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Relation between the geology and the effects of the earthquakes of May 1960 in the city of Ancud and vicinity, Chiloé

INSTITUTO DE INVESTIGACÍONES GEOLOGICAS, SANTIAGO, Chile
U. S. NAVAL ORDNANCE TEST STATION, CHINA LAKE, CALIF.

Abstract

The rocks and sediments of Ancud and suburbs have been grouped into five units. The volcanic rocks are included in a unit, or geologic formation, called the Volcanic Complex of Ancud. The complex is tentatively considered to be Tertiary or Quaternary in age and is formed of andesites, welded tuffs, breccias and agglomerates that outcrop in almost all the hilly region, the coast, and the surroundings of Ancud. In some sections terraces have been formed over the rocks of this complex. The outcrops of the complex are weathered to a reddish-yellow material easily disaggregated with a hammer. Terrace deposits are observed in some of these terraces to be poorly cemented gravel, clay, and limonite. The alluvial deposits are distributed in the stream courses of Rio Pudeto and in Matadero and La Toma Creeks; they are composed of unconsolidated gravels, sands, silts, and clays. Beach deposits are observed along the length of the coast, and are principally a poorly consolidated aggregate of gravels and sands. Artificial fill is deposited in the coastal zone; its materials are gravels, sands, silts, bricks, wood, debris, etc.

There is a relation between the geology and the damage produced by the earthquakes in works of man. The areas underlain by the Volcanic Complex of Ancud are those that best resisted the effects of the earthquakes, and these are suggested as the most suitable for the expansion of the city. The most serious damage from the earthquakes are observed over the alluvial deposits and at the edges of one of the terraces. The last observation suggests that these areas are unfavorable, and that any future plans for construction over these should be preceded by a study of soil mechanics. It is indicated that the geographic position of Ancud with respect to tidal waves is unfavorable. It is suggested that the construction be prohibited near the sea, and that absolutely indispensible coastal buildings should be carefully studied with regard to a possible repetition of the phenomena.