About tsunamis

Waitakere City Council warning sign, New Zealand. In places with a high tsunami risk, like the coasts of New Zealand, there may be warning sirens, but if you feel tremors, play safe and go uphill.

In August 1805, The Sydney Gazette and New South Wales Advertiser reprinted a letter from Norfolk Island, a Pacific sub-colony, describing a ‘freak tide’ on 8 May 1805. At 3.30 pm, almost at low tide, all the water drained from a channel that usually had 2 to 3 fathoms (4 to 6 metres) of water in it, and in two minutes it was left dry. Suddenly, the water rushed back and came inland, reaching the military barracks, usually 20 metres above the high tide mark. Then, the water all ran out again.

You can read the story here: The Sydney Gazette and New South Wales Advertiser, 4 August, 1805, 1 – 2, https://trove.nla.gov.au/newspaper/article/626869

The “town of Sydney” that is mentioned there is now called Kingston.

This event was later called a ‘tidal wave’. In Japan, where this happened more often, people called it a ‘harbour wave’, or in Japanese, a tsunami. A tsunami is far bigger inside a harbour than it is out in the deep ocean, and it actually has nothing to do with tides. It is caused by movements in or under the sea.

English speakers adopted the Japanese name when we began to understand what causes these waves. Out at sea, a tsunami passes under boats without anybody on board noticing. The wave is not just on the surface: the disturbance reaches deep into the water and, as it approaches shallow water, the bottom of the wave touches the sea floor, it gets slowed down, and the water starts to pile up.

On Norfolk Island in 1805, the first warning of looming trouble came when the sea level fell. This often happens with a tsunami, and there will always be people who walk out onto the exposed seafloor when the sea goes out. If you ever see this kind of fall in sea level (with or without a starter wave), move quickly to high ground, because the several following waves will be larger!

In Sri Lanka, when the 2004 Boxing Day tsunami hit, the biggest wave was the third or fourth, which gave a British geologist time to warn people to leave the beach. In the ocean, those waves were a metre high, but their tremendous speed—almost 1000 kilometres per hour—is converted into extra height in shallow waters.

We live on top of a 65 metre hill (for primitive societies, that means 200 feet above sea level. Nonetheless a dodgy insurance agent tried to sell us tsunami insurance. and several fellow-residents scurried to sign, until I pointed out that any tsunami hitting us had to come through, 70 metres high, meaning that most of Sydney would be devastated, and the insurance company would be bankrupt, so we would never get any money back.

That aside, I told them, our coastline was unlikely to allow any surge to come our way. The most dangerous place to be during a tsunami is in a steep-sided inlet like a fiord or in a wedge-shaped bay. These shapes funnel the wave’s energy in, increasing the force and the danger.

In 1946, a magnitude 8.1 earthquake in the Aleutian Islands in the northern Pacific Ocean produced a tsunami that killed 96 people in Hilo, a town on a wedge-shaped bay on the eastern side of the island of Hawaii. The people of Hilo had two later tsunami warnings, in 1952 and 1957, but these waves were small and people had stopped worrying about mere waves. A few years later, in 1960, a magnitude 9.5 earthquake in Chile sent another huge wave into Hilo’s bay, killing 61 people. Seven hours later, the wave reached Japan where 142 people were killed.

Some of my Californian friends have just (December 5, 2024) been through a tsunami scare, and while they got off safely this time, there may be another quake that does produce a quake in the near future. Note the may. All I offer is a reasonable hypothesis.

A tsunami can happen when there is an earthquake resulting from a large part of the seafloor moving suddenly up or down; or when there is an earthquake or volcano that makes a large mass of rock tumble into the sea; or when a large solid piece of a meteor falls into the sea.

These days, people receive alerts if a tsunami is coming, but there was an insufficient warning system in place when the 2004 Boxing Day tsunami happened in the Indian Ocean, after a quake with a magnitude of about 9.1. In hindsight, that event might have been predictable, because a week or so earlier, there had been a powerful earthquake with a north to south sideways movement of the Australian Plate, south of New Zealand.

Sideways movements make no waves, but they transfer force and, sooner or later, the northern end of the Indian Plate had to slide under the Burma micro-plate, bumping it upwards. When the slide happened, 30 cubic kilometres of water were moved, leading to waves that were about one metre high, out at sea, but up to 15 metres high as they came ashore. With no warning, about 230,000 people died across 14 countries. 

This may perhaps happen off California: every seismic movement transfers stress to to a new place.

At other times nobody may notice. On 16 January 2022, Little Manly beach in Sydney harbour had repeated surges from a tsunami triggered by a volcano near Tonga. Only the present writer was aware of the continued one-minute ebb-and-flow, and my camera caught it: I managed to crop out members of the public and children, but they all remained blissfully 

To the left of those pictures lies North Head, covered in a deep layer of aeolian (wind-blown) sand from the last ice age, but there is at least one Australian geologist who claims the sand was deposited there by a tsunami. If I were to assess this, I would need to use rude words. Let me just say excreta tauri...

This draws on my recent and about-to-be-pitched What on Earth: behind earth science.

So to my Californian friends, I advise a degree of vigilance.