PERTH'S EARTHQUAKE RISK

ON APRIL 24, 2006, I SEND THE FOLLOWING EMAIL TO TREVOR JONES OF GEOSCIENCE AUSTRALIA ON THE ISSUE OF THE EARTHQUAKE RISK FOR PERTH AND THE SOUTH WEST OF WESTERN AUSTRALIA:

Some weeks ago, I received the December edition of the AUSGEO news which featured an article by yourself and Miriam Middlemann about the Cities Project Perth natural hazard risk. I subsequently went to your report on the Geoscience Australia website to see what comments had been made about earthquake risk as I have some comments to pass on which may be of interest to you about reasonably recent earthquakes in the southern Swan Coastal Plain around Bunbury and Busselton which may affect your assessment of Perth's risk rating. I should point out that I'm a geologist and have worked in the south west of WA for most of my life.

Here's the three comments that I hope will be of interest:

1) Several years ago, I was speaking with Ray Mewett (now deceased) who told me of an earthquake in the Dunsborough in the 1940s. He had lived in the area for all of his life and he advised that, in the 40s, he felt an earthquake one night while he was at home in the Dunsborough area. The next morning, he went out to inspect his farm and found a low (about 30 cm), north-south striking escarpment had been formed through one of his properties that lay close to what I know is the Dunsborough fault. Some time later, I met Ray's son (sorry, his name escapes me) to whom I related this story. His response was that he'd never heard his father talk about the escarpment but he can clearly recall being told that, after the earthquake, a well was found to have gone dry. His father spent some time and money trying to find where the water had gone to but without success, so that well lay abandoned thereafter.

2) Several years ago, I was studying the 1990 Department of Agriculture report written by Tille and Lantzke on the soils of the Busselton/Margaret River region. As shown on the attachment, the soil unit Ad2 is Abba Deep Sandy Dunes, a prime host of heavy mineral deposits such as ilmenite and zircon which have been extensively mined in the past in this region. Tille and Lantzke define Ab2 as "gently sloping low dunes and rises (0-5% gradients) with deep bleached sands". The north-south displacement in the Ad2 soil unit, which corresponds to the geological unit Bassendean sands with an estimated age of less than 100,000 and which I have coloured yellow on the attachment, is striking and strongly suggestive of an earthquake with some 2000 metres of lateral displacement along what I understand has been mapped by Geological Survey of WA geologists as the Busselton fault.

3) In the 1980s, while exploration manager for Westralian Sands Ltd (now Iluka Resources) at Capel, I directed a drilling team to explore the region north-east of Bunbury in what is known as the Kemerton industrial park area. On the western side of this industrial part, a rival company Cable Sands (WA) Pty Ltd - now part of the BeMax group - was known to have found the northern extension of the mineral sand deposits contained within the Bassendean sands. The drill crew drilled along a public road reserve and the geologist in charge of that rig subsequently told me that they had duly found the strand line (our name for a mineral sand deposit) but its base was several metres lower than expected. From memory, its base lay close to 0.0 AHD whereas it should have been somewhere between 4 and 10 m above AHD. We assumed that this was because of some post-depositional movement. It was only some time later that, when looking at a topographic map of the general area, I noticed that there was a unique topographic feature to the east of this lowered mineral sand deposit that was found nowhere else on the southern part of the Swan Coastal Plain. The feature was Benger Swamp, a large (5 km circumference) circular wetland located just to the west of the Darling escarpment which marks the eastern limit of the coastal plain. All other wetlands on the coastal plain are either very small or are elongated to reflect their history of being formed in swales between shoreline-dependent dune systems. My conclusions were that, at some stage since the Kemerton mineral sand deposit was formed, there had been a very significant lowering of a roughly triangular or wedged-shaped section of the coastal plain as shown in the attached drawing (apologies for its poor quality).

Taken together, these three examples suggest to me that there is far more tectonic movement affecting the Swan Coastal Plain on which Perth is largely sited than has previously been considered. I believe that the earthquake risk to Perth is therefore significantly higher than what recent seismic records indicate.

I don't know how you can use this information, regardless of whether you agree with my conclusion or not, but I hope it is of interest nonetheless.