Earthquake Actions and Older Australian Buildings: A Practical Guide for Asset Owners

Seismic risk sits at the back of most Australian asset owners' minds, somewhere behind roof leaks and fire compliance. Then an insurer requests a seismic assessment, or a prospective tenant's due diligence checklist includes it, and the question becomes urgent without any context to frame it.

This post explains what the relevant Australian standard actually covers, what a structural assessment of an existing building typically examines, and how to judge whether a specialist investigation is a proportionate response or an overreaction.

Why Australia Has a Seismic Standard at All

Australia is not Japan. The continent sits in the middle of the Indo-Australian tectonic plate rather than on a plate boundary, which means large earthquakes are infrequent. But infrequent is not the same as impossible. The 1989 Newcastle earthquake, magnitude 5.6, killed 13 people and caused damage estimated at over $4 billion in today's dollars. It remains one of the costliest natural disasters in Australian history, and it happened in a city where almost no one had considered seismic risk.

AS 1170.4, *Structural design actions: Earthquake actions in Australia*, is the standard that governs how new structures are designed to resist seismic loading. It classifies the country into seismic hazard zones based on probability of ground motion, assigns buildings to importance levels based on occupancy and consequence of failure, and specifies the structural systems and detailing requirements that follow from those classifications.

The standard is referenced by the National Construction Code, which means it applies to new construction and to alterations that trigger a building approval. It does not automatically apply to existing buildings that have not been modified. That distinction matters enormously for asset owners.

How Hazard Zones Work in Practice

AS 1170.4 assigns a hazard factor (Z) to locations across Australia. Brisbane sits at Z = 0.08. Sydney is Z = 0.08. Melbourne is Z = 0.08. Newcastle, following the 1989 event, is Z = 0.11. Perth is Z = 0.09. These are not alarming numbers by global standards, but they are not zero, and the structural implications depend heavily on the building's importance level and the soil conditions beneath it.

Soil amplification is a factor that catches many owners off guard. Soft or deep alluvial soils can amplify ground motion significantly compared to rock. A building on soft fill near a river may experience considerably higher effective loading than a structurally identical building on competent rock a kilometre away, even though both sit in the same hazard zone.

Importance levels under AS 1170.4 range from Level 1 (minor structures) through to Level 4 (post-disaster facilities like hospitals and emergency services). Most commercial office buildings and retail centres fall at Level 2 or Level 3 depending on occupancy numbers and function. Higher importance means higher design loads and more stringent detailing requirements.

What 'Existing Building' Means in This Context

Buildings constructed before AS 1170.4 was introduced in 1993, or before its predecessor provisions in the 1980s codes, were not designed with explicit seismic loading in mind. That does not mean they are necessarily unsafe; gravity load design and wind load design provide some inherent lateral resistance. But it does mean the structural system was never checked against seismic demand, and certain vulnerabilities that seismic design specifically addresses may be present.

When a structural assessment examines an existing building for seismic adequacy, it is not simply comparing the building to the current standard for new construction. That comparison would condemn a large proportion of Australia's built stock. Instead, the assessment is asking a more targeted question: does this building have characteristics that make it significantly more vulnerable than a typical structure of its era, and are those characteristics relevant to the hazard at this site?

What Assessments Actually Examine

Structural Irregularity

Regular structures, those with consistent stiffness and mass distribution in plan and elevation, tend to behave predictably under lateral loading. Irregular structures concentrate demand at the points of discontinuity. A building with a soft storey, where one level is significantly more flexible than those above (often a ground-floor carpark or open retail podium), can experience disproportionate deformation and damage at that level during an earthquake.

Other irregularities include re-entrant corners in plan, significant changes in lateral load-resisting elements between floors, and heavy plant or mechanical equipment concentrated at upper levels. None of these automatically make a building dangerous, but each requires specific consideration.

Diaphragm Behaviour

Floor and roof diaphragms are the structural elements that collect lateral forces and distribute them to the vertical load-resisting system, whether that is shear walls, braced frames, or moment frames. In older construction, diaphragm connections were rarely detailed for seismic load transfer. Timber diaphragms in older commercial buildings, precast concrete floor systems with limited topping, and steel deck without adequate shear connectors are all areas where diaphragm performance under seismic loading warrants scrutiny.

