Earthquake risk in Australia 30 years after the 1989 Newcastle Earthquake

Paul Somerville, Risk Frontiers

Newcastle Workers Club. Source: livinghistories.newcastle.edu.au

The 1989 Newcastle Earthquake and its Impact

The Newcastle earthquake occurred at 10:27am local time on December 28, 1989. It had a magnitude Mw of 5.42 (Allen et al., 2018), the epicentre was approximately 15 km SW of the Newcastle CBD (near Boolaroo) and it occurred at a depth of about 11 km.

The earthquake claimed 13 lives: nine people died at the Newcastle Workers Club (pictured above), three people were killed along Beaumont Street in Hamilton and one person died of shock in Broadmeadow. Melchers (2012) showed that collapse of the Newcastle Workers Club would have been unlikely if there had not been significant deficiencies in the structure as built. The number of people in the city on the day of the earthquake was lower than usual, due to a strike by local bus drivers. It is estimated that about 500 people may have died on a normal day.

The earthquake caused damage to over 35,000 homes, 147 schools and 3,000 commercial and other buildings, with significant damage (over $1,000) to 10,000 homes and structural damage to 42 schools within the immediate Newcastle area. About 300 buildings were demolished. Approximately 300,000 people were affected by the earthquake and 1,000 made homeless. 160 people required hospitalisation but the Royal Newcastle Hospital was rendered inoperable by the earthquake. Insured losses are estimated to be $4.25 billion normalised to 2017 values (McAneney et al., 2019).

The effects of the earthquake were felt over an area of about 200,000 sq. km, with isolated reports of shaking felt up to 800 km from Newcastle. Damage to buildings and facilities occurred over a 9000 km2 region. The damage was most severe on soft sediments from the Hunter River, with shaking intensity of MMI VIII observed at many locations.

Lessons learned

As pointed out by Woodside and McCue (2017), the Newcastle earthquake demonstrated that all the basic principles of earthquake engineering design that have been learned abroad also apply to Australia. Specifically, the damage was due to:

  • Failure of unreinforced masonry, especially the failure of galvanised brick ties due to corrosion from the lime mortar
  • The failure of non-structural elements such as ceilings and chimneys
  • The effects of eccentricity and soft stories on the performance of buildings
  • Inadequate seismic design including tying together of the structure.

As described by Brunsdon and Bull (2019), the involvement by New Zealand engineers in the Newcastle earthquake response and recovery prompted a closer look at New Zealand’s earthquake preparedness, particularly through the professional engineering lens. In conjunction with the preceding Loma Prieta earthquake and subsequent Northridge and Kobe earthquakes, the Newcastle earthquake strongly influenced subsequent work in New Zealand, notably the development of capabilities in post-earthquake assessment and placarding and urban search and rescue. As a result, New Zealand was much better prepared to deal with the many challenges presented by the Canterbury Earthquake Sequence of 2010/11, and significant post-earthquake support of urban search and rescue in Christchurch was provided by Australian engineers who had been trained by their New Zealand counterparts.

Contribution to the development of seismic provisions in the Australian Building Code

Prompted initially by the Mw 6.68 Meckering earthquake of 1968 and further by the three Mw 6.3 to 6.6 Tennant Creek earthquakes of 1988, Standards Australia in 1988 decided to revise the Australian Building Code standard AS 2121. The appointed subcommittee first met on 12 December 1989 in Adelaide, about two weeks before the Newcastle earthquake on 28 December 1989 (Woodside and McCue, 2011). The Newcastle earthquake provided impetus to this task, and the revised code was introduced as AS1170.4 in draft form in 1991 and published in 1993. The performance objective was and still is for life safety or better in a rare event, currently defined as one whose ground motion has an annual probability of exceedance (AEP) of 1:500 (return period of 500 years). The code peak accelerations, up to about 0.1g in some cities, are exceeded close to earthquakes having magnitudes above about Mw 4.5.

