Earthquakes

Stress, faults and elastic rebound

Plates don't slide past each other smoothly. Where rocks meet along a fault (a crack where rocks have moved), friction locks them together. The plates keep moving, so the rocks on each side bend and store energy, like a ruler bent over the edge of a desk. When the stress becomes greater than the strength of the rock or the friction on the fault, the rocks suddenly slip and spring back. This is called elastic rebound, and the stored energy is released as seismic waves.

• The focus (or hypocentre) is the point underground where the rupture starts.
• The epicentre is the point on the surface directly above the focus. Shaking is usually strongest near it.
• Earthquakes are shallow (less than 70 km deep), intermediate (70–300 km) or deep (300–700 km). Deep earthquakes happen only in slabs sinking at subduction zones. Shallow earthquakes do the most damage.
• Smaller aftershocks follow as the rocks around the fault adjust. They can go on for months or years.

Three kinds of seismic wave

• P waves (primary) are longitudinal waves: the rock is pushed and pulled in the direction the wave travels, like a slinky. They are the fastest, about 6 km/s in the crust, and can travel through solids, liquids and gases.
• S waves (secondary) are transverse waves: the rock moves side to side, at right angles to the direction of travel. They travel at about 3.5 km/s in the crust and cannot pass through liquids. (This is how we know the outer core is liquid.)
• Surface waves travel along the surface after the others arrive. They are the slowest but have the largest movement, and do most of the damage to buildings.

Finding the epicentre

A seismometer records ground movement on a seismogram. Because P waves outrun S waves, the gap between them grows the further the waves travel. For an earthquake in the crust, each second of S–P gap means the earthquake is roughly 8 km away. One station gives only a distance, so the earthquake could be anywhere on a circle around it. Two circles cross in two places. A third station picks out the single point where all three circles meet. This method is called triangulation (strictly, trilateration).

How big was it?

Scientists measure earthquakes in two ways:

• Magnitude measures the energy released at the source. It has one value for each earthquake. Seismologists now use the moment magnitude scale (Mw), which replaced the older Richter scale for large earthquakes. Each step of 1 means about 10 times bigger ground movement on a seismogram and about 32 times more energy. So a magnitude 8 releases about 1000 times as much energy as a magnitude 6.
• Intensity describes how strongly the shaking was felt and the damage it did at a particular place, using the Modified Mercalli scale (I to XII). One earthquake has many intensities: high near the epicentre and lower further away. Soft, wet ground such as old swamp or river sand shakes more than solid rock.

Earthquakes in Australia

Australia sits in the middle of a plate, far from any boundary, but the plate is being squeezed as it pushes north into Indonesia, New Guinea and the Pacific. That stress is released on old faults inside the continent. Geoscience Australia records about 100 earthquakes of magnitude 3 or more in Australia each year, and one of magnitude 5 or more every year or two on average.

• Meckering, WA, 1968 (magnitude 6.5): it split the ground along a 37 km fault scarp and destroyed the town.
• Tennant Creek, NT, 1988: three earthquakes of magnitude 6.3 to 6.7 within 12 hours.
• Newcastle, NSW, 1989 (magnitude 5.6): Australia's deadliest earthquake. 13 people died, many of them when the Newcastle Workers Club collapsed.
• Petermann Ranges, NT, 2016 (magnitude 6.1) and Woods Point, Victoria, 2021 (magnitude 5.9), which was felt in Melbourne and damaged buildings in Chapel Street.