Acehtsunami

On April 18, 1906, soon after 5:00 a.m., a great earthquake strike San Francisco and a long narrow band of towns, villages, and countryside to the north-northwest and south-southeast. Many buildings were ruined; hundreds of people were killed; electric power lines and gas mains were broken. Fires bust out and burned wildly for days, utterly out of control because of severed water mains.

The land had broken down open for more than 270 miles along a great fault - the San Andreas rift. The country on the east side of the rift had moved southward relative to the country on the west side of the rift. The supreme displacement had been 21 feet about 30 miles northwest of San Francisco

Causes

The tsunami can be generated by any disturbance that displaces a large mass of water, such as an earthquake or landslide

Tsunamis can be generated when the sea floor abruptly deforms and vertically displaces the overlying water.. Around the margins of the Pacific Ocean, for example, denser oceanic plates slip under continental plates in a process known as subduction. Subduction earthquakes are particularly effective in generating tsunamis.

Submarine landslides, which often accompany large earthquakes, as well as collapses of volcanic edifices, can also disturb the overlying water column as sediment and rock slump downslope and are redistributed across the sea floor. Similarly, a violent submarine volcanic eruption can uplift a water column and generate a tsunami.

Large landslides and cosmic-body impacts can disturb the water from above, as momentum from falling debris is transferred to the water into which the debris falls. Generally speaking, tsunamis generated from these mechanisms, unlike the Pacific-wide tsunamis caused by some earthquakes, dissipate quickly and rarely affect coastlines distant from the source area.

Wednesday, November 29, marks the 20th anniversary of the 1975 Kalapana earthquake, the largest earthquake to strike Hawaii since 1868. Residents were awakened at 3:35 a.m. by a magnitude-5.7 earthquake located a few miles inland of Lae’apuki on Kilauea’s south coast. A little more than one hour later, at 4:47 a.m., the main shock struck with a magnitude of 7.2. The epicenter was located at Kamoamoa, just a few miles east, and closer to the shoreline, from the foreshock.
Much of the south coast of Hawaii Island slid horizontally towards the ocean and subsided. The maximum horizontal displacement, near Keauhou Landing several kilometers east of Halape, was about 26 feet. The displacements decreased to the east and west from this area.

Similarly, subsidence of the coast was greatest in this same area and amounted to about 11.5 feet. The amount of subsidence rapidly decreased to the west, and, at Punalu’u, the shoreline actually uplifted by about 4 inches. To the east, subsidence was about 9.8 feet at Halape, 3.6 feet at Kamoamoa, 2

On March 9, 1957, at 14:22 GMT, an earthquake occurred south of the Andreanof Islands, in the Aleutian Islands of Alaska. A Pacific-wide tsunami was triggered by the earthquake, which had a surface-wave magnitude of 8.3, an epicenter of 51.5° N, 175.7° W, and a focal depth of 33 km. Even though no lives were lost, the Hawaiian Islands suffered the greatest with damage costs approximately $5 million (1957 dollars).

The Island of Kauai, Hawaii, was hit twice as hard by this tsunami than by the Aleutian Islands tsunami in 1946. Houses were washed out and destroyed at Wainiha and Kalihiwai. At Haena, the waves reached heights of 16 m. In addition to that bridges were destroyed and sections of highways were flooded. At Hilo, Hawaii, the run-up was reached 3.9 m and damaged buildings. In Hilo Bay, Cocoanut Island was covered by 1 m of water and the bridge connecting it to shore was destroyed.

Although the northwest side of the Hawaiian Islands received high levels of water, the rest of the islands only received elevated water levels on average of 2 to 3 m. Both the 1946 and 1957 tsunamis occurred in the same general location (the Aleutian Islands). Even though the 1957 earthquake released more energy than the earthquake of 1946. The tsunami generated by this 1957 event caused less damage than the tsunami of 1946. This uncertainty of the potential destructive power of a tsunami forces Pacific Tsunami Warning System to issue warnings even when a tsunami may have little or no effect.