In many locations in Australia, wind forces, rather than earthquake forces, govern code-based structural design, and so many practicing engineers here do not develop a full understanding of the nature of the forces presented by earthquakes. It is one thing to design a structure to resist the steady force of the wind on the side of a building, and quite another to design it to resist the forces that result from an earthquake, which are equivalent to having the rug you are standing on pulled sideways from under you. Unless the building is strong enough that its roof can follow the abrupt horizontal movement of its foundation within a separation of a few percent of its height as the ground moves back and forth, it will collapse. This requires the careful detailing of connections between columns, beams, floors and walls so that even if the building is damaged in a strong earthquake it does not collapse. In contrast, buildings can easily be designed to withstand the strongest winds even without structural damage let alone collapse.

Motivated by the relatively small (Mw 6.2) Christchurch earthquake of February 22, 2011, which caused major damage and rendered the CBD unusable for a long period of time because it occurred directly underneath the city, Goldsworthy and Somerville (2012) argued for the adoption of a lower probability event (1:2,500 AEP or 2,500 year return period instead of 1:500 AEP or 500 years return period) in Australia in conformance with developments in building codes in Canada, New Zealand and the United States. Unlike the mainly empirical approach to code development based primarily on the past performance of structures in earthquakes, this new generation of codes uses the framework of performance-based design to quantitatively estimate the capacity of buildings to withstand strong ground motion.

Recent developments in seismicprovisions in the Australian Building Code

Major improvements were made in the national seismic hazard map of Australia by Geoscience Australia (NSHA18; Allen et al., 2019). Revision of the magnitudes of historical Australian earthquakes led to the conclusion that for a given magnitude, earthquakes are about half as frequent in Australia as had been previously thought. However, the NSHA18 hazard map was not adopted in the most recent revision of AS1170.4 on August 15, 2019 (Standards Australia, 2019), which contains a minimum peak ground motion level of 0.08g for design. The large reductions in probabilistic seismic hazard estimates in NSHA18 mean that the ground motion levels embodied in AS1170.4 – 2019 are roughly equivalent to an AEP of 1:2,500 (return period of 2,500 years) in most of the capital cities, as shown by Allen et al. (2019), thus largely fulfilling the objective proposed by Goldsworthy and Somerville (2012).

Development of catastrophe loss modeling for the insurance industry

Catastrophe loss modeling for the insurance industry was in its infancy when the Newcastle earthquake occurred. Through the founding of Risk Frontiers in 1994, enabled by the sponsorship of the insurance industry in Australia, the Newcastle earthquake spurred the development in Australia of quantitative methods of estimating catastrophic losses from natural disasters based on validation against comprehensive catalogues of historical losses. Risk Frontiers now has a complete set of catastrophe loss models for all perils in Australia as well as several others in the Asia Pacific region.

Cautionary notes

The beneficial outcome of NSHA18 described above is offset by the fact that in Australia, due to the lack of attention given to seismic design, the performance of some buildings is likely to be poor even in a small event. In Australia, material codes such as the Steel Structures code (Standards Australia, 1998) and the Concrete Structures code (Standards Australia, 2009) do not require designers to use capacity design principles in their design. The implementation of these design principles in New Zealand since the 1980s, in line with the performance requirement for “near collapse” or better under a 2,500 year return period event, is what probably saved many lives in the Christchurch earthquake. Australian building codes do not address single story dwellings.

To further deter complacency, note that there have been 30 known earthquakes with magnitudes larger than the 1989 Newcastle earthquake since 1840, nine of which had magnitudes of Mw 6.2 (the size of the 2011 Christchurch earthquake) or larger. Several Australian capital cities, including Adelaide, Canberra and Melbourne, have known faults in their vicinity that are capable of generating damaging earthquakes. Australian earthquakes have sometimes occurred in clusters; the three Mw 6.3 to 6.6 earthquakes occurred in one day in the 1988 Tennant Creek sequence. Australian earthquakes have also been followed by long aftershock sequences like that of the Canterbury sequence; one occurred off the east coast of Tasmania near Flinders Island from 1884 to 1886 with magnitudes as large as Mw 6.4.

