COSEISMIC COASTAL UPLIFT AND CORALLINE ALGAE RECORD IN NORTHERN CHILE- THE 1995 ANTOFAGASTA EARTHQUAKE CASE

Coralline algae that may be predominant in the upper part of the infra littoral zone along rocky shorelines proved to be a useful indicator o f rapid coastal uplift. As these encrusting algae cannot survive desic cation, even for short periods at low tide, they can provide estimates of positive vertical motions like those which may accompany seismic e vents. The desiccation of a fringe of the algal encrustment, at the ba se of the intertidal area (=infralittoral fringe), combined with effec ts of solar radiation rapidly bills the organisms. This mortality resu lts in a conspicuous alteration of the pigmentation from pink/beige/re ddish to white. After the July 30, 1995 Antofagasta earthquake (M(w) 8 .1), in northern Chile, such a white fringe appeared in some parts of the bay of Antofagasta and surroundings. The width of the dead algae f ringe varied from 0 to more than 1.8 m. The widest observed widths are related to local parametres (exposition to wave splash, geometric dis position) that account for an amplification of the width of the dead a lgae fringe, and must be identified. Thus, a careful study of each loc ality led us to determine the extent of the coastal areas that had bee n uplifted, and to reconstruct, with a precision of the order of 2 cm, the amount of the vertical deformation along the Antofagasta Bay and southern Mejillones Peninsula. It was thus shown that the coastline bo rdering the town of Antofagasta suffered practically no coseismic upli ft, while areas to the south and to the west of Antofagasta Bay proved to have been uplifted by as much as 25 and 40 cm, respectively. The m aximum uplift (80 cm) was seen at the southwestern tip of the peninsul a of Mejillones. These precise reconstructions are of great help for t he calibration of geodetic studies performed independently and for the modelling of the coseismic deformation at a regional scale. Copyright (C) 1996 Elsevier Science Ltd