articolo Mc Cue.pdf Key words recent faults – Australia – intraplate seismicity – multiple ruptures 1. Introduction Since 1968, five of Australia’s largest earth- quakes have ruptured the ground surface (McCue, 1990; Crone et al., 1997). These five surface ruptures comprise nearly half of all known historical surface ruptures during earth- quakes in stable continental regions throughout The Lake Edgar Fault: an active fault in Southwestern Tasmania, Australia, with repeated displacement in the Quaternary Kevin McCue (1), Russ Van Dissen (2), Gary Gibson (3), Vagn Jensen (4) and Bruce Boreham (5) (1) Australian Seismological Centre, Canberra, Australia (2) Institute of Geological and Nuclear Sciences, Lower Hutt, New Zealand (3) Seismology Research Centre, Richmond, Victoria, Australia (4) University of Tasmania, Hobart Tasmania, Australia (5) Department of Industry, Tourism and Resources, Canberra, Australia Abstract The Lake Edgar Fault in Western Tasmania, Australia is marked by a prominent fault scarp and is a recently reac- tivated 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. 10 000 years). This is the only known case of surface faul- ting 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 move- ment on the Lake Edgar Fault. the globe (Johnston et al., 1994). The fault scarps produced by these earthquakes are quite impressive features in the landscape – extend- ing for some tens of kilometres in length and up to several metres in height. These scarps pre- serve information related to the earthquake’s location, size and the timing of the past event or sequence of events. There were eight large shallow earthquakes with a magnitude of 6.0 or more in Australia between 1901 and 1967 and another eleven between 1968 and 2000. The largest, in 1906, had a magnitude of MS 7.2. None of the pre- 1968 20th century events are known to have caused surface ruptures. More than a dozen fault scarps of Late Quaternary age have been discovered to date in 1107 ANNALS OF GEOPHYSICS, VOL. 46, N. 5, October 2003 Mailing address: Mr. Kevin McCue, Australian Seis- mological Centre, PO Box 324, Jamison Centre ACT 2614, Australia; e-mail: asc@netspeed.com.au 1108 Fig. 1. Simplified geological map of Tasmania showing the location of the Lake Edgar and Gell River faults (after Burrett and Martin, 1989). Kevin McCue, Russ Van Dissen, Gary Gibson, Vagn Jensen and Bruce Boreham Australia (McCue, 2001) so there is little doubt that the past 35 years are not so unusual in rela- tion to the frequency of large earthquakes. The study of these fault scarps and related geology will provide critical information for defining Australian earthquake source zones, determin- ing the long-term average recurrence rate of large earthquakes, and assessing earthquake hazard in Australia. Such study should also contribute to the debate on whether the source regions of the five fault scarps produced since 1968 are still capable of producing further large earthquakes. The purpose of this paper is to present results from our two field studies of the pre-his- toric Lake Edgar Fault scarps of Southwestern Tasmania (fig. 1). We describe the features of the fault, and then present evidence for repeat- ed surface-faulting earthquakes on the Lake Edgar Fault within the Quaternary (the last ca. 2 Ma). We conclude with some speculative comments regarding possible spatial and tem- poral relations between surface-faulting earth- quakes 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 sup- ports 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. 2. Lake Edgar Fault 2.1. Fault geometry The Lake Edgar Fault is located about 70 km inland from Tasmania’s western and southern coastlines in the Tasmanian Wilderness World Heritage Area and a similar distance west of Hobart the capital city (fig. 1). Being a World Heritage listed area, no trenching is allowed, access is difficult and sampling restricted. The fault was first noted and partially map- ped by Carey and Newstead (1960) who identi- fied it as a recently reactivated fault, initially of Cambrian age (ca. 540 Ma). It is within a Pre- cambrian inlier or region called the Jubilee re- gion within which the subsurface continuity of