Monitoring and predicting hazards

Hazard or disaster?

A natural hazard is an event, such as an earthquake or eruption, that could harm people. It becomes a disaster when it seriously affects a community that can't cope. The same size earthquake can kill thousands in one place and nobody in another, depending on how many people live nearby, how buildings are made, and whether there is warning. Science can't stop earthquakes, but it can reduce the risk.

The tools of monitoring

Predicting earthquakes

No one can predict an earthquake, meaning say exactly when, where and how big it will be. Scientists can forecast earthquakes as probabilities. By measuring how fast a fault is loaded (from GPS), and digging trenches across the fault to date past earthquakes, they can estimate the chances over decades. For example, the Alpine Fault in New Zealand has about a 75% chance of a large earthquake in the next 50 years. Forecasts guide building codes and emergency planning.

Some countries, including Japan, Mexico and the USA, have earthquake early warning systems. Seismometers near the fault detect the fast P waves and send an alert by phone, TV and radio before the slower, more damaging S and surface waves arrive. The warning is only seconds to a minute long, but it is enough to stop high-speed trains, open fire station doors, and let people take cover. During the 2011 Tōhoku earthquake, all Japan's bullet trains stopped safely.

Forecasting eruptions

Volcanoes usually give warning signs as magma rises: swarms of small earthquakes, the ground swelling, and more gas. In 1991 scientists monitoring Mt Pinatubo in the Philippines saw all three and persuaded authorities to evacuate tens of thousands of people. The eruption was the second-largest of the 20th century, but evacuations saved an estimated 5000 or more lives. Warnings are not always enough. In 2019 the alert level at Whakaari / White Island in New Zealand had been raised, but tours continued, and a sudden steam-driven eruption killed 22 people.

Tsunami warnings

A tsunami is a series of waves caused when a large volume of water is suddenly moved, most often by an undersea earthquake that lifts the sea floor at a subduction zone. Landslides and eruptions, like Hunga Tonga in 2022, can also cause them.

In the deep ocean a tsunami travels at the speed of a jet plane but may be less than a metre high, so ships barely notice it. Its speed depends on the depth of the water: speed = √(9.8 × depth), in metres per second. As it reaches shallow water it slows down, the back of the wave catches up, and it can grow to tens of metres high. In 2011 the Tōhoku tsunami ran up to about 40 m above sea level on the Japanese coast.

After 2004, Australia set up the Joint Australian Tsunami Warning Centre (JATWC), run by the Bureau of Meteorology and Geoscience Australia, which has operated since 2007. When a large undersea earthquake happens, it:

1. uses seismometers to find the location and magnitude within minutes
2. models how a tsunami would travel and which coasts are at risk
3. checks sea-level data from DART buoys (a pressure sensor on the deep sea floor sends readings to a buoy and then by satellite) and coastal tide gauges
4. issues warnings to emergency services and the public, by coastline, with expected arrival times.

Natural warnings are just as important. If you are at the coast and feel strong shaking that lasts a long time, or see the sea suddenly drain away or hear a loud roar from the ocean, go to high ground straight away. Don't wait for an official warning.

Building for earthquakes

Earthquakes rarely kill people directly; collapsing buildings do. Engineers reduce the risk by:

• Building codes that set how strong buildings must be in each area. Australia's earthquake code was strengthened after the 1989 Newcastle earthquake.
• Base isolation: the building sits on rubber-and-steel bearings that let the ground move underneath while the building moves much less. The Te Papa museum and Parliament House in Wellington stand on base isolators.
• Cross-bracing and steel-reinforced concrete that let buildings bend without breaking. Unreinforced brick buildings are the most dangerous; many of the people killed in Newcastle and Christchurch were hit by falling masonry.
• Dampers, such as the giant 660-tonne pendulum in the Taipei 101 skyscraper, which reduce swaying.
• Planning: avoiding building on soft, wet ground that shakes more and can turn to liquid (liquefaction), and placing hospitals and schools on safer sites.