Why did the 1989 Newcastle earthquake occur?

The Day Newcastle Shook: Why the ’89 Earthquake Happened

December 28, 1989. I remember it like it was yesterday, even though I wasn’t in Newcastle myself. At 10:27 AM, the city was rocked by an earthquake – a 5.6 magnitude jolt that no one saw coming. It wasn’t just a tremor; it was a full-blown disaster. Thirteen people lost their lives, over 160 were injured, and the damage bill soared to a staggering A$4 billion. So, what caused this catastrophe? Let’s dig in.

The thing is, Australia isn’t exactly known for earthquakes. We’re not sitting on some major fault line like California or New Zealand. That’s because Newcastle’s quake was an intraplate earthquake. Picture Australia smack-bang in the middle of the Indo-Australian Plate – a pretty stable spot, usually. Intraplate earthquakes happen within these plates, far from the action at the edges.

Now, figuring out exactly what triggers these intraplate quakes is a bit of a puzzle. Unlike those plate boundary earthquakes, the causes here are less clear-cut. Think of it like this: the Earth’s crust has weak spots, and stress builds up over time. Eventually, something’s gotta give, and the earth “snaps”. That’s the earthquake. Several things probably played a role in Newcastle:

• Old Wounds: Australia’s got a long geological history, and that means ancient fault lines crisscrossing the continent. These old faults can be reactivated when stress builds up. The Hunter-Mooki Fault System is a prime suspect in the Newcastle quake, though the link isn’t crystal clear.
• The Pressure Cooker: Even in a stable plate, stress is constantly building. The plate’s movement, what’s going on deep in the Earth’s mantle, even erosion – it all adds up. When the pressure becomes too much for the rocks along a fault to handle, boom! Earthquake.
• Going Up: The Newcastle earthquake was a “thrust fault”. The earthquake source is defined as being on a thrust fault trending NW-SE dipping 75° to the NE, with a depth of focus at 11.5 km.

But it wasn’t just the earthquake itself; Newcastle’s geography made things worse. The city center was partly built on soft soil and reclaimed land – basically, former swampland. This kind of ground amplifies the shaking, like jelly wobbling on a plate. The damage was most severe on soft sediments from the Hunter River, with shaking intensity of MMI VIII observed at many locations. Imagine the chaos!

Then there’s the coal mining question. Did it play a part? It’s a controversial topic. Some say yes, others, like Dr. David Denham, reckon the quake was too deep to be linked to mining. But a United States academic claimed in early 2007 that coal mining in the region triggered the earthquake. The debate rages on.

And let’s not forget the building codes – or lack thereof. Back in ’89, Newcastle didn’t have strict earthquake-resistant building standards. The codes issued in 1979 didn’t mandate earthquake-resistant design or the strengthening of older buildings. That’s why buildings like the Newcastle Workers Club collapsed, tragically claiming lives.

The Newcastle earthquake was a harsh lesson for Australia. It forced us to rethink our approach to earthquake preparedness. The revised code was introduced as AS1170. 4 in draft form in 1991 and published in 1993. We updated building codes, invested in more research, and raised public awareness.

Australia might not be earthquake central, but Newcastle proved that we’re not immune. Understanding what happened that day is vital to keeping our communities safe and resilient in the future. It’s a reminder that even in the most stable places, the earth can still surprise us.