Do earthquakes occur in Melbourne? If you live in Melbourne, is your building earthquake safe?
Just this morning at around 9:30am in Melbourne Victoria, a 5.8 magnitude earthquake hit 129km east of the Melbourne CBD (22/09/21).
The tremor was felt in large areas of Melbourne and even as far as Sydney. You may have seen you building shake in the tremor and think to yourself is your house, building or structure ok.
This article will provide a beginners guide to buildings responses to seismic events. For a more thorough Structural Engineering perspective, take a look at THIS article which discusses why buildings collapse due to earthquakes.
Has there Been an Earthquake in Melbourne?
If you were awake at 9:30am this morning (22/09/21) you would say a resounding yes!! But how often and how large are earthquakes in the Melbourne region?
Here is as summary of the more substantial tremors we have received around Melbourne over the years.
|How long Ago
|Depth below surface
|Location (in Victoria)
|6 Years Ago
|6 Years Ago
|7 Years Ago
|8 Years Ago
|9 Years Ago
|9 Years Ago
|10 Years Ago
|10 Years Ago
|11 Years Ago
A really great website for tracking earthquakes (and Victoria/Melbourne in particular) can be found at this link here…
Are Melbourne Buildings Earthquake Proof?
I often get asked this question a lot in Victoria, “Are Melbourne Buildings Earthquake proof?”. The truth of the matter is that there are very rare instances of “Earthquake Proof” buildings in Australia. It must be noted that “Earthquake Proof” And “Earthquake Resistant” are terms that are often interchanged when talking about buildings and seismic design, this is technically incorrect.
- Earthquake Proof Building: Is a building that largely remains un-damaged after a moderate seismic event. Significant cost is usually required in order to construct a building “Earthquake Proof”. Even when designing to the strictest requirements to the Australian Seismic Design Code, a building still may require moderate repairs to keep it running operational after an earthquake event. Some earthquake proof buildings are designed to fail in selected locations during an earthquake in order to help dissipate the shaking forces produced during a seismic event. These elements are called “structural fuses” and are designed to be un-bolted and replaced after an earthquake. There are a very small percentage of these types of buildings in Australia. Having said that, Earthquake Proof is often not a term used in Structural Engineering, rather we prefer to talk in teams of the level of a buildings “Earthquake Resistance”.
- Earthquake Resistant Building: Is a building which has been designed with seismic activity in mind and designed to remain standing after an earthquake. This is what the majority of the buildings in Melbourne are if constructed since the 1980’s and more so since the early 2000’s (we will talk more about this later on). The main objective of the Australian Seismic Design Code (and other similar overseas codes) is to ensure that a building remains stable enough after an earthquake for its occupants to evacuate. The main intent is to preserve life safety, not to preserve the building. The building may need to be knocked down and will be irreparable following such an event.
This subtle difference between Earthquake Proof and Earthquake Resistant was brought the the publics attention in Christchurch New Zealand in 2011. As a practicing Structural Engineer, I was present in Christchurch during the post-earthquake clean up. The majority of my day was spent inspecting damaged buildings and either signing their death warrant for demolition or specifying strengthening and repair measures.
The sentiment of the locals was quite evident. There was widespread surprise at the amount of recently constructed buildings which needed to be torn down due to significant damage during their 2011 earthquake. However with the exception of a small percentage of buildings, all of these structures were designed to the required Seismic Code and performed their role. This role being to allow their occupants to evacuate during the earthquake even if the building itself was not serviceable afterwards.
Is my Building ok After an Earthquake in Melbourne?
A few factors come into play to answer this question. If you are concerned about your structure and have experienced significant movement during the earthquake event your building may have experienced damage. If you can see visible cracking in your structure or settlement/movement, you should seek advice from an experienced and qualified Structural Engineer. Here are some elements to consider to determine if your building is at risk from the effects of an earthquake.
What Year your Building was Constructed
Structural Engineers must cater for seismic loading for structures built today, but that wasn’t always the case…
The first revision of the Australian Seismic Design Code was released in 1979. Since 1979, research and understanding of the effects of earthquakes on buildings have improved which has resulted in revisions being made to the original 1979 code.
