Central Chile: An example of quasi-static crustal behavior

Indications of transient crustal displacement associated with the 3 March 1985, M-s = 7.8, Central Chile earthquake are evidenced by various observational devices. Almost half a meter of coastal uplift at localities close to the epicentral region was detected by repeated leveling lines. A tide gauge at Valparaiso revealed minor coseismic coastal subsidence (similar to 10 cm) continuously developing after the earthquake. Two limnigraphs, 27 km apart, that were situated at the extremes of Rapel Lake to the south of the leveling line, have recorded continuously the equipotential lake level for more than 10 years, providing a permanent very-long-base tiltmeter. The water level difference at the two limnigraphs as a function of time resembles a ramp function, beginning approximately at the time of the earthquake occurrence and gradually developing over a period of 10 months with a maximum amplitude of 120 mm or 4.4 mu radians in tilt. The shape of the time-dependent tilt is mimicked by the sea level signal recorded at Valparaiso, similar to 100 km away from Rapel Lake, showing a maximum coastal subsidence of 0.6 m. Comparisons of sea level changes produced by the 1971, M-s = 7.5, earthquake indicates that they represent rupture in different portions along the seismogenetic region as well as a different rupture mode. Gravity surveys carried out in three different pre-and post-seismic epochs, along the segment of the leveling line which shows major coseismic uplift, indicate that the whole region has subsided, post-seismically, 10 cm in 5 years. These observations are interpreted in terms of a variable slip dislocation model. Inversions show that it is the more than 2 m of fault displacement in 10 months of post-seismic movement along the contact between the Nazca and South America plates, which is interpreted to be responsible for the time-dependent elevation changes.