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.

 A tsunami (pronounced soo-nahm-ee) is a series of huge waves that happen after an undersea trouble, such as an earthquake or volcano eruption. (Tsunami is from the Japanese word for harbor wave.) The waves travel in all directions from the area of disorder, much like the ripples that happen after throwing a rock. The waves may travel in the open sea as fast as 450 miles per hour. As the big waves approach shallow waters along the coast they grow to a great height and crash into the shore. They can be as high as 100 feet. They can cause a lot of destruction on the shore. They are sometimes mistakenly called “tidal waves,” but tsunami have nothing to do with the tides.

Hawaii is the state at greatest risk for a tsunami. They get about one a year, with a damaging tsunami happening about every seven years. Alaska is also at high risk. California, Oregon and Washington experience a destructive tsunami about every 18 years.

Did you know:

In 1964, an Alaskan earthquake generated a tsunami with waves between 10 and 20 feet high along parts of the California, Oregon and Washington coasts.

In 1946, a tsunami with waves of 20 to 32 feet crashed into Hilo, Hawaii, flooding the downtown area.

Tsunamis are unlike wind-generated waves, which many of us may have experiential on a local lake or at a coastal beach, in that they are characterised as shallow-water waves, with long periods and wave lengths. The wind-generated swell one sees at a California beach, for example, spawned by a storm out in the Pacific and metrically rolling in, one wave after another, might have a period of about 10 seconds and a wave length of 150 m. A tsunami, on the other hand, can have a wavelength in excess of 100 km and period on the order of one hour.

As a effect of their long wave lengths, tsunamis behave as shallow-water waves. A wave becomes a shallow-water wave when the ratio between the water depth and its wave length gets very small. Shallow-water waves move at a speed that is equal to the square root of the product of the speeding up of gravity (9.8 m/s/s) and the water depth. Let’s see what this implies: In the Pacific Ocean, where the typical water depth is about 4000 m, a tsunami travels at about 200 m/s, or over 700 km/hr. Because the rate at which a wave loses its energy is inversely associated to its wave length, tsunamis not only propagate at high speeds, they can also travel great, transoceanic distances with limited energy losses. The earthquake-generated 1960 Chilean tsunami, for example, travelled across over 17,000 km across the Pacific to hit Japan. The wave crests bend as the tsunami travels —- this is called refraction Wave refraction is caused by segments of the wave moving at different speeds as the water depth along the peak varies.

A tsunami can be generate by any disturbance that displace a huge water mass from its equilibrium position. In the case of earthquake-generated tsunamis, the water column is anxious by the uplift of the sea floor. Submarine landslide, which frequently accompany large earthquakes, as well as collapse of volcanic edifices, can also concern the overlying water column as sediment and rock droop downslope and are redistributed across the sea floor. Similarly, a violent underwater volcanic eruption can create an impulsive force that uplifts the water column and generate a tsunami. Conversely, underwater landslides and cosmic-body impacts concern the water from above, as momentum from falling wreckage is transferred to the water into which the debris falls. Gernerally tsuna-mis generated from these mechanism, unlike the Pacific-wide tsunamis caused by some earthquakes, dissipate quickly and sometimes affect coastlines distant from the source area. What happens to a tsunami as it approach land?

As a tsunami leaves the deep water of the open ocean and travels into the shallower water next to the coast, it transforms. If you read the “How do tsunamis differ from other water waves?” section, you exposed that a tsunami travels at a speed that is interrelated to the water depth — hence, as the water depth decrease, the tsunami slows.The tsunami’s energy flux, which is depends on the both its wave speed and wave height, remains nearly invariable. As a result, as the tsunami’s speed diminish as it travels into shallower water, its height grows. Because of this shoaling effect, a tsunami, imperceptible at sea, may grow to be numerous meters or more in height near the coast. When it finally reaches the coast, a tsunami may appear as a rapidly rising or falling tide, a series of breaking waves, or even a bore.

It is likely that one or more severely damaging earthquakes, which equal or exceed the 1994 Northridge earthquake in magnitude, will strike the United States within the next decade. Repeats of the 1906 San Francisco and the 1964 Alaska earthquakes loom somewhere in the future for California and Alaska. Although most people associate them with the nation’s West Coast, earthquakes pose a significant risk in at least 39 states. The New Madrid, Missouri, earthquake of 1811 was as powerful as the 1906 San Francisco earthquake and was felt across the entire eastern United States. The National Research Council has estimated that a repeat of the 1811 New Madrid earthquake could result in hundreds to thousands of lives lost and over $100 billion dollars of damage in a 26-state area. In areas such as the Midwest that experience earthquakes infrequently, the earthquake hazard awareness, vulnerability, and risk sensitivity of the residents is low. Even in areas that have frequent earthquakes, preparedness is often highly variable.