Precambrian rocks is thought to be likely (Turner, 1989). Precambrian rocks underlie about 20% of Tasmania. According to Turner the inliers are separated by either folded strata of latest Precambrian to Devonian age or relatively flat lying Carboniferous to Cainozoic strata. The fault has a northerly trend and is up- thrown to the west. Carey and Newstead also reproduced a copy of an aerial photograph show- ing two small shallow lakes formed by ponding of streams against the upthrown block (fig. 2). Note the fans emanating westward from the ranges to the east. From the photograph we infer that the lakes were once joined, the larger rem- The Lake Edgar Fault: an active fault in Southwestern Tasmania, Australia, with repeated displacement in the Quaternary Fig. 2. Airphoto of the Lake Edgar Fault prior to the filling of Lake Pedder, north up the page. Creeks draining from mountains to the east have ponded against the 2 m high scarp, forming a lake which has since partially dried leaving two small lakes, the southern one is Lake Edgar. 1109 nant was called Lake Edgar, hence the name of the fault. Carey and Newstead commented on the youthfulness of the fault scarp based on the observation that the ponds had not yet silted up despite the high average annual rainfall of more than 2500 mm/yr. These ponds were drowned when the Gordon, Scott’s Peak and Edgar Dams were built in the early 1970s creating Lake Pedder. In the region of the Scott’s Peak and Edgar Dams, the Edgar Fault separates an Older Pre- cambrian graphitic phyllite to the east from a Precambrian metasiltstone, argillite, quartz sand- stone conglomerate to the west. Wedges of dolomite and limestone on its western side are truncated against the fault. During investigation of the Edgar Dam site about 1964 and prior to dam construction, geologists with the then Hy- dro-Electric Commission dug an 80 m long trench west from the scarp and excavated a num- ber of co-linear pits. Their scope was to investi- gate possible water leakage along the scarp that cuts through the right abutment of Edgar Dam (Roberts et al., 1975). According to the authors «the Edgar Fault is a sinistral wrench fault trend- ing approximately north-south with an original movement of approximately 12 km. It passes within 76 m of the western end of Edgar Dam». They note that the «original fault movement drag- ged a narrow, possibly discontinuous, sliver of dolomite north from the main body a maximum distance of 1200 m.». Roberts et al. (1975) also drilled an oblique hole to intersect the fault at about 100 m depth that showed it has a steeply dipping reverse com- ponent. Starting in 1994 we made two field surveys of the scarp with assistance from Hydro Tas- mania and Tasmanian Parks and Wildlife Service (McCue et al., 1996; Van Dissen et al., 1997). A post-reservoir filling aerial photograph looking north towards the small pond at the southern base of Edgar Dam is shown in fig. 3. An active fan flowing out of the ranges to the east is clear- ly dissected by the fault scarp. Carey had traced the scarp for nearly 12 km (7 miles) in the 1960s. On our second visit in 1997 we used a helicop- ter to track the surface rupture and found that it curved around to the southeast from its pre- viously mapped southern end for another 13 km (fig. 4). We subsequently confirmed this using 1960s-vintage black and white aerial photographs. The water-filled trench was in good shape in 1994, 30 years after its completion, the walls were still vertical with only small collapse zones. With continuous water pumping we were able to reoccupy the trench near the centre of the surface fault and on the second field trip in 1997 briefly expose the overthrust glacial gravel layer at the former land surface. The inferred fault plane has a dip of about 60°-70° to the west at this near- surface location. McKavanagh (in Van Dissen et al., 1997) made three levelling traverses across the scarp as shown in fig. 5, north up the page. The central 1110 Fig. 3. Aerial photo of the Lake Edgar Fault, north up the page. A post-last-glaciation fan from the mountains to the east has planed and then been dis- sected by the last movement on the fault. Water ponded against the