Coseismic coastal uplift and coralline algae record in Northern Chile: The 1995 Antofagasta earthquake case

Abstract

Coralline algae that may be predominant in the upper part of the infralittoral zone along rocky shorelines proved to be a useful indicator of rapid coastal uplift. As these encrusting algae cannot survive desiccation, even for short periods at low tide, they can provide estimates of positive vertical motions like those which may accompany seismic events. The desiccation of a fringe of the algal encrustment, at the base of the intertidal area (=infralittoral fringe), combined with effects of solar radiation rapidly kills the organisms. This mortality results in a conspicuous alteration of the pigmentation from pink/beige/reddish 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 fringe varied from 0 to more than 1.8 m. The widest observed widths are related to local parameters (exposition to wave splash, geometric disposition) that account for an amplification of the width of the dead algae fringe, and must be identified. Thus, a careful study of each locality led us to determine the extent of the coastal areas that had been 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 bordering the town of Antofagasta suffered practically no coseismic uplift, 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 maximum uplift (80 cm) was seen at the southwestern tip of the peninsula of Mejillones. These precise reconstructions are of great help for the calibration of geodetic studies performed independently and for the modelling of the coseismic deformation at a regional scale.