One Hundred Years of Earthquake Recording in Australia

Abstract A comprehensive new catalog of Australian earthquakes has been compiled and used to review the seismicity of Australia. The catalog contains 27,000 events, of which 17,000 are considered to be mainshocks. The catalog is complete for all Australian events with a magnitude greater than M L 5.5 since 1910, M L 5 since 1960, M L 4 since 1970, and M L 3.5 since 1980. It is complete for events in southern Australia above M L 3.5 since 1965 and M L 2 since 1980. Before the development of local magnitude scales for Australia, around 1990, the Richter magnitude scale was generally used. At 600 km (a typical hypocentral distance for much of Australia) the Richter formula overestimates magnitude by around 0.5 units. Thus, the results in the catalogs before and after the early 1990s are potentially discrepant. Most well-located Australian earthquakes are in the southern areas of the continent, where the seismometer density is greatest. In general, the location uncertainty of Australian earthquakes is high. Only 60% of events are located with an uncertainty of 10 km or less. This percentage is smaller for earthquakes before 1980, and before 1960 very few events were located to within 10 km. The hypocentral depths of Australian earthquakes range mostly between 8 and 18 km, except for the southwest corner of the continent where they are typically shallower than 5 km. The seismicity in some areas of Australia has been steady for at least 100 yr (including the southeast corner, the Flinders Ranges, and the northwest corner). In contrast, seismicity in the southwest corner jumped by at least a factor of 6 in the 1940s and has been steady since then. Much of the rest of Australia is characterized by episodic seismicity. These episodes begin with a period of high activity lasting 1–10 yr and they are normally associated with a large ( M >6) earthquake. Following the large earthquake, there is often a period of moderate activity lasting from a few years to several decades. Before and after each episode is a quiescent period of low activity lasting 0.1–10 ka, during which the seismicity is more than an order of magnitude lower than during the period of high activity. Frequency-magnitude relations were calculated using events since 1970 from the new catalog. Gutenberg–Richter a - and b -values were calculated on an 85-km grid, and maps of the probability of an earthquake of M ≥4.9 occurring per year were derived. These results are very similar to the Global Seismic Hazard Assessment Program (GSHAP) map for Australia. The results were used to define four large (>20,000 km 2 ) seismogenic zones (Fig. 1). There are also several other small zones, some of which appear to reflect recent episodes, while others appear to be long-lived. The expected number of earthquakes M ≥5 and M ≥6, strain rate, and deformation rate is given for the four large zones, the remainder of Australia, and the whole Australian continent. The combined estimates of strain using seismic Global Positioning System (GPS) and Satellite Laser Ranging (SLR) data suggest east–west compressive deformation across southern Australia of 0.65±2.0 mm per year, likely to be in the 0.5–1.0-mm-per-year range.

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