downstream face of Edgar Dam (off the top of the page) and the fault can be seen at the top of the photo. The prominent white east-west scar is a former airstrip, the trench and exploration pits are a few hundred metres parallel to and south of the airstrip (see fig. 5). Kevin McCue, Russ Van Dissen, Gary Gibson, Vagn Jensen and Bruce Boreham 1111 The Lake Edgar Fault: an active fault in Southwestern Tasmania, Australia, with repeated displacement in the Quaternary Fig. 4. Mapped surface expression of the Lake Edgar Fault, Tasmania, west side up. The three arrows show our levelling traverses. The relationship of the fault to two of the dams impounding Lake Pedder, Edgar Dam and Scotts Peak Dam is shown. traverse was alongside and parallel to the trench where the scarp is 2.5 m high. The trench and series of exploratory pits can just be seen in this figure on the western side of the scarp as a thin white line and line of white dots. Seventy metres north of the trench at the second traverse, the scarp is 4 m high. Our last traverse was 500 m south of the trench where the scarp approached its maximum height of 6 m. The unusual variability in the scarp height was a puzzle at first. But air photos such as that in fig. 3 show alluvial fans flowing west out of the ranges to the east and cutting across the cen- tral part of the scarp at its lowest point. Fan surges have planed off the scarp at least once and the fan has been cut in turn by subsequent fault ruptures. We interpret the 6 m high section to be a measure of the unaltered cumulative fault dis- placement. Our interpretation is shown in car- toon form in fig. 7 and discussed in more detail below. Hale and Roberts (personal communication) identified another co-linear fault scarp, the Gell River Fault to the north of the Lake Edgar Fault scarp and about 30 km or a fault dimen- sion away. Maximum crustal thickness in Tas- mania is nearly 28 km with about 5 km of relief on the Moho (Richardson, 1989). Our aerial examination showed that the Gell River Fault scarp is more dissected and eroded than the Lake Edgar scarp and apparently older, but clear evidence that there was at least a third large Quaternary earthquake in Central West- ern Tasmania. 2.2. Evidence for repeated Quaternary displacement Geomorphology – Deposits and landforms from four distinct glaciations have been identi- fied in the Lake Edgar region (Colhoun and Fitz- 1112 Kevin McCue, Russ Van Dissen, Gary Gibson, Vagn Jensen and Bruce Boreham Fig. 5. Aerial photograph (north up the page) showing Edgar Dam and the location of surveyed cross sections (three arrows). The trench is the fine white line of exposed glacial gravels opposite the central arrow. Cross-sec- tion profiles are on the right. simmons, 1990). The glaciations are thought to date from the Latest Pleistocene (youngest gla- cial maximum, 14.000 to 25.000 years BP, the glaciation finished by 10.000 years BP), Middle Pleistocene (middle two glaciations), and Early Pleistocene or older (oldest glaciation, ca. 2 Ma). The Lake Edgar Fault displaces till of the oldest glaciation, the scarp height across these highly weathered deposits is about 6 m. The fault also cuts younger alluvial fans that cor- relate to the younger glaciations. Along the central portion of the fault and across the youngest fan the scarp is about 2.5 m high. Across the next oldest fan the scarp is about 5 m high. Older fans have higher scarps which indicates that the fault has generated at least two surface-faulting earthquakes within the Quaternary, possibly within the last 25.000 years; the youngest being within the last ca. 10.000 years. Trench exposure – The 80 m long trench was excavated west from the fault scarp and near its centre where the youngest extensive fan that we consider Latest Pleistocene in age ( ca. 10.000 years) is draped over the scarp. Nearly three decades after the trench was dug, we pumped it dry and cleaned-down its walls and were rewarded with clear stratigraphic evidence of at least two surface-faulting earthquakes. A simplified trench log of the sunlit southern face of the trench is shown in fig. 8. White fan gravels and underlying basement of sands and peats/lignites comprise