Abstract During 1976 and 1977, 120 earthquakes occurred in South Australia which were large enough to be recorded by the University of Adelaide seismograph network, and subsequently located. Only four epicentres lie outside the recognised seismic zones of the Adelaide Geosyncline, Eyre Peninsula and the Southeast Zone; only four of the earthquakes had magnitudes (ML) greater than 4. Fault‐plane solutions for three of the events within the Geosyncline suggest a direction of principal stress which is a northeast southwest compression. Notes Present address: The Geophysical Observatory, Box 323, Port Moresby, Papua New Guinea.
Abstract The overall picture of the seismicity of South Australia has been made clearer in recent years by information from a small network of seismograph stations operated by the University of Adelaide. Much of the activity is concentrated along the Flinders Ranges from south of Gladstone to Hawker with further activity extending southwards along the Mt Lofty‐Olary arc to Kangaroo Island, and north to Leigh Creek. Another active region occurs on Eyre Peninsula. All South Australian earthquakes up to the end of May 1967, with instrumentally determined epicentres, are listed and shown on a map, and the area within which an earthquake of magnitude 2–5 could be located using the present network is shown. Magnitudes are given based on a local scale which arose out of the Richter scale for Southern California and which uses maximum amplitudes from Benioff short‐period seismographs. Magnitude frequency relationships are also shown.
Two large (∼ 500 t) chemical explosions, detonated in 1970 and 1971, in northern Australia each produced seismic amplitudes equivalent to about a magnitude 4½ earthquake. Arrivals from these explosions were recorded to a distance of 25° by 36 seismograph stations and provided an opportunity to measure seismic velocities over a large part of the Australian continent. A Pn velocity of 8·27±0·01 km s−1 and a crustal thickness of approxmately 40km were obtained from stations in the central and western regions of the continent. Stations to the east of the shot point give a lower Pn velocity of 8·17±0·02 km s−1 and indicate a significant difference in travel times in this direction. A change in slope of the P wave travel-time curve, to 8·85±0·08 km s−1 at a distance of 1400 km has not been observed before in Australia but corresponds to similar observations made from large explosions in North America. The S wave travel times also show a change in slope, near 1400 km, to 4·84 km s−1, but between 400 and 1400 km they indicate some complexity, not seen on the P wave curves, which may indicate a low velocity S layer in the upper mantle.
abstract Experimental determinations of the velocity of compressional waves in plates of varying thickness are presented. Measurements indicate that if the thickness of the plate is less than about 0.2 of the wavelength, the concept of propagation in a “thin” 2-dimensional plate is valid. For thicknesses greater than this and up to about 0.75 of the wavelength, the velocity steadily increases until the “3-dimensional” propagation velocity is reached.
Abstract The South Australian seismograph network has been extended since 1968 so that most earthquakes of Richter magnitude ML ≥ 1.9 within the state are located accurately. Recurrence relations have been derived which define the seismicity of the known active areas. The seismic energy release has also been used to indicate the spatial variation in earthquake activity. Epicentre trends have been noted, and may be related to hypothesized movements on intracontinental plate boundaries.
Abstract A moderately large earthquake (for South Australia) with magnitude ML = 4.6 occurred in the mid‐north near Spalding on January 7, 1971 (January 6, G.M.T.). It caused minor damage and was felt up to 80 km from the epicentre. Four aftershocks were located and another four events are inferred to have been associated with this sequence.
Abstract Data on Australian earthquakes up to 1966 are presented. The most active area in the continent is the Adelaide seismic zone. The epicentres follow quite closely the trend of the Flinders and Mt Lofty Ranges, and appear to be associated with the South Australian rift zone. Some seismic zones such as that in southwestern Australia may be associated with structural boundaries at depth in the crust and upper mantle. On the other hand there is little seismicity along some marked faults such as the 1000‐km long Darling Fault in Western Australia. All reliable depth determinations for Australian foci so far place them within the crust, mostly the upper crust. Such shallow crustal continental earthquakes should perhaps be regarded as a special class.
The general shape of the phase shift curve for a seismometer-galvanometer combination has been derived from elementary mechanics. Direct experimental determination of phase shift for seismographs using recording oscilloscopes or pen recorders is also discussed.
abstract Experimentally determined Rayleigh-wave dispersion curves of group velocity are given for five paths from NTS to stations in the network operated by the Seismographic Station at U.C. Berkeley. Periods observed range from 4 to 14 seconds. Although, as expected, two different paths from NTS to the western edge of the Sierra Nevada resulted in similar curves, efforts to find empirical curves appropriate to the Great Valley and the Coast Ranges on the assumption of provinces with parallel boundaries were not successful. Estimates of group velocity across the Great Valley along the path NTS to BRK indicate velocities, in the period range 5–9 seconds, considerably lower than would be expected from crustal models so far suggested.
Abstract Although the largest earthquake since European settlement in South Australia occurred near Beachport in the Southeast in 1897, the recorded activity since then, even with the establishment of the University of Adelaide Seismograph Network and the Bureau of Mineral Resources station at Bellfield in the Victorian Grampians, has been small. Most of the known epicentres lie within the boundary of the Lower Cretaceous sediments marking the edge of the Otway Basin and there is some correlation with the belt of recent volcanism in the Mt Gambier‐Mt Burr region.
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