The 1989 Newcastle earthquake, with a revised Mw of 5.42, caused a loss equivalent to $4.25 billion if it were to recur today (McAneney et al., 2019). This is the largest earthquake loss among all of the Australian natural disaster losses spanning 1967 to the present listed by these authors. Although weather related disasters have historically caused larger losses than the 1989 Newcastle earthquake, larger earthquakes could cause larger losses than those of any weather-related disaster.

Challenges for the way forward

The 1989 Newcastle earthquake and the 2011 Christchurch earthquake present challenges for improving the outcomes of future earthquakes in Australia. We need ongoing training of emergency responders in search and rescue, and of engineers in assessing the safety and placarding of buildings in the immediate aftermath of the earthquake. Extending beyond prescriptive code formulas, we need to foster among practicing structural engineers a better understanding of the principles that underly earthquake resistant design. Given the high level of vulnerability of Australian cities to earthquakes, building design and construction need to consider not only the integrity of individual buildings and infrastructure and the life safety of their occupants, but also the role that they play in providing the functionality and viability of whole communities, with advanced focus on recovery. It took several years for Newcastle to recover from its relatively small magnitude earthquake. Almost ten years on, Christchurch is still struggling to regain the functionality that its residents took for granted before the 2011 earthquake. We must do what we can to avoid that fate.

A good way to advance preparedness and mitigation activities is to develop plans for response to and recovery from significant scenario earthquakes in major cities. These plans need to involve emergency responders, structural engineers, architects, city planners, community organisations, and the members of relevant government departments (such as building officials) and elected representatives of the affected cities, states and nation. Members of the public at large also need to be aware of what to do if they experience an earthquake. The message to “drop, cover and hold on” is promoted and practiced in annual “ShakeOut” exercises around the globe.

References

Allen, T. I., Leonard, M., Ghasemi, H, Gibson, G. (2018). The 2018 National Seismic Hazard Assessment for Australia – earthquake epicentre catalogue. Record 2018/30. Geoscience Australia, Canberra. http://dx.doi.org/10.11636/Record.2018.030.

Allen, T., J. Griffin, M. Leonard, D. Clark and H. Ghasemi (2019). The 2018 National Seismic Hazard Assessment: Model overview. Record 2018/27. Geoscience Australia, Canberra. http://dx.doi.org/10.11636/Record.2018.027

Brunsdon, David and Des Bull (2019). Reflections on Thirty Years of Significant Earthquakes in Australasia and Beyond: Earthquake Engineering into the Future. Proceedings of the 2019 Annual Conference of the Aistralian Earthquake Engineering Society, Newcastle, November 30 – December 2, 2019

Goldsworthy, Helen and Paul Somerville (2012). Reassessment of Earthquake Design Philosophy in Australia after the Christchurch Earthquake, Risk Frontiers Briefing Note 232, February 2012.

McAneney, John, Benjamin Sandercock, Ryan Crompton, Thomas Mortlock, Rade Musulin, Roger Pielke Jr & Andrew Gissing (2019). Normalised insurance losses from Australian natural disasters: 1966–2017, Environmental Hazards, 18:5, 414-433, DOI: 10.1080/17477891.2019.1609406
(https://www.tandfonline.com/doi/full/10.1080/17477891.2019.1609406

Melchers, Robert E. (2010). Investigation of the Failure of the Newcastle Workers Club, Australian Journal of Structural Engineering, 11:3, 163-176, DOI: 10.1080/13287982.2010.11465064

Standards Australia (1979). The Design of Earthquake Resistant Buildings, AS2121-1979
Standards Australia (1998). AS4100-1998: Steel Structures.
Standards Australia (1993). Minimum design loads on structures: Part 4 – Earthquake Loads, AS1170.4-1993.
Standards Australia (2009), AS3600-2009: Concrete Structures.
Standards Australia (2019). Structural design actions Part 4: Earthquake actions in Australia. AS1170.4-2019

Woodside, John and Kevin McCue (2017). Early History of Seismic Design and Codes in Australia. Australian Earthquake Engineering Society, 2017.