After an earthquake, you usually cannot tell whether a building is safe to occupy just by looking at it. A structure can look completely normal and still have been shaken beyond what it was designed for, and it can look alarming yet be structurally sound. The reliable way to know is to measure how the building itself responded to the shaking and compare that against its own design limits. Without that, the decision falls back on a visual inspection and conservative assumptions, which is slow, and often means closing a sound building for far longer than necessary.
This is one of the hardest calls a building owner, facilities manager, or council faces in the minutes and days after a significant quake. Here is what actually determines the answer, and how measured data changes it.
In this article
- Why you cannot tell from the outside
- What actually determines whether a building is safe to reoccupy
- The cost of guessing
- How measured data answers the question
- In New Zealand: an answer in minutes
- What to do if your building is not instrumented
- Frequently asked questions
Why you cannot tell from the outside
Earthquake damage is not always visible, and visible damage is not always structural. Cracked plaster and fallen ceiling tiles look dramatic but may be cosmetic. Meanwhile, a building can suffer a real change in its structural behaviour with nothing obvious on the surface. The shaking a building actually experiences also varies enormously from place to place. In central Wellington you can move from solid rock to reclaimed land in a few hundred metres, which is why some buildings were badly damaged in the 2016 Kaikoura earthquake while others nearby were untouched. A regional shaking estimate does not tell you what your building took.
What actually determines whether a building is safe to reoccupy
Three things decide it, and only one of them is visible:
How hard the building was actually shaken, at its location. Not the regional magnitude, not a shaking map, but the ground and structural acceleration at that specific site.
How the structure responded, compared to what it was designed for. Did the motion stay within the building's serviceability and ultimate design limits, or exceed them? That comparison is what separates "carry on" from "get out."
Whether anything changed that you cannot see. A step change in a building's fundamental frequency after a significant quake, sometimes by 10 to 15 percent, can occur with no visible damage at all. That kind of change only shows up in measured data.
| Visual inspection alone | Measured monitoring | |
|---|---|---|
| Speed | Hours to days, longer in a widespread event when engineers are stretched | Minutes |
| Basis | What is visible, plus conservative default assumptions | How the building actually moved, versus its design limits |
| Catches hidden change? | No | Yes |
| Result | Often an over-cautious closure, or uncertainty | A clear, defensible stay or evacuate decision |
The cost of guessing
Guessing is expensive in both directions. Reoccupy a genuinely compromised building and you put people at risk. Close a sound one and you lose trading days, disrupt operations, and carry cost for no reason.
The Christchurch Civic Offices are a clear example. After the February 2011 earthquake, the Council evacuated its headquarters, home to more than 1,250 staff, for several months while structural assessments were completed. The repairs turned out to be minor and architectural. The evacuation was largely unnecessary, but with no measured record of what the building had experienced, there was no defensible alternative at the time. Once the building was instrumented, it ran through thousands of aftershocks and two magnitude 6 quakes in December 2011 with zero further evacuations, because the decisions were made on data.
How measured data answers the question
Instrumenting a building means placing accelerographs through it, typically at the base, at mid-height, and at the roof, so the system records how the whole structure responded, not just how strong the shaking was at ground level. The moment an event finishes, that record can be compared against the building's own design limits to produce a clear answer: is the measured response within tolerance, or not?
The value is that the answer is specific to your building and available fast. You are not waiting for an engineer to reach the site and work from visual cues and safe defaults. You have evidence of how the structure actually performed, which is exactly what a defensible reoccupation decision needs.
In New Zealand: an answer in minutes
For New Zealand building owners, this is what Sentinel does. Sentinel measures the actual shaking at your building, compares it against your building's own design limits under NZS 1170.5 rather than a regional average, and sends a clear white, amber, or red status with an action attached, straight to your phone by app, SMS, and email, within minutes of an event. White means business as usual. Amber means inspect for hazards. Red means evacuate. Anyone on site can read it and act.
It is built on New Zealand's building code and backed by a nationwide sensor network, and it already protects buildings for councils, hospitals, airports, and property portfolios around the country, including Christchurch Airport and Wellington City Council. It is a subscription service, so there is no up-front hardware cost or installation project for the building owner to carry.
What to do if your building is not instrumented
If your building has no monitoring, your reoccupation decision after a significant event will rest on a rapid assessment by a suitably qualified engineer, working from visible evidence and conservative assumptions, and on any placard or notice that assessment produces. That is the right thing to do, but in a widespread event engineers are in high demand, so it can take time, and it will tend toward caution.
The lesson from every major New Zealand sequence is the same: the time to instrument a building is before an event, not after one. Monitoring installed today is what gives you a fast, evidence-based answer when it matters, and a continuous record of how your building behaves over its life.
Frequently asked questions
How long after an earthquake is a building safe to enter?
There is no fixed time. It depends on how the building responded and on assessment, not on the clock. An instrumented building can give a stay or evacuate answer within minutes of an event. A building relying on visual inspection may wait hours or days for an engineer, longer in a widespread event where engineers are stretched.
Can you tell if a building is damaged just by looking at it?
Not reliably. Visible damage can be cosmetic, and serious structural change can occur with nothing obvious on the surface, including a measurable shift in the building's dynamic behaviour. A visual inspection is a necessary step, but on its own it cannot confirm how hard the structure was shaken or whether it stayed within its design limits.
What do the white, amber, and red building statuses mean?
In Sentinel, white means the measured shaking stayed within the building's limits, so it is business as usual. Amber means possible non-structural damage, so inspect for hazards before normal use. Red means possible structural damage, so evacuate and activate your emergency response plan. Each status comes with a clear action.
Who decides whether a building is safe to reoccupy?
Ultimately the building owner and their engineer, informed by assessment. Monitoring does not replace that judgement; it gives it evidence to work from, so the decision is faster and more defensible, and unnecessary closures are avoided.
What is Sentinel?
Sentinel is Canterbury Seismic's New Zealand monitoring service. It measures the shaking at your building, compares it against your building's own NZS 1170.5 design limits, and sends a clear white, amber, or red status with an action, within minutes, by app, SMS, and email. It is a subscription service, with no up-front hardware cost for the building owner.
Canterbury Seismic Instruments designs and manufactures seismic recording instrumentation from Christchurch, New Zealand, and provides the Sentinel earthquake response service for New Zealand buildings. Get in touch to talk through what monitoring could look like for your building.
