Acehtsunami

On Monday, November 18, 1929, the earth rumbled and the waters rose on the Burin Peninsula in southern Newfoundland. A tsunami (a Japanese word meaning ‘harbour wave’), struck the peninsula’s shores. It was caused by an underwater earthquake offshore on the floor of the Atlantic Ocean. The tsunami came as a complete surprise to the residents of the Burin Peninsula. Most tsunamis occur in the region that encircles the Pacific Ocean.

The underwater earthquake originated at 44°69′ north latitude and 56° west longitude, along two fault planes about 250 kilometres (153 miles) south of the Burin peninsula. The tremor measured 7.2 on the Richter scale.

The disaster began around 5:00 p.m. as a violent earth tremble that lasted five minutes. Everyone was instantly both baffled and alarmed. People in St. John’s, 402 kilometres (250 miles) from the epicentre, thought the rumblings were the result of an accident in the shafts at the Bell Island mines in Conception Bay. Recovering from their initial fear, most inhabitants tried to put the tremor out of their minds as they continued their dinner preparations.

At around 7:30 p.m. a tsunami swept ashore on the Burin Peninsula. The waves, travelling at the astounding speed of 129 km/h (80 mph) from the epicentre, hit the peninsula at a speed of 105 km/h (65 mph). It affected more than 40 coastal communities. The November 22, 1929 editorial in the St. John’s Daily News described the event as follows:

Suddenly without warning, there is a roar of waters. Louder than that of the ordinary waves on the shore, it breaks on their ears, and then, with a shuddering crash, a fifteen foot wall of water beats on their frail dwelling, pouring in through door and window and carrying back in its undertow, home and mother and children!

All communication was cut off with the outside world. Moreover, there was at the time no road connecting Burin Peninsula to the rest of the province. Once the wave receded, overwhelmed survivors were forced to invent their own rescue plans.

Three days after the tsunami, on Thursday, the coastal steamer Portia made a scheduled stop at Burin’s altered port. An SOS message to St. John’s resulted in the arrival of the SS Meigle, filled with doctors, nurses, blankets and food.

The loss of property, originally estimated between $150,000 and $250,000, reached over $1 million in the aftermath. The boats, fishing gear, supplies and other industrial equipment of half of the wage earners were destroyed. Tsunami victims were not reimbursed for lost foodstuffs or winter fuel. Compensation was allowed for house repairs and lost boats.

The first official disaster fund for the emergency was established by a committee in St. John’s on November 25, 1929. The value of donations to the South Coast Disaster Committee, from the rest of Newfoundland, Canada, the United States and Britain, totalled more than $250,000.

The tsunami did irreparable damage, affecting 10,000 people in over 40 settlements. In the Burin Peninsula, 27 deaths were attributed to the tsunami; another victim died in 1933 from injuries sustained during the disaster. A tsunami generated by the same earthquake was also reported to have struck Cape Breton, Nova Scotia, drowning one individual there.

 Tsunamis act very differently from typical surf swells; they spread at high speeds and can travel great transoceanic distances with little energy loss. A tsunami can cause damage thousands of miles from its origin, so there may be several hours between its creation and its collision on the coast, more time than it takes for seismic waves to arrive.

Tsunamis have extremely long periods, 2 minutes to over one hour, and long wavelengths, in excess of 100 km. (Compare a typical wind-generated swell one sees at a surf beach, which might be spawned by a faraway storm and rhythmically roll in, one wave after another, with a period of about 10 seconds and a wavelength of 150 m.)

Typically undersea earthquakes give rise to between 3 and 5 distinct waves (crests), the second or third of which are usually the largest.

In instances where the leading edge of the tsunami is its trough, the sea will recede from the coast half the wave’s period before the wave’s arrival. If the slope is shallow, this recession can exceed 800 m. People unaware of the danger may remain at the shore due to curiosity, or for collecting fish from the dry sea bottom.

In instances where the leading edge of the tsunami is its first peak, low-lying coastal areas are flooded before the higher second wave reaches them. Again, being educated about a tsunami is significant, to realize that when the water level drops the first time, the danger is not yet over.

A wave becomes a shallow-water wave when the ratio between the water depth and its wavelength gets very small. Since a tsunami has a large wavelength, tsunamis act as a shallow-water wave even in deep oceanic water. Shallow-water waves move at a speed that is equal to the square root of the product of the acceleration of gravity (9.8 m/s2) and the water depth. For example, in the Pacific Ocean, where the typical water depth is about 4000 m, a tsunami travels at about 200 m/s (about 712 km/hr or 442 mi/hr) with little energy loss even for far distances, while at a water depth of 40 m, the speed is 20 m/s (about 71 km/hr or 44 mi/hr), much slower, but still difficult to outrun.

In deep water, the energy of a tsunami is constant, a function of its height and speed. Thus, as the wave approaches land, its height increases while its speed decreases. While in deep water a person at the surface of the water would probably not even notice the tsunami, the wave can increase to a height of 30 m and more as it approaches the coastline and compresses. Tsunamis can cause harsh destruction on coasts and islands, even at locations remote to the source event, where that event itself is not even noticable without instruments.

Tsunamis propagate outward from their source, so coasts in the “shadow” of affected land masses are regularly fairly safe. However, tsunami waves can diffract around land masses (as shown in this Indian Ocean tsunami animation as the waves reach southern Sri Lanka and India). They also need not be symmetrical; tsunami waves may be much stronger in one direction than another, depending on the nature of the source and the surrounding geography.