Abstract
The Lake Edgar Fault in Western Tasmania, Australia is marked by a prominent fault scarp and is a recently reactivated fault initially of Cambrian age. The scarp has a northerly trend and passes through the western abutment of the Edgar Dam, a saddle dam on Lake Pedder. The active fault segment displaces geologically young river and glacial deposits. It is 29 ± 4 km long, and dips to the west. Movement on the fault has ruptured the ground surface at least twice within the Quaternary and possibly the last ca. 25 000 years; the most recent rupture has occurred since the last glaciation (within the last ca. 10000 years). This is the only known case of surface faulting in Australia with evidence for repeated ruptures in the Late Pleistocene. Along its central portion the two most recent surface-faulting earthquakes have resulted in about 2.5 m of vertical displacement each (western side up). The Lake Edgar Fault is considered capable of generating earthquakes in the order of magnitude 61/2-71/4. The Gell River Fault is another fault nearby that was apparently also active in the Late Pleistocene. It has yet to be studied in detail but the scarp appears to be more degraded and therefore older than the most recent movement on the Lake Edgar Fault.
Highlights
Since 1968, five of Australia’s largest earthquakes have ruptured the ground surface (McCue, 1990; Crone et al, 1997)
The purpose of this paper is to present results from our two field studies of the pre-historic Lake Edgar Fault scarps of Southwestern Tasmania
We describe the features of the fault, and present evidence for repeated surface-faulting earthquakes on the Lake Edgar Fault within the Quaternary
Summary
Since 1968, five of Australia’s largest earthquakes have ruptured the ground surface (McCue, 1990; Crone et al, 1997). We conclude with some speculative comments regarding possible spatial and temporal relations between surface-faulting earthquakes on the Lake Edgar Fault and on a second active fault approximately 40 km to the north, the Gell River Fault. This part of Southwestern Tasmania supports temperate rainforest and button grass plains, and experiences ten times the average rainfall of the semi-arid regions of southwest Western Australia and Central Australia where the five other scarp-producing events, including the 1968 Meckering and 1988 Tennant Creek earthquakes, occurred. From the photograph we infer that the lakes were once joined, the larger rem-
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