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A few weeks ago a small earthquake shook areas of Sydney around Campbelltown. The tremor prompted an article in The Fifth Estate where experts warned that most people vastly underestimate the risks of an earthquake in Australia. Prior to this I had been researching the risk of an earthquake in Sydney as part of an assignment. I found that whilst the likelihood of a Sydney earthquake are small, the consequences could be great.

Earthquake Sources

Although far from tectonic boundaries Australia is more seismically active than the interiors of other continental plates. Although these intra-plate earthquakes are less common than those that occur along plate boundaries, earthquakes with magnitude 7 or more can happen occasionally. Elsewhere in the world earthquakes of this magnitude, when they strike near major population centres, can cause significant damage and loss of life.

There are numerous faults throughout the urban area of Sydney. Although most of these show little sign of recent activity recurrence intervals on faults in Australia can measure in the order of tens to hundreds of thousands of years. At the foot of the Blue Mountains lies the 28 km long Kurrajong Fault Complex. This has been the subject of quite a bit of research as it could produce large earthquakes of magnitude 7 or greater. The likelihood of a large earthquake on this fault is highly uncertain, though it is probably smaller than 1 in 100,000 per year.

Built Environment

Sydney has a very low density. Of its 1.5 million dwellings 55% are separate houses and only 10% are in apartment buildings of 4 or more storeys. Some construction in the city survives from the early 1800s. Houses built prior to the 1930s are likely to be of poorer unreinforced masonry construction and more prone to structural damage or collapse in the event of an earthquake. Houses built since the 1980s should include modern seismic resistant elements based on the mapped hazard for Sydney at the time. Asbestos was a common construction material from 1945 to 1980 and could be disturbed by even light earthquake damage.

Sydney Airport handles 35.6 million passengers and 470,000 tonnes of cargo per year. In addition to the Airport a large amount of heavy industry is clustered around Botany Bay. This includes Port Botany, the Kurnell Refinery and Desalination Plant. All these assets are on unconsolidated or reclaimed land (notably the runways of Sydney airport) more likely to experience strong earthquake shaking.

The Earthquake Threat to Sydney

Site classes for parts of Sydney (values D, DE and E show weaker soils and thus greater amplification of earthquake shaking)

Sydney is located in a moderate earthquake hazard zone for the Australian continent. A peak ground acceleration of 0.05g has an annual exceedance probability of 1 in 500. The likelihood of of different peak ground accelerations being exceeded in the Sydney area is given in the table below. Earthquake shaking can be much worse in soft, sandy or watery soils – indeed it’s on these soils that damage is often most severe. The amplification of ground acceleration can be up to a facor of three. Although most of Sydney sits on relatively firm ground there are many areas on softer soils. The most important of these is Botany Basin, which extends north from Botany Bay almost as far as the Sydney CBD. The Botany Basin is a low-lying swampy area with a water table close to the surface. There are other smaller areas of unconsolidated sediments and fill, particularly along the foreshore of Sydney Harbour. These areas are likely to experience greater ground shaking in an earthquake than older areas which sit on consolidated rock.

An earthquake on the Kurrajong fault complex could potentially produce even stronger shaking but is less likely.

Although these earthquakes are rare emergency planning should not be discounted. Emergency planning for flood and tsunami in NSW considers events this rare and rarer.

The combination of unconsolidated sediments and a high water table in the Botany Basin means that it may be susceptible to liquefaction. No investigations of this potential have been undertaken.

Impacts of a Sydney Earthquake

The threat of an earthquake in Sydney has received significant attention from the global reinsurance industry. In 2005 Munich Re included a Sydney earthquake as one of its top 10 scenarios for a 1000 year PML.³ Swiss Re considers an earthquake in Sydney to be one of its reference losses; an event with likelihood of 1/1,000 would have an economic loss of $20 billion.(18)

Other insurance risk modellers have prepared scenarios for an earthquake in the Sydney Basin. The one shown, estimates a loss of $11.3 billion for an earthquake of similar size to the Newcastle quake occurring 40 km south of Sydney.

A significant hazard is the collapse of unreinforced masonry awnings in older commercial districts. This was extensive in both the 1989 Newcastle and 2011 Christchurch earthquakes and led to fatalities. These awnings are prevalent in many commercial areas throughout Sydney.