Earthquakes release the strain built up in the earth’ otentially damaging earthquakes are caused by sudden movements along faults. Earthquakes may result in offsets of up to thirty feet which extend up to hundreds of miles along the length of the faults. The 1906 San Francisco earthquake and the 1964 Alaska earthquake were of this scale. Lesser earthquakes, like the 1971 San Fernando earthquake, the 1989 Loma Prieta earthquake are intermediate in magnitude but were still felt over thousands of square miles. Even in relatively well-studied areas surprises can occur. The 1994 Northridge earthquake, which occurred along an unrecognized, buried fault, is a prime example. In the Central and Eastern United States, where earthquakes are less frequent than in the West, there are potentially more surprises; because the risk is less well understood, mitigation practices are less commonly implemented and the potential for damage, should an earthquake occur, is much greater.

Earthquake effects include violent ground shaking and earthquake-induced ground failure such as liquefaction (the sudden conversion of soil to a liquid mass due to shaking as occurred in the 1995 Kobe earthquake), landslide, or ground surface rupture. Submarine earthquakes can induce damaging tsunami (seismic sea waves or “tidal” waves), which can travel undiminished thousands of miles before bringing destruction to coastal areas. Earthquakes may also cause permanent changes in sea-level elevation through local ground subsidence or uplift.

The principal threat from earthquakes is shaking damage and the collapse of buildings and other structures that have been inadequately designed or constructed to resist seismic forces. Major earthquakes can severely interrupt regional or national economic activity by damaging lifelines such as roads, railways, water, power, and communication lines. Seismic damage interrupts the flow to users of vital resources and services, thereby increasing the risk to life safety and impeding economic growth. Ground failure hazards such as subsidence, landslides, liquefaction, and settlement also cause damage to structures and lifelines, and are a major threat to dams, waterfront structures, highway facilities, and buried lifelines.

Although much remains to be learned about the most effective and economical techniques for enhancing the seismic safety of structures, many proven cost-effective measures are already being applied in the United States. Considering that little to no strong earthquake ground motion data was collected prior to the 1933 Long Beach earthquake, there have been great accomplishments in the design and construction of earthquake-resistant structures. Because of improved building codes, land use planning, and preparedness, the losses in the San Francisco Bay area from the 1989 Loma Prieta earthquake and in the Los Angeles area from the 1994 Northridge earthquake were much lower than would have occurred in a less well-prepared region .

The current legal requirements for constructing buildings, highways, bridges, and other lifelines in earthquake-prone regions vary greatly from one region to another, or even from one local jurisdiction to another, despite the fact that seismic safety can often be incorporated in new buildings and lifelines at little or no extra cost for design, construction, or operation. Local action to provide earthquake mitigation measures depends largely upon the awareness and education of public officials, engineers, planners, the business community, and the general populace.

While the United States has lost comparatively few lives in earthquakes in recent years, the number can be reduced further. The cost of earthquake damage is still unacceptably high. All regions that are prone to earthquakes must begin to undertake mitigation measures to reduce future human and property losses. While earthquakes are inevitable natural hazards, they need not be inevitable disasters. Our nation can reduce losses of life, casualties, property losses, and social and economic disruptions from future earthquakes through prudent actions

A tsunami cannot be prevented or precisely predicted—even if the right magnitude of an earthquake occurs in the right location. Geologists, oceanographers, and seismologists analyse each earthquake and based upon many factors may or may not issue a tsunami warning. However, there are some warning signs of an impending tsunami, and there are many systems being developed and in use to reduce the damage from tsunami. One of the most important systems that is used and constantly monitored are bottom pressure sensors. These are anchored and attached to buoys. Sensors on the equipment constantly monitor the pressure of the overlying water column. This is deduced through the calculation:

P = \rho gh

where
P = the overlying pressure in Newtons per metre square,
ρ = the density of the seawater= 1.1 x 103 kg/m3,
g = the acceleration due to gravity= 9.8 m/s2 and
h = the height of the water column in metres.

Hence for a water column of 5,000 m depth the overlying pressure is equal to
\! P = \rho gh=(1.1 * 10^3 \frac{kg}{m^3})(9.8 \frac{m}{s^2})(5.0 * 10^3 m)=5.4*10^7 \frac{N}{m^2}=54MPa
or about 5.7 Million tonnes per metre square.[citation needed]

In instances where the leading edge of the tsunami wave is the trough, the sea will recede from the coast half of the wave’s period before the wave’s arrival

Natural hazards impact on every Australian State and Territory. These hazards include bushfires, cyclones, earthquakes, floods, landslides, severe weather, tsunami, and volcanoes. The phenomena threaten lives and damage private and public assets as well as disrupt water, power, transport, and communication services. These hazards and their associated impacts also can seriously affect employment and incomes to industry, agriculture, commerce and public administration.

In Australia, natural hazards are estimated to cost an average of A$1.25 billion annually (BTE, 2001), but the cost of individual hazards can be much greater. For example, in 1989 an earthquake cost the community in the New South Wales city of Newcastle an estimated A$4.5 billion.Natural hazards cannot be averted, but their consequences can be minimised by implementing mitigation strategies and reducing the potential impact to areas which are most vulnerable.As part of its extensive work on natural hazard risk research, Geoscience Australia monitors and assesses earth-surface processes which pose a risk to Australia. It gathers data and develops tools for use by governments and other authorities to help them make Australia as safe as possible from natural hazards.