Connection Adequacy

Seismic loading is cyclic and reverses direction. Connections that perform adequately under gravity or even wind loading can fail under the repeated tension and compression of earthquake motion. Unreinforced masonry parapets, precast panel connections, and beam-to-column connections in older steel frames are common areas of concern. The assessment looks at how elements are tied together and whether those connections can transfer the forces that seismic loading generates.

Non-Structural Risk

This is the category that most owners underestimate. In moderate earthquakes, structural collapse is relatively rare in Australian conditions. Injuries and business disruption more commonly result from falling non-structural elements: ceiling systems, light fittings, partition walls, facade panels, mechanical plant, and building services. A thorough assessment considers not just the primary structure but the non-structural inventory and how it is secured.

This is particularly relevant for buildings with suspended ceilings installed in earlier decades, unreinforced masonry infill walls, and heavy plant on rooftops or upper floors without positive restraint.

When Is an Assessment Proportionate?

Not every older building needs a seismic assessment, and not every assessment that is requested needs to be extensive. The question is whether there is a credible reason to examine seismic performance, and whether the scope of that examination matches the actual risk.

Reasonable triggers for a seismic assessment include:

• An insurer requesting it as part of a policy renewal or claims process
• A lender requiring it for refinancing or acquisition due diligence
• A major tenant whose lease requires evidence of structural adequacy
• A planned change of use that increases the importance level under AS 1170.4
• A significant alteration or addition that triggers NCC compliance for the whole building
• A building with known irregularities in a higher-hazard zone or on soft soil
• Post-earthquake inspection following a regional seismic event

An assessment is not warranted simply because a building is old. Age alone is not a reliable predictor of seismic vulnerability. A well-constructed reinforced concrete frame building from the 1970s may perform better than a poorly detailed building from the 1990s.

The proportionate response to a trigger is usually a desktop review followed by a targeted site inspection, not an immediate full structural investigation. The desktop review examines available drawings, the structural system, the site hazard, and the building's importance level. If that review identifies specific concerns, a more detailed investigation follows. If it does not, the findings can be documented and the assessment concluded.

This sequencing matters because it controls cost and avoids generating alarm where none is warranted. The instinct to order a comprehensive investigation immediately is understandable, but it often produces a report that identifies theoretical vulnerabilities without quantifying their actual significance at that site under realistic loading.

The Extent and Severity Question

A seismic assessment that simply lists potential vulnerabilities without quantifying them is of limited use to an asset owner. The relevant questions are: how significant is this vulnerability at this site, given the actual hazard? What is the likely consequence if it is not addressed? And what would remediation actually cost compared to the risk it mitigates?

Answering those questions requires measured data, not assumptions. Structural monitoring can establish baseline behaviour. Non-destructive testing can confirm connection details where drawings are absent or incomplete. Material testing can verify concrete strength or masonry bond capacity where design records are unavailable. These inputs allow an assessment to move from a list of concerns to a ranked, evidence-based picture of actual risk.

That evidence base is what makes remediation decisions defensible. Without it, a contractor pricing seismic upgrades will price the worst case. With it, interventions can be targeted to the elements that actually matter.

A Practical Starting Point

If a seismic question has been raised about a building you manage or own, the first step is not to commission a full structural investigation. It is to establish whether the concern is credible given the building's location, construction type, and the trigger that raised it.

That initial scoping conversation, with a structural engineer who understands both the standard and the practical realities of existing building assessment, will tell you whether a detailed investigation is warranted, what it should cover, and what a proportionate scope looks like.

TRSC works with asset owners across Queensland, New South Wales, and Victoria on exactly this kind of assessment, from initial desktop review through to detailed investigation and remediation design where the evidence supports it. If a seismic question has landed on your desk, visit [https://trsc.au](https://trsc.au) or get in touch to discuss what a proportionate response looks like for your asset.