Shaking amplified by local ground conditions and the possibility of liquefaction pose a potential hazard to the large industry around Botany Bay. Possible secondary impacts on travel and freight shipped through Sydney Airport and Port Botany could have flow-on economic effects to otherwise unaffected businesses. Damage to chemical plants or the Kurnell Oil refinery could lead to leaks, posing a risk to the surrounding population and having environmental impacts.

What could be done?

Research and Risk Assessment

Despite ongoing work in the Sydney Basin on earthquake hazards there is still substantial gaps in knowledge of the risk. Further research is required to address these knowledge gaps.

The seismology of possible earthquake producing faults is still not well understood. Further research on these sites would help define the probability of an earthquake occurring.

More crucial is a proper assessment of earthquake vulnerability, particularly in areas known to have a combination of old buildings and unconsolidated soils. The methodology developed by Geoscience Australia for a report on earthquake hazards in Newcastle and Lake Macquarie could be replicated across Sydney, with initial priority given to the following areas:

• The soft soils around Botany Bay
• Areas with high concentrations of old dwellings in the inner city
• Areas along Sydney harbour that have been reclaimed

Assessment of liquefaction potential in the Botany Basin and other similar areas should also be undertaken. Priority should be given to the areas that host key infrastructure, such as Sydney Airport.

A better understanding of the risk could help motivate governments to undertake mitigation and preparedness measures and prioritise areas for assessment.

Mitigation

Retrofitting of individual homes is likely to be expensive and not cost effective, however attention should be given to emergency infrastructure such as hospitals, important public buildings like schools and important heritage assets. City of Sydney Council has conducted seismic retrofitting of some of its town halls.

Special assessment should be made of masonry awnings in older commercial districts, particularly in areas where the ground conditions may lead to strong shaking. These proved to be particularly prone to collapse in the Newcastle earthquake.

Key industrial and aviation assets surrounding Botany Bay should be engaged to undertake detailed seismic risk assessments including addressing liquefaction. Retrofitting should be considered where indicated by these risk assessments. These facilities should also audit their business continuity and emergency plans to ensure minimal disruption to operations in the event of an earthquake.

Assessment of the seismic safety of bridges on Sydney’s extensive road and rail network should also be conducted.

Response Planning

The geology and urban geography of Sydney could create specific challenges for an earthquake response in Sydney. The low density of Sydney, potential for widespread shaking and areas likely to experience higher intensities due to local ground conditions will create widespread low-moderate damage with pockets of heavier damage. The low likelihood of an earthquake and lack of lived experience also pose challenges to community education and engagement. New South Wales may wish to develop a State Earthquake Emergency Sub-Plan similar to that used in Victoria. Alternatively Supporting Plans for key functions that may also be required in other disasters could be developed.

Key functions that will need to be addressed include Damage Assessment, Search and Rescue and Debris Management. Other elements of an earthquake response are already provided for in other emergency plans, however these should be reviewed to ensure that they would function appropriately in an earthquake emergency.

Damage Assessment

Damage assessment is required during the immediate response for activities like search and rescue and for the longer term recovery to determine whether buildings are safe to re-inhabit.

Use of new technologies could be used for rapid damage assessment. Creating social media and other technological solutions solely for earthquake is unlikely to be feasible, but the capacity to cope with reports of earthquake damage could be incorporated into other tools developed for hazards such as hailstorms.

Prioritising areas for longer term assessment could also be challenging, as will sourcing enough qualified engineers to conduct assessments. Rules of thumb for initial checks by unqualified personnel may need to be developed. This could identify buildings that need further inspection.

Search and Rescue

Although a substantial portion of Fire and Rescue NSW and the NSW State Emergency Service have search and rescue capabilities, including the rescue of people from collapsed and damaged structures there exists no plan for a large scale dispersed search and rescue operation.

Coordination and communication between the different rescue services will be critical to ensure that the worst affected areas receive the resources they need, yet are not overwhelmed with emergency services personnel.

Debris Management

Some areas will generate large amounts of debris as a result of destroyed structures and those needing to be demolished. Existing landfill sites are not likely to be adequate for the possibly large debris loads. This debris may also contain hazardous materials including asbestos. Careful management and disposal of this debris will be required to ensure that emergency service and waste management personnel and the broader community are not exposed.

Tags: Australia, Botany Bay, critical infrastructure, earthquakes, NaTech, research, Sydney, Sydney Airport

By casuscalamitas in Disaster Risk Management on 6 November 2013.

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