the main units exposed in the trench (fig. 6). The old- est unit is a dark brown peat/lignite that con- tains diagenetic quartz laminae. These lami- nae, which probably formed parallel to the The Lake Edgar Fault: an active fault in Southwestern Tasmania, Australia, with repeated displacement in the Quaternary Fig. 6. The trench showing the white glacial fan grav- els overlying basement. One of the authors (RvanD) is pointing out a gravel filled tension crack Fig. 7. Cartoon showing the sequence of scarp de- velopment; scarp formation, scarp planation by gla- cial fans and subsequent re-faulting. Time 4 is current time. 1113 once horizontal bedding in the peat, are now deformed and locally attain dips of 45°-80° (fig. 8). Near the fault, fine white sand appears to have intruded into the peat/lignite. We inter- pret this sand unit as a sand-blow (Amick et al., 1990; Obermeier, 1994), a liquefaction feature resulting from strong earthquake ground shaking. The fan gravels comprise, in part, the youngest extensive fan, and are warped over the scarp that, at this locality, is about 2.5 m high (we take this to represent the amount of vertical displacement associated with the most recent surface-faulting earthquake). Two lines of evidence indicate older faulting: i) The very fine, white, silty sand interpret- ed as a sand-blow is truncated by, and thus old- er than, the overlying gravel. The shaking that resulted in the emplacement of the sand-blow must be older than both the gravel and the sur- face-faulting that later deformed the gravel. ii) Quartz laminae in the peat/lignite are more deformed than the overlying gravel, suggesting that the laminae were already deformed by at least one earthquake prior to the deposition, and subsequent deformation, of the younger gravel. Several gravel filled tension cracks can be seen in the over-thrust block (fig. 8). 2.3. Dating the last event Several charcoal samples were collected from pre and post-earthquake soil horizons to bracket the age of faulting (fig. 8), but the C14 dating results were disappointing. The fibrous dark brown peat on the ground surface of the footwall block at the fault was too modern and the samples from the body of the hanging wall block, a fine dark brown silty fine sand were too old (> 39.600 years old). As Carey said, the scarp must be recent for the ponds to have survived silting up in this high rainfall environment. The scarp itself is still very clear (figs. 4 and 5), in fact it is more prominent than the 1968 Meckering earthquake scarp in Western Australia which has been severely degraded, thanks in large part to farm manage- ment practices. The 1988 Tennant Creek earth- quake scarp in the Northern Territory has also weathered extensively due to the sandy soil and infrequent but heavy downpours of rain. 1114 Kevin McCue, Russ Van Dissen, Gary Gibson, Vagn Jensen and Bruce Boreham Fig. 8. Simplified log of the trench excavated through the Lake Edgar Fault. The fault trace is at the eastern end of the trench. The disturbed quartz laminae and truncated liquefaction sand lens are evidence of a previous fault- ing episode while the gravel filled tension cracks in the overthrust block are typical features of reverse faulting. The last large earthquake on the lake Ed- gar Fault would have been felt throughout Tasmania and in Southern Victoria. The his- toric earthquake record of the last nearly 200 years contains no such reports so the earth- quake predates the arrival and spread of Eu- ropean immigrants in Tasmania nearly 200 years ago. The event then was sometime be- tween 200 years ago and the end of the last ice age ca. 10.000 years ago. We consider that based on the subjective information of C14 dat- ing of sediment deposition on the footwall block, on the non-silting up of Lake Edgar and on the sharp scarp morphology, the causative earthquake probably occurred nearer 200 than 10.000 years ago. 2.4. Earthquake magnitude estimate An approximate magnitude for causative earthquakes can be estimated, based on empiri- cal relations between surface rupture parame- ters and earthquake magnitude. The mapped length of the Lake Edgar scarp is 29 ± 4 km, at least 25 km but no more than 33 km