Historic Earthquakes

Leeward Islands
1974 October 08 09:50:58 UTC
Magnitude 7.5

The Leeward Islands Earthquake of October 8, 1974

by James W. Dewey
U.S. Geological Survey

"On Wednesday, 8th February, 1843, the very day after the meeting of the Vestry, the island was visited by a most terrific and destructive Earthquake. . . .

Everyone within the Church after the first shock was compelled to escape for their lives. The Tower was rent from the top to the bottom, the north dial of the clock precipitated to the ground beneath with a dreadful crash, and the east parapet wall of the Tower thrown upon the roof of the Church. Almost the whole of the north est wall of the north gallery fell out in a mass. The north east wall was protruded beyond the perpendicular. . . .

Thus within the space of three minutes, the Church was reduced to a pile of crumbling ruins, the walls that were left standing being rent in every part, the main roof only remaining sound, being supported by the hardwood pillars."

-- The Minutes of the Vestry
Cathedral Church of St. John the Divine
Antigua

The Leeward Island earthquake of October 8, 1974, produced the strongest shaking in several of the Leeward Islands since the great earthquake of February 8, 1843.

The 1974 earthquake had a magnitude of 7.5, making it one of the largest earthquakes of the year, worldwide, in terms of seismic energy released at the earthquake source. Fortunately, the source was tens of kilometers from the nearest inhabited land. For this reason, damage was generally unspectacular, and there does not seem to have been loss of life.

Damage was costliest on Antigua. The Cathedral of St. John, built at the site of the cathedral destroyed in the 1843 earthquake, suffered extensive damage to its masonry exterior. The new deepwater harbor facility at St. Johns, Antigua, was damaged by settling of the fill on which it was constructed. Equipment and buildings of the West Indies oil refinery, on the outskirts of St. Johns, were damaged, and thousands of barrels of crude oil leaked from tanks.

Elsewhere, sparsely inhabited Barbuda was strongly shaken, On St. Kitts, the tower of the church of St. Thomas fell through the roof of the church causing serious damage, although the level of shaking which produced the tower collapse was probably less than the shaking at Barbuda and Antigua.

The damage data from the Leeward Islands earthquake are valuable as guides to the interpretation of historical earthquakes. For the 1974 earthquake, we have damage data for both modern reinforced-concrete structures and for unreinforced masonry buildings which are more than a century old. For the historical earthquakes, we have damage information only for the unreinforced masonry structures. What the historical earthquakes might have done to modern structures must be inferred from the damage pattern of the 1974 earthquake. For example, it is clear that the 1983 earthquake caused destruction over a much wider area than the 1974 earthquake. In 1843, sever damage occurred at locations damaged by the 1974 earthquake and, in addition, at Montserrat, Nevis, English harbor (Antigua), and Guadeloupe (south of Antigua), which experienced little damage in the 1974 shock. In general, the duration of strong ground shaking in earthquakes increase with the size of the zone of damage caused by the earthquakes. It is probable, therefore, that strong ground motion in the 1843 earthquake lasted significantly longer than strong ground motion in the 1974 earthquake. Modern structures in the vicinity of the Leeward Island which are susceptible to damage from sustained strong or moderately strong ground motion would likely to be damaged much more from a repetition of the 1843 earthquake than they were in the earthquake of October 8, 1974.

Abridged from Earthquake Information Bulletin, March-April 1975, Volume 7, Number 2.