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From October 5–8,1985, a tropical wave centered about 25 km northeast of Ponce, Puerto Rico, produced as much as 560 mm of rainfall in 24 hours and as much as 70 mm in one hour. This extraordinarily heavy rainfall triggered a rock-block slide that destroyed much of the Mameyes residential area, on the northwest outskirts of Ponce.

The Mameyes landslide failed in three distinct phases between 3:00 and 4:00 a.m. (local time) on October 7, 1985. The first two phases of sliding involved translational sliding of two 12-m-thick slabs of calcareous-sandstone bedrock along bedding-plane surfaces parallel to the slope surface. The third phase of sliding involved the toppling failure of a block that disaggregated and formed a rock fall on the western part of the slide. Subsidiary flow failures onto the toe and from the downstream face of the toe were triggered by the heavy rainfall and the rupture of a water pipe that emptied as much as 4 million liters of water onto the slide.

The Mameyes landslide is approximately triangular in plan view; the maximum width and length are both about 250 m. The total area of the landslide is ~35,000 m2, and the slab of bedrock and soil that failed is about 12 m thick and comprises ~300,000 m3. The landslide moved downslope 30 m, parallel to the 17°–24° bedrock dip. At most, an estimated 120 homes were destroyed by the landslide, and at least 129 people were killed, though only 39 bodies were recovered. This death toll is the largest from a single landslide in North American history.

Calculations of the prelandslide slope stability at the site indicate that when the water table was below the potential shear surface, the factor of safety against failure was about 1.26. A water table about 6 m above the shear surface, which would saturate half the thickness of the slide block, reduces the factor of safety to 1.0. A water table at this level is a reasonable result of the conditions preceding failure.

The landslide area has been stabilized by engineering measures and was developed as a memorial park to the landslide victims.

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