depending on its continuation into heavily wooded hilly country at its northern end. Vertical offset across the fault ranges from 2.5 m for the last faulting event to an average of 2 m (three events) or 3 m (two events). McCue (1990) plotted magnitude M against rupture length L (km) and maximum vertical displacement u (m) of the five Australian earth- quakes ranging from M 5.1 to 6.8 that are known to have ruptured the surface. He obtained by least squares fit M = 4.11 (± 0.18) + 1.65 (± 0.15) log.L M = 5.04 (± 0.17) + 0.75 (± 0.10) u. The first equation yields magnitudes of 6.4 to 6.6 for the range of the observed Lake Edgar Fault scarp length while the second yields magnitudes of 6.5 to 7.3 for the imputed uplift. Wells and Coppersmith (1994) computed the regression of subsurface rupture length L (km) on magnitude M for 167 earthquakes for all slip types. Magnitudes ranged from 4.7 to 8.2. M = 4.38 + 1.49 log.L. This equation yields magnitudes of 6.5 to 6.6 for the range of the observed scarp length. We consider that the Lake Edgar Fault is capable of generating earthquakes in the mag- nitude range 61/2 -71/4. Calculations based on seismic moment con- siderations also yield magnitude estimates in the order of magnitude 7. 3. The Gell River Fault and possible relations with the Lake Edgar Fault About 30 km north of the Lake Edgar Fault lies another fault with apparent Late Pleistoce- ne movement, the Gell River Fault which has a north-northeast trend and is ca. 10 km long (figs. 1 and 9). The active trace of the Gell River Fault appears to be more subdued than that of the Lake Edgar Fault; this could suggest that the most recent displacement of the Gell River Fault is older than that on the Lake Edgar Fault. Separating the two faults are two large bodies of ultramafic rock, including serpenti- nite and talc. The ultramafic rocks appear to be bounded by the northern extension of the Lake Edgar Fault (Burrett and Martin, 1989). The Gell River Fault is parallel to local geological structure, though based on existing mapping, it is not clear whether it is part of the structure that con- trols the location of the active trace of the Lake Edgar Fault. We speculate that the recent activity on the Lake Edgar and Gell River Faults, and their geographic position relative to each other, is influenced by the occurrence of relatively weak ultramafic rocks along a major pre-existing fault. The more easily deformed talc and serpen- tinite between the two faults may, in essence, act to concentrate strain in the Lake Edgar and Gell River regions (Malcolm Somerville, personal com- munication). This strain may be released by creep in the weak ultramafic rocks. To the north and south, where the rocks are presumably 1115 The Lake Edgar Fault: an active fault in Southwestern Tasmania, Australia, with repeated displacement in the Quaternary stronger, strain is released episodically as earth- quake rupture along the Gell River and Lake Edgar Faults respectively. 4. Conclusions The evidence of glacial fans offset by fault- ing and disproportionate displacement along the fault scarp led us to develop a model of fault growth, planation and subsequent faulting. This model demonstrates that the Lake Edgar Fault has generated at least two reverse surface-fault- ing earthquakes within the Quaternary, the most recent of these being younger than ca. 10.000 years by which time the glaciers had melted. These earthquakes were probably in the order of M 61/2 -71/4 in size, and produced, along the central portion of the fault, at least 2.5 m of ver- tical displacement. There was at least one other Late Pleistoce- ne earthquake in Central Western Tasmania on the Gell River Fault. The spatial relationship between the Lake Edgar and Gell River faults, and temporal rela- tionships between surface-faulting earthquakes on these two faults, are probably influenced by one, or both, of the following: i) the existence of a pre-existing fault which is oriented favourably, both with respect to strike and dip, for movement in the contem- porary stress field, and ii) the existence of large bodies of ultramafic rocks, including easily deformed talc and serpenti- nite, along the pre-existing