Earthquake Actions and Older Australian Buildings: A Plain-Language Guide for Asset Owners

Seismic risk sits at the bottom of most Australian asset owners' priority lists, and for large parts of the country that is a reasonable position. Then an insurer asks for a seismic assessment, a lender flags it as a condition of refinancing, or a major tenant's due diligence team raises it during lease negotiations. Suddenly a topic that felt academic becomes a transaction blocker.

This post explains what the relevant standard actually requires, what an existing-building assessment looks at, and how to judge whether commissioning one is proportionate to your situation.

Why Australia Has a Seismic Standard at All

Australia sits on the Indo-Australian tectonic plate, away from the Pacific Ring of Fire. That geography means seismic hazard here is lower than in New Zealand, Japan, or the western United States. It does not mean zero.

The 1989 Newcastle earthquake killed thirteen people, injured more than 160, and caused damage estimated at over $4 billion in today's dollars. Newcastle sits in a region that, at the time, engineers and owners considered low-risk. The event prompted a fundamental rethink of how Australian standards treated seismic actions, and the current framework traces its origins to that reassessment.

AS 1170.4:2007 (Structural Design Actions, Part 4: Earthquake Actions in Australia) is the standard that defines seismic design requirements for Australian buildings. It assigns every location a hazard factor based on ground motion data, and it classifies buildings by importance level and site sub-soil class. Those three inputs drive the design requirements for new structures.

How AS 1170.4 Fits the NCC Framework

The National Construction Code references AS 1170.4 as the basis for seismic design of new buildings. When a building permit is issued today, the structural engineer of record must demonstrate compliance with the loading standard, which includes earthquake actions alongside wind, gravity, and other loads.

For most of Queensland, New South Wales, and Victoria, the hazard factor is low enough that seismic actions rarely govern structural design decisions. In practice, wind loads tend to control lateral design for most building types in these states. There are exceptions: certain soil conditions amplify ground motion, and some building configurations are more vulnerable regardless of the base hazard.

The NCC framework applies to new work. Existing buildings sit outside its direct scope unless they undergo a change of use, a material alteration, or a structural modification that triggers a compliance assessment. That distinction matters because most of Australia's commercial and industrial building stock was designed under earlier codes that either did not include seismic provisions or applied them differently.

What Changes When You Are Assessing an Existing Building

An existing-building seismic assessment is not a code-compliance exercise in the same way a new-build design is. The question shifts from "does this building meet AS 1170.4" to "what is the actual seismic risk, and is it acceptable given the building's use, occupancy, and the consequences of failure?"

That framing matters because older buildings cannot always be economically brought to current code standards, and in many cases that is not what the situation demands. The assessment is about understanding vulnerability, not issuing a pass or fail mark against a standard written for new construction.

Four areas tend to drive seismic vulnerability in older Australian buildings.

Structural Irregularity

Buildings that are irregular in plan or elevation behave differently under lateral loading than regular structures. A building with a large setback at upper floors, an asymmetric core, or a soft storey (a floor level that is significantly more flexible than those above it) concentrates seismic demand in ways that standard analysis may underestimate. Older commercial buildings with open ground-floor retail or car-parking levels are a common example of soft-storey configurations.

Irregularity is not automatically a problem, but it requires careful analysis. An engineer reviewing an existing building needs to identify these conditions early, because they influence how the rest of the assessment is structured.

Diaphragm Performance

Floor and roof diaphragms transfer lateral loads from the building's mass to its vertical lateral-force-resisting elements, whether those are shear walls, moment frames, or braced bays. In older construction, diaphragms are often timber floors, unreinforced concrete slabs, or precast concrete systems with connections that were not designed for seismic demand.

A diaphragm that cannot transfer loads effectively will cause the lateral system to perform well below its theoretical capacity, even if the walls or frames are adequate in isolation. Assessing diaphragm integrity and connectivity is one of the less visible but more consequential parts of an existing-building review.

Connections and Continuity

Seismic loading reverses direction rapidly. Connections that perform adequately under gravity or one-directional wind loading can fail under the cyclic demands of an earthquake. Older precast concrete buildings, masonry structures with steel roof systems, and buildings with mixed structural systems often have connection details that were not designed with seismic continuity in mind.

This is particularly relevant for unreinforced masonry (URM) construction, which is common in older commercial precincts across Brisbane, Sydney, and Melbourne. URM walls have little tensile capacity and can fail in out-of-plane bending or at wall-to-floor connections. The consequences of that failure mode are serious, and it is one of the first things an engineer looks for in a heritage or pre-1990s building assessment.

