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Samar Island, Philippines, is located on the eastern side of the central or Visayan Islands. Here the Talavera limestone, locally as much as 400 m thick, of Miocene age unconform-ably overlies an argillized lower Tertiary tuff. It grades into and is fringed by upper Miocene to Pliocene clastic rocks called the Barili Formation.

By the end of Pleistocene time, the Talavera limestone had developed karst with maximum relief of 300 m. Internal drainage saturated the argillized tuff, and earthquake mobilization caused a limestone block 18 x 25 km to glide northward on a slope of 0.6° for about 5 km. This beheaded the delta in Maqueda Bay, moved the drainage divide from the center to within 1 km of the west coast, and dammed the Ulot and Tubig Rivers. Behind the dams, estuary-like lakes were formed and filled with sediment. Subsequent headward erosion of these rivers cut gorges and drained the lakes.

There would be no way of controlling a slide of this size, and some engineering projects could actually cause conditions allowing such a slide to be triggered by an earthquake. This illustrates that in a region of composite hazards it is necessary to consider features an order of magnitude larger than would otherwise be of concern.

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