Source

Tsunamis

Unknowns

Source of the 1755 Lisbon tsunami—Offshore faulting related to collision of the Nubian (African) and Eurasian plates29

The 1755 tsunami was noted in the Caribbean, from Barbados northwestward to Cuba. Its maximum estimated Caribbean height is 7 m. The tsunami is unknown from ports along the U.S. Atlantic seaboard, probably because of shielding by submarine hills that directed the transatlantic waves northwestward toward Newfoundland and southwestward toward the Caribbean and Brazil.30

How did the 1755 tsunami affect Puerto Rico and the Virgin Islands? Does it account for overwash of Anegada, in the British Virgin Islands northeast of Puerto Rico (Fig. 3-3e)? How often can tsunamis like the one in 1755 be expected?

Seattle fault—One of several faults capable of displacing waters of Puget Sound.31

Six-meter uplift along the Seattle fault generated a tsunami in Puget Sound during an earthquake about 1,100 years ago.32 The same earthquake set off slides in Lake Washington.33

How often does the Seattle fault produce earthquakes like the one 1,100 years ago? Do tsunamis result from slip limited to the Seattle fault’s backthrusts, which have a post-glacial history of repeated earthquakes?34

SLIDES—Most slides that set off tsunamis have been triggered by earthquake or, less commonly, by volcanic eruption. Several grand examples:

• Lituya Bay, Alaska, 1958—An earthquake-induced rockslide in 1958 set off a giant wave that trimmed trees to an altitude of 525 m.35

• Sunda Strait, Indonesia, 1883—The explosion of Krakatau triggered a tsunami that killed an estimated 35,000 persons.36

• North Sea, 8,000 years ago—The Storegga slide displaced 2,400-3,200 km3 of ocean-bottom materials37 and generated waves known from tsunami deposits in Norway and Scotland.38,39

• Big Island of Hawaii, 120,000 years ago—Flank collapse produced tsunami run-ups to heights of hundreds of meters.40 A catastrophic ancestor to the local Hawaiian tsunamis that killed 46 persons in 1846 and 2 in 1975.41

Slide-generated tsunamis rarely amount to much on distant shores. Compared with the areas of ocean floor displaced by faulting during great subduction zone earthquakes, their source areas are usually compact. Slides therefore yield tsunami waves of short period that diminish rapidly with distance. This decrease helps limit the hazards to the U.S. Atlantic coast from flank collapse in the Canary Islands, off West Africa.42

Alaskan slides during the 1964 earthquake—Slides at Chenaga,43 Kenai Lake,44 Seward,45 Valdez,46 and Whittier47

The separate tsunamis from the Chenaga, Seward, Valdez, and Whitter slides together account for 79 of the 106 Alaskan deaths from tsunamis that the 1964 earthquake triggered (Fig. 3-2e). Most of the slides resulted from shaking-induced failures of deltas.

What do these slides imply for Puget Sound deltas as potential tsunami sources?31



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