Non-Structural Risk

Not every seismic loss comes from structural collapse. Falling ceilings, unsecured plant and equipment, unbraced partition walls, and poorly anchored facades can injure occupants and disrupt operations even in moderate earthquakes. For buildings with continuous occupancy, high public throughput, or sensitive operations, non-structural risk can be as consequential as the structural risk.

A thorough assessment considers both. The non-structural review often identifies cost-effective interventions that meaningfully reduce risk without requiring structural modifications.

When an Assessment Is Proportionate

Not every building warrants a detailed seismic investigation. The decision depends on several factors working together.

Hazard location is the starting point. Buildings in higher-hazard zones, including parts of Western Australia, South Australia, and the ACT region, face greater baseline risk than those in the lower-hazard areas of coastal Queensland. Within any zone, site soil conditions matter considerably; soft or liquefiable soils amplify ground motion relative to rock sites.

Building age and construction type shape vulnerability. Pre-1990 construction predates the modern seismic provisions. URM buildings, older precast concrete frames, and structures with known irregularities carry higher inherent vulnerability. Post-2000 construction designed under modern codes is generally better positioned, though connection details and non-structural provisions still warrant review for high-occupancy uses.

Occupancy and consequence set the threshold for acceptable risk. A warehouse with low occupancy and replaceable contents sits in a different risk category than a hospital, a school, or a high-rise apartment building. AS 1170.4 formalises this through importance levels, but the same logic applies to existing-building assessments: the higher the consequence of failure, the lower the acceptable risk threshold.

Trigger events often drive the decision. Insurers, lenders, and sophisticated tenants increasingly request seismic assessments as part of their due diligence. If a transaction is contingent on one, the question is not whether to commission it but how to structure it so the findings are useful rather than just a compliance document.

A proportionate response to most of these triggers is a desktop screening assessment first. This involves reviewing available drawings, identifying the structural system and construction type, mapping the site hazard, and forming a preliminary view of vulnerability. If the screening identifies low risk and no significant irregularities or URM elements, that may be sufficient. If it identifies concerns, a more detailed field investigation follows.

What a Field Investigation Adds

Where the desktop screening raises questions, a site investigation provides the evidence to answer them. This typically involves accessing structural elements to verify construction details, using non-destructive testing to confirm material properties where drawings are absent or unreliable, and assessing connection conditions directly.

For older buildings without as-built documentation, 3D scanning can capture geometry that would otherwise require intrusive investigation. Knowing the actual dimensions of walls, the position of structural elements, and the configuration of connections allows analysis to proceed on measured data rather than assumptions.

The output of a field investigation is not a list of defects to remediate. It is a calibrated picture of where vulnerability is concentrated, how severe it is, and what interventions would address it proportionately. That evidence base is what allows asset owners to make informed decisions rather than reacting to worst-case estimates.

The Cost of Not Knowing

Seismic assessments are sometimes perceived as a risk in themselves: commission one and you might find something that requires expensive remediation. That concern is understandable but often misplaced.

An assessment that finds low risk provides documented evidence for insurers, lenders, and tenants. An assessment that finds moderate risk typically identifies targeted interventions, many of which are far less expensive than the worst-case figures that circulate without data. The alternative, carrying undocumented vulnerability through a transaction or an event, tends to be the more expensive outcome.

The evidence gathered through investigation is also what prevents over-remediation. Without measured data, contractors and engineers default to conservative assumptions. With it, the scope of any required work can be defined precisely.

Getting the Scope Right

A seismic assessment commissioned without a clear scope brief often produces a report that answers the wrong question. Asset owners are best served by engaging an engineer early in the process to define what the assessment needs to demonstrate, who the audience is (insurer, lender, internal risk committee), and what decisions the findings will inform.

For Queensland assets, Form 15 certification by an RPEQ engineer may be required where structural modifications are proposed following an assessment. Understanding that requirement before work begins avoids delays at the certification stage.

If your building is approaching a transaction, a lease renewal with a sophisticated tenant, or an insurance renewal where seismic has been flagged, the starting point is a conversation about what the assessment actually needs to achieve. TRSC works with asset owners across Queensland, New South Wales, and Victoria on exactly these questions. More information is available at [https://trsc.au](https://trsc.au).