fault which may con- centrate strain in the Lake Edgar and Gell River areas. The Lake Edgar Fault is the only identified Australian Fault with unequivocal evidence for repeated movement within the Late Pleistocene. Acknowledgements We are indebted to Dr. Malcolm Somerville (deceased), and thank Byron McKavanagh, and Dr. Albert Goede who participated in one of the field studies. This study benefited from discus- sions with numerous people; we would speci- fically like to acknowledge those with Glyn 1116 Fig. 9. Gell River Fault between arrows, oblique aerial photo looking west. Kevin McCue, Russ Van Dissen, Gary Gibson, Vagn Jensen and Bruce Boreham Roberts (formerly of the Hydro-Electric Corpo- ration), Tom Laudon (University of Wisconsin), and Ruth and David Wilson (Hydro-Electric Corporation). We also wish to thank Hydro Tasmania, particularly Andrew Pattle, the dam safety officer, and the Tasmanian Parks and Wildlife Service. This study was sponsored by Hydro Tasmania as part of an ongoing program of assessing the seismic risk for their dams. Some financial support was also provided by the then Department of Industry, Technology and Commerce. We thank Kelvin Berryman as well as Des Darby, Kathleen Hodgkinson, and Mark Rattenbury for their suggestions to improve an earlier draft of this manuscript. REFERENCES AMICK, D., G. MAURATH and R. GELINAS (1990): Char- acteristics of seismically induced liquefaction sites and features located in the vicinity of the 1886 Charleston, South Carolina Earthquake, Seismol. Res. Lett., 61, 117-130. BURRETT, C.F. and E.L. MARTIN (1989): Geology and min- eral resources of Tasmania, Geol. Soc. Australia, Spec. Publ., 15, pp. 574. CAREY, S.W. and G. NEWSTEAD (1960): Tasmania Univ- ersity Seismic Net, Publication 84 (Geology Depart- ment, University of Tasmania). COLHOUN, E.A. and S.J. FITZSIMMONS (1990): Late Cainozoic glaciation in Western Tasmania, Australia, Quat. Sci. Rev., 9, 199-216. CRONE, A.J., M.N. MACHETTE and J.R. BOWMAN (1997): Episodic nature of earthquake activity in stable conti- nental regions revealed by paleoseismicity studies of Australian and North American Quaternary faults, Aust. J. Earth Sci., 44, 203-214. JOHNSTON, A.C., K.J. COPPERSMITH, L.R. KANTER and C.A. CORNELL (1994): The earthquakes of Stable Continental Interiors, EPRI Report TR-102261-V1 US. MCCUE, K.F. (1990): Australia’s large earthquakes and recent fault scarps, J. Struct. Geol., 12, 761-766. MCCUE, K.F. (2001) Earthquake Epicentres in Australia 1841-2000 and Recent Fault Scarps, 1:10M map pub- lished by Geoscience Australia. MCCUE, K.F., B. BOREHAM, R. VAN DISSEN, G. GIBSON, V. JENSEN and B. MCKAVANAGH (1996): A paleoseismology case study: the Lake Edgar Fault scarp in Tasmania, in Proceedings of the 13th AGC, Canberra, 19-23 February 1996, GSA Abstracts 41, Geoscience for the Community. OBERMEIER, S.F. (1994) Using liquefaction-induced fea- tures for paleoseismic analysis, U.S. Geol. Surv. Open- File Rep. 94-663, chapter A, pp. 58. RICHARDSON, R.G. (1989) Crustal thickness, in Geology and Mineral Resources of Tasmania, edited by C.F. BURRETT and E.L. MARTIN, Geol. Soc. Australia, Spec. Publ., 15, 465-467. ROBERTS, G.T., B.A. COLEB and R.H.W. BARNETT (1975): Engineering geology of Scotts Peak Dam and adja- cent reservoir watertightness, Aust. Geomech. J., 39-45. TURNER, N.J. (1989): Precambrian, in Geology and Mineral Resources of Tasmania, edited by C.F. BURRETT and E.L. MARTIN, Geol. Soc. Australia, Spec. Publ., 15, 5-46. VAN DISSEN, R., K.F. MCCUE, G. GIBSON, V. JENSEN, M. SOMERVILLE, B. BOREHAM, B. MCKAVANAGH and A. GOEDE (1997): The Lake Edgar Fault: evidence for repeated Quaternary displacement on an active fault in Southwest Tasmania, in Proceedings of the Seminar «Earthquakes in Australian Cities - Can we Ignore the Risks?», Australian Earthquake Engineering Society, Brisbane 2-3 October 1997. WELLS, D.L. and K.J. COPPERSMITH (1994): New empirical relationships among magnitude, rupture length, rupture width, rupture area, and surface displacement, Bull. Seismol. Soc. Am., 84, 974-1002. 1117 The Lake Edgar Fault: an active fault in Southwestern Tasmania, Australia, with repeated displacement in the Quaternary k k k k k k