Macroseismic effects, locations and magnitudes of some early Tasmanian earthquakes

The Richter magnitude, ML, for historical earthquakes can be obtained from the Modified Mercalli intensity, I, and hypocentral distance, R (km) using the formula ML = 1.13 In R + 0.6671 - 2.60. A magnitude is calculated from each intensity contour with a mean radius greater than 35 km, and the arithmetic mean of these magnitudes is designated MI. It approximates ML, usually to half a magnitude unit or better. For Tasmanian and Victorian events where the MMIII contour is not included in the magnitude determination, a correction of -0.2 should be applied to MI. For New South Wales earthquakes, the correction is -0.1. Isoseismal maps prepared from felt reports in contemporary newspapers were used to determine the epicentres and magnitudes of what are probably the five largest western Tasmanian earthquakes from 1853 to 1957: Circular Head, 21 November 1859, MI 5.4; South West Tasmania, 3 February 1880, MI 5.5; Queenstown, 4 May 1908, MI 4.8; West Coast, 4 November 1911 , MI 4.8; and West Coast, 1 March 1924, MI 5.2. All were felt with a maximum intensity of at least MMVI. The South West Tasmania event, the largest historic western Tasmanian earthquake, may have occurred on the Lake Edgar fault, but its epicentre is not well-constrained. The Tasmanian earthquake swarm of 1883-1892 consisted of around 2000 events felt in the northeastern Tasmanian region. The activity occurred at the rate of at least one event per month from April 1883 to May 1887, and there were at least two events each year up to and including 1892, when the series ended. Isoseismal maps were drawn from contemporary newspaper reports and publications, and epicentres and magnitudes determined for the three largest events: 26 January 1892, MI 6.9; 12 May 1885, MI 6.8; and 13 July 1884, MI 6.4. All three earthquakes were felt from southeastern New South Wales in the north to Hobart in the south, and each caused minor damage in Launceston. Their epicentres were located in the Tasman Sea off the northeastern tip of Tasmania. The MI 6.9 (±0.4) event of 1892 is the largest historic earthquake recorded in eastern Australia. In 1883, Alfred Barrett Biggs of Launceston, Tasmania, built and operated the first seismoscopes used in Australia. From his measurements and the epicentres determined above,the instrumental Richter magnitudes of the 1884 and 1885 events have been calculated as ML 6.2 and 6.5, respectively, in good agreement with their intensity-deduced magnitudes, MI 6.4 and 6.8.