to use S1 as well as P1 data. To guard as far as possible against misinterpretation of phases, a technique described by Richter (1958, p. 698) was used, which may be summarized briefly as follows:
The outstanding arrivals on all records are measured. Trial “S-P” time intervals are derived, and using the P1 and S1 velocities of 6⋅03 km./sec. and 3⋅61 km./sec. which were found experimentally from timed quarry explosions in the Snowy Mountains (Doyle, Everingham and Hogan, 1959), “P-O” intervals are calculated from the relationship (P−O) = 1⋅49 (S−P) which holds for these velocities. Here P and S are the arrival times of the P1 and S1 waves and O is the time of origin of the earthquake.