Generally the design requirements have become more stringent and more refined as time has gone by. The most significant update to the seismic design code occurred in the early 1990’s, after that a further significant update was made in the early 2000’s.
Therefore your buildings vulnerability to earthquake shaking may be dependant on when the building was constructed, here is a table summarising the timeline for buildings constructed in Australia:
|Year Building Was Constructed
|Seismic Design Allowance
|Your building is unlikely to have been specifically designed for seismic loading.
|Between 1979 – 2004
|Your building has been designed for seismic loading with un-refined approaches.
|2004 until Today
|Your building has been designed with the latest seismic code using the latest understanding of seismic effects on structures.
Is your Building Constructed from Earthquake Friendly Materials?
Some building materials perform better than others in an earthquake. The more ductile your construction material the better.
Ductility is the structures ability to dissipate energy from earthquake shaking through yielding and/or deformation. Brittle materials are those that will fracture and break under small deformations while ductile materials bend and remain in-tact at high deformations.
The following table outlines a list of materials ranked in terms of their ductility.
|Timber / Lumber1.
- Timber is inherently a brittle material under torsion/shear/bending, however when detailed adequately, the connections of timber members can be incredibly ductile.
How Heavy your Buidling is
The seismic load applied to a given building is directly influenced by the mass of the building. Lighter buildings perform better in earthquake events compared to heavier buildings.
This means that buildings constructed of timber or structural steel are less prone to damage from an earthquake compared to masonry and concrete (all other things being equal).
Can your Building Sway and Move During and Earthquake?
Flexibility is somewhat related to ductility to a certain extent. It is all about energy dissipation under a given deformation.
How flexible a structure is is determined by how easily the building is set into motion given an applied load. Movement in a structure allows the energy to dissipate through inertia rather than forces within the structural elements.
Is your Soil Prone to Liquefaction Caused by an Earthquake?
A cutting edge seismically designed building is useless if it is founded upon poor ground conditions.
Soil can drastically lose its strength when liquefaction occurs. Liquefaction is when an otherwise solid soil structure behaves temporarily as a liquid (usually produced by ground shaking).
Soils prone to liquefaction are those which are loosely packed (un-consolidated) and contain a high moisture content. When the ground shakes, the loosely packed soil begins to consolidate (the soil particles begin to settle and fill the air voids within the soil mass). As consolidation occurs, any water which is present within the voids deep in the soil becomes pressurised. As the water is pressurised there is no where to go but upwards through the soil mass. The upwards pressure of the water causes the soil structure on the upper layers to become buoyant, resembling the behaviours of a viscus material (liquid).
The effects of liquefaction can be devastating to structures. Structures most effected by liquefaction are those founded on shallow foundations. Liquefaction can ultimately cause excessive differential settlement which may overstress the shallow foundations causing structural damage.
Is my House Earthquake Safe
Conventional housing construction in Melbourne and Australia performs quite well under a seismic event. Conventional housing construction being defined as:
- 1 or 2 levels of Construction
- Primary structural framing comprising timber or cold formed metal studs.
- Founded on a stiffened raft slab in soil conditions not pone to liquefaction.
Similar construction types in Christchurch New Zealand were proven to perform very well during their most recent earthquake events of 2010 and 2011. The design standards agree with this as there is no requirement to specifically design and detail a domestic type structure for earthquake in Australia provided that:
- The building has been designed for lateral wind forces as per the appropriate codes and
- The building height is less than 8.5m.
This is because the effects of wind for this type of construction and height are more significant than those for an earthquake event.
It is important to understand that the intent of the Seismic Design Code is for buildings to remain standing after an earthquake. If your building is designed in accordance with the Seismic Design Code, it is likely to sustain significant damage in a major earthquake, however will be strong enough to remain standing to allow its occupants to evacuate.
How can I Make my House Earthquake Safe
You can make your house earthquake safe by adopting good practice design which has been proven to perform well in seismic conditions:
- Adopt light-weight construction materials such as timber and steel instead of concrete
- Specify your structural framing to be comprised of ductile materials such as structural steel or well detailed timber
- Install appropriate foundations if you building lies on soil which is prone to liquefaction (preferably deep foundations to rock level)