Seismicity of Western Australia
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INTRODUCTIONEarthquakes are more common in Australia than many people realise. The majority of earthquakes located in Australia over the years would be in Western Australia, though this is due in part to the fact that it is Australia's largest state. Maps of the most recent earthquakes in WA are shown on a separate page. In order to compute earthquake risk in Australia, using a technique known as the Cornell-McGuire method, Brian Gaull and Marion Leiba (1990) divided Australia into a number of earthquake source zones ( see map below). 13 of these source zones are in Western Australia. While ideas on seismicity have evolved since then, this map gives an idea of the major sources of seismicity in Western Australia and Australia in general. Notable is the low seismicity attributed to the Northern Territory. Earthquakes in this region since 1987 have dramatically changed the level of seismic risk attributed to this region of Australia. The map below shows known earthquakes of magnitude 5.5 and above in Western Australia from 1885 to March 2004
The most significant concentration of seismic activity is probably off the northwest coast of the state. The on-shore region of the northwest also has elevated seismicity. There is also an apparent concentration of earthquakes in the south west of the state, which is of particular interest to seismologists. The region is now generally known as the SouthWest Seismic Zone. As yet there is no clear scientific reason why the northwest and southwest regions should possess higher than average levels of seismicity.
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Background to "The South West Seismic Zone" The Australian Federal Government established a geophysical observatory at Mundaring, on the eastern outskirts of Perth, in 1957. In the 1960's, a seismologist at the Observatory, Ian Everingham, identified a region, roughly between Geraldton and Albany, that was apparently more prone to earthquakes than most other regions of WA. He named this zone the "Yandanooka - Cape Riche Lineament". This was later referred to by geologist Ray Gordon as "The South West Seismic Zone", a name which is now established in the literature. Since the 1960's, the zone has become more extensive, and less well defined than originally proposed. However, earthquakes which have occurred since then have confirmed Everingham's observation that this is a region of higher than average seismicity. In particular, two major earthquakes have occurred in the northern part of this zone - at Meckering, in October 1968 (magnitude 6.9), and at Cadoux, in June 1979 (magnitude 6.1). Both of these earthquakes caused major damage, running into millions of dollars. BELOW - the Yandanooka - Cape Riche lineament as originally defined by Ian Everingham.
The exact boundaries of the Southwest seismic zone are still imprecise, and the reasons why this zone should exist at all are also not understood. The zone exists within an Archaean Shield structure called the Yilgarn Block. The identified geological subdivisions within this Precambrian structure do not show an obvious relation to the seismicity.
Fortunately, Perth is outside of the earthquake zone. No earthquake of magnitude greater than 2 is known to have occurred in the Perth metropolitan region. Large blasts associated with engineering or farming activity can sometimes be confused with natural seismic activity. The closest confirmed seismic activity to Perth has been at Wooroloo, approximately 50 km east of Perth, where several earthquakes of magnitude just over 2 have been recorded. Several earthquakes of magnitude approximately 4 have been recorded close to York, approximately 100 km east of Perth. York is thought to be on the western margin of the SWSZ. The Meckering and Cadoux earthquakes both caused ruptures at the Earth's surface - a rather rare occurrence. Only about 12 earthquakes in Australia's modern history have caused surface rupturing. These include a major earthquake near Tennant Creek (NT) in 1988, an earthquake at Marryat Creek in northern SA in 1986 and an earthquake near Calingiri ( in the SWSZ) in 1970. BELOW - map of earthquakes in the SW of WA 1997 - 2000 -Blue ; 2001 - March 2004 -Red An interesting aspect of the seismicity of Western Australia is the lack of topographic relief in the seismogenic areas, as the accompanying plot on a DEM ( courtesy of Geoscience Australia) shows. Note the fact that the Darling Scarp is well defined, but not associated with seismicity. BELOW - Locality map for central Wheat-belt area of WA
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SEISMICITY IN THE EASTERN GOLDFIELDS REGIONIt has been suggested that earthquakes in this region may fall in a poorly defined zone which trends towards the northeast, and may reach the WA - SA border. The most significant earthquake in the region was the magnitude 5.6 event near Norseman in 1985. 10 aftershocks of magnitude > 4, and nearly 60 of magnitude 3 - 3.9 were recorded from this event to 1999, and more may well be recorded. The largest event close to Kalgoorlie was a magnitude 4.5 event in October 1987. It was located at 11 km northeast of Coolgardie, but this location could be +/- 20 km. A magnitude 3.7 event occurred approximately 30 km SE of Kambalda on the morning of 31 May, 2004, and was felt widely in the region. Although it was close to an active mine, it is not believed to be mining induced seismicity. Below - map of earthquakes, Mag 4.5 or greater, Jan 1970 - Sept 2004 (28 - 36 Deg S, 120 - 126 Deg E) Details of the major events
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SEISMICITY OF NORTHERN WESTERN AUSTRALIA Relative to the rest of the Australian continent, northern Western Australia is quite seismically active. Much of this seismicity occurs offshore, but large earthquakes have also been recorded onshore. One of the largest earthquakes known in the Austalian region occurred off the northwest coast on 19 Nov 1906. It was given a magnitude of 7.75 by Gutenburg & Richter (1954) and the location of Stover (1966) puts it at 19.1 south, 111.8 east. It was felt over the entire western half of Western Australia. In recent times, a magnitude 6.3 event occurred at Collier Bay, approx 200 Km NE of Broome in Aug 1997, and a magnitude 5.1 event occurred NW of Exmouth in October 2000. The map below shows all known earthquakes in the region, of magnitude 5.0 or more. Earthquakes of magnitude 6.0 or greater are shown in yellow EARTHQUAKES OF MAGNITUDE 6.0 OR GREATER
Important earthquakes in the north of WA were felt events at Broome in 1929, Derby in 1964, Broome in 1979 and again at Derby in 1988. A number of large events occurred near Lake MacKay in 1970, and aftershocks occurred for many years. The most recent large event was near Cockatoo Island, in Yampi Sound, in 1997 ( magnitude 6.3). The first seismic station in the north of WA was installed at Kununurra in 1965. The accuracy of earthquake locations in the north of WA was improved greatly by the installation of stations at Marble bar in 1976 and at Nanutarra in 1980. SEISMIC STATIONS IN THE NORTHWEST
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