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October 24, 2019

The Magnitude of Earthquakes

On Sunday 14th July 2019, Broome was rattled with a Magnitude 6.6 earthquake.

Broome

M6.6
Stock tumbles from supermarket shelves in Broome
(Photo – Scott Banfield)

On Tuesday 16th July 2019, Bali was struck by a Magnitude 5.7 earthquake. At least one person was injured and 38 buildings were damaged including homes and Denpasar’s main hospital.

M5.7
Tiles fell off the damaged roof of a school building, Bali
(Photo – Sonny Tumbelaka/AFP)

In December 1989 Newcastle (NSW) was struck by a Magnitude 5.6 earthquake. It killed 13 people, injured more than 160 and damaged 50,000 buildings.

Newcastle-quake

M5.6
The 1989 Newcastle quake killed 13 people,
injured more than 160 and damaged 50,000 buildings
(Photo – Newcastle Library Collection)

You could be forgiven for assuming the lower the Magnitude, the worse the earthquake!

The truth is, the magnitude of an earthquake has absolutely nothing to do with the effects caused by them. Clearly!

We could never say “Your building will resist a magnitude 8.0 earthquake”, for example.

The magnitude means nothing in terms of damage and loss of life, and it certainly means nothing to a structural engineer. We are more interested in ‘Peak Ground Acceleration (PGA)‘ at the location of the structures.

When an earthquake occurs, there’s acceleration at the ground surface – that’s the surface shaking – sideways and vertically. How much depends on the depth and distance to the source of the quake, and the type of ground (rock, sand, silt, clay).

There’s more to it. The ‘period‘ of the earthquake vibrations is significant. Also the ‘natural frequency‘ of the structures. Some structures will be affected, some won’t, depending on the period of the earthquake and the natural frequency of the structures and building elements.

Confused? You’re excused!

The location of the Newcastle earthquake, the soil in the region, the particular nature of the quake (specifically the frequency of the vibrations), and the types of structures – all came together to produce the devastating effects.

On the other hand, the Bali earthquake occurred some 90 km below the surface and around 80 km offshore from Denpasar.

Similarly, the Broome earthquake was a long way offshore.

Do we need to design for earthquakes in Perth and regional Australia?

Yes, and we do! Codes require us to design for earthquake forces (or displacements) on all structures in all areas of the country – at least to a point.

But good engineers recognise that the design is in the detail.

We really can’t design a building to resist the next big ‘quake. We can design it so it doesn’t catastrophically collapse, so even if the forces are bigger than expected, we get to save lives. That’s our job – to design within the realms of acceptable risk.

“Earthquakes Don’t Kill People, Buildings Do.”

The reality is, after a big enough earthquake, ‘normal’ structures should have “failed” in such a way that they don’t bury people, but they would more than likely need to be demolished and rebuilt.

There are subtle differences between an earthquake-friendly design and one that has no chance in a shake of even mild proportions. That’s the difference you don’t see, but you do get, when you choose a good structural engineering team!

Have a Project that You Want to Discuss?

Book a meeting with us and we will assist you with the structural design and analysis of your structure to help you produce a structure that is fit-for-purpose and capable of resisting all applied loads without failure during its intended life.

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