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

INSTITUTO DE INVESTIGACIONES GEOLOGICAS, SANTIAGO, Chile
US NAVAL ORDNANCE TEST STATION, CHINA LAKE, CALIF.

Abstract

The rocks and sediments of the region have been divided into eight units: metamorphic rocks of pre-Mesozoic age, composed of mica schists; volcanic rocks, columnar andesites, tentatively assigned to the Tertiary, that outcrop in a small area to the west of Castro; fluvioglacial deposits, uncemented sands and gravels that in almost all cases overlie three terraces of Pleistocene age that are indicated on the map as morphologic units; slope deposits, friable hybrid material neither classified nor consolidated, that cover one of the most dangerous areas in which to construct; beach deposits—unconsolidated coastal sediments; unconsolidated alluvial sediments distributed in the beds of rivers and in the river deltas presently in formation; and deposits of artificial fill, gravels, sands, bricks, trash, etc.

Damage to structures is clearly related to the geology. In areas of artificial fill and of slope deposits, catastrophic destruction was caused almost exclusively by geologic and morphologic factors. Some areas with a favorable morphology, such as the intermediate and high terraces, are formed of sound material, such as metamorphic rocks, and had little or no damage.