On 22 September at 9:15 am, Central Victoria experienced the largest earthquake that has occurred within the state in recorded history. The magnitude 5.8 earthquake was felt by most people within 200 km of the epicentre and some people in South Australia, Tasmania, and NSW (usually those near the top of tall buildings).
A few examples of significant damage occurred in the capital city of Melbourne, especially in the Chapel St, Prahran area with a notable roof collapse and chimney damage. Fortunately, there were no reports of injuries or fatalities.
What are the closest dams to Woods Point?
The epicentre was about 15 km northeast of Woods Point in the Eastern Highlands, a remote area with a limited population. Minimal damage was reported from this area. The Thomson and Upper Yarra Reservoirs are located within 40 km (direct distance) of the earthquake epicentre with no damage detected (Melbourne Water community update, 23 September 2021).
Upper Yarra Reservoir is formed by an earthfill and rockfill embankment dam. Constructed in 1957, it stores water from the catchment to provide into the drinking water supply network. It is an essential link between Melbourne Water’s supply network and its largest dam, Thomson Reservoir. Upper Yarra Dam recently underwent an upgrade and remediation works to maintain the integrity of the dam wall.
Thomson dam is also a rockfill embankment with a vertical earthfill core built in 1984. As Melbourne’s largest reservoir, it has a capacity of 1,068 billion litres or two times Sydney Harbour. Downstream releases from Thomson Reservoir pass through a small hydropower plant, generating electricity and feeding it into the state power grid.
How did the Woods Point earthquake compare to historical data?
Earthquake locations are determined using the travel time of seismic waves from the earthquake to the seismographs, which depends on the velocities of seismic P (sound) and S (shear) waves through the earth crust, which can vary considerably depending on rock types.
Although this earthquake was the largest within Victoria in 150 years, several earthquakes with magnitudes 6.0 or larger (including two of magnitude 6.8) have occurred in the northeast of Flinders Island between 1884 and 1892.
In addition, Dan Clark from Geoscience Australia has found geological evidence of prehistoric earthquakes in the relatively flat region north of Echuca, with three events having a magnitude of approximately 7.0 during the past 100,000 years. Over the past 6 million years, the geologically recent tectonic motions have produced the Eastern Highlands (where the Woods Point earthquake occurred) and the Strzelecki Ranges of southwest Gippsland.
The importance of designing for earthquake effects
We can observe that past earthquakes have been responsible for behaviours such as sliding and lateral spreading of embankment dams, crest settlements, and in some instances, liquefaction and embankment failure.
Although Australia is generally considered to have relatively low seismicity, the Woods Point earthquake is a reminder that the evaluation of earthquake effects on buildings, dams and infrastructures is of paramount importance when planning the design of these structures.
Authors: Sarah Moein (Associate Engineer Seismologist) and Gary Gibson (Principal Seismologist) from ATC Williams. Gary is also a Principal Research Fellow at the University of Melbourne; read more about the authors here
Source: Melbourne Water