A severe and locally damaging tsunami generated on Kamchatka by a magnitude 8.2 earthquake struck the Hawaiian Islands at 1:00 P.M. Property damage from these waves was estimated at $800,000 to $1,000,000; however, no lives were lost. The waves beached boats, caused houses to collide, destroyed piers, scoured beaches, moved road pavement, etc. A farmer on Oahu reported 6 cows killed. In Honolulu harbor, waves tore a cement barge from its moorings and hurled it against the freighter Hawaiian Packer. At Pearl Harbor, Oahu, the tsunami was evidenced by the periodic rise and fall of the water, but no damage was done. Loomis (1967) reports wave heights of 9.1 m at Kaena Point, Oahu. Pararas-Carayannis and Calebaugh, (1977) report much damage on Oahu’s north coast including Waialua. A boathouse worth $13,000 was demolished in Hilo when water 2.4 m high swept over the wharf. One span of the bridge to Coconut Island was destroyed. The highest wave on Hawaii of 3.5 m above MLLW (or 3.2 m above the tide stage) was reported here and at Reed’s Bay. The Naniloa Hotel had flood damage. Houses were knocked from their foundations. Coast Guard buoys weighing 11 metric tons were ripped loose from their moorings. Damage in Hilo, Hawaii was estimated at $400,000. Damage on Maui was greatest in the Kahului-Spreckelsville area. The wave caused the tide gaga at Kahului to go off scale and stop recording. Pararas-Carayannis and Calebaugh report 10.4 m at Haena Point, Kauai, but this is identical to the value for the March 3, 1957 tsunami and is probably a misplaced value. They also report much damage to the north coast of Kauai.

A magnitude 8.2 earthquake off the Kamchatka Peninsula produced a tsunami that was observed in Alaska. At Massacre Bay, Attu the wave had an amplitude of 2.7 m and a period of about 17 minutes. This record was observed on the tide staff as the gage was not operating initially and the record was clipped. Low-lying areas were flooded. At Sweeper Cove, Adak the tsunami had an amplitude of about 1.1 m and slightly overflowed the banks of the harbor. At Dutch Harbor, Unalaska the schools were closed, and the people evacuated to higher ground, but the wave was only 0.6 m high. It was widely recorded elsewhere throughout Alaska with amplitudes of 0.3 m or less.

On April 1, 1946, at 12:29 GMT, an earthquake occurred in the Aleutian Islands of Alaska. A Pacific-wide tsunami was triggered by the earthquake, which had a surface-wave magnitude of 7.8, an epicenter of 52.8° N, 163.5° W, and a focal depth of 25 km. Before the tsunami dissipated it took the lives of more than 165 people and cause over $26 million (1946 dollars) in damage.

One of the structures affected by the tsunami was the newly built Scotch Cap Lighthouse on Unimak Island, Alaska. At the lighthouse 5 men lost their lives and the run-up reached 35.0 m. The Hawaiian Islands were one of the hardest hit locations by the tsunami. Pololu Valley recorded the highest run-up of 12.0 m on the Island of Hawaii. However, Hilo was the city that received the most damage on the Island of Hawaii. The tsunami arrived at Hilo 4.9 hours after it originated in the Aleutian Islands and the run-up was measured at 8.1 m. Hilo received $26 million in damage and 96 people died.

Scotch Cap Lighthouse on Unimak Island, Alaska as it looked before the earthquake and tsunami. The structure was built in 1940. It was 40 feet above the sea and was five stories high. Photograph Credit: U.S. Coast Guard. Source: National Geophysical Data Center.

All that remained of Scotch Cap Lighthouse on Unimak Island, Alaska after the tsunami. Photograph Credit: U.S. Coast Guard. Source: National Geophysical Data Center.

The large number of deaths from this event brought the realization that a warning system was necessary to ensure the safety of the populace. On August 12, 1948, a plan was approved and the Seismic Sea Wave Warning System was established. Later its name would be changed to the Pacific Tsunami Warning System

Tsunami Related Fatalities