Dual seismogenic behavior: The 1985 Central Chile earthquake

Repeated leveling lines evidenced almost half a meter of uplift at coastal localities nearby the epicentral region of the March 3, 1985, Ms=7.8, Central Chile earthquake. A tide gauge at Valparaiso and two limnigraphs, 27 km apart, situated at the extremes of Rapel Lake, to the south of the leveling line, have been recording continuously sea level and the equipotential lake level for more than 10 years, providing a permanent very-long base tiltmeter. 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; it has a rise rime of approximately 10 months with a maximum amplitude of 120 mm or 4.4 mu radians in tilt. The overall shape of the time-dependent tilt is mimicked by the sea level signal recorded at Valparaiso, about 100 km away from Rapel Lake, showing a maximum coastal subsidence of 0.6 m. Gravity surveys carried out in three different pre and postseismic epochs, along the segment of the leveling line which shows major coseismic uplift, indicate that the whole region has subsided postseismically, 10 cm in five years. Simple two-dimensional models for both co and postseismic stages indicate that at least 1.2 m of average fault displacement in ten months of postseismic movement along the contact between Nazca and South America plates is responsible of the time dependent elevation changes, 25% less of what was deduced with the same method for the coseismic stage.