A New Interpretation of the 1968 Fernandina Caldera Collapse and its Implications for the Mid‐Oceanic Ridges

Abstract

The earthquake swarm which accompanied the Fernandina Caldera collapse is shown to consist of two different types of earthquake: lowstress events occurring on the main caldera fault and higher stress events on minor faults. The difference in scale of these faults allows the two types of earthquake to be distinguished by magnitude. Events occurring on the main caldera fault have a high ‘b’ value, those on minor faults a normal ‘b’ value. The fall in magma chamber pressure which led to the collapse is shown to have been quite small and hence a model of the volcano is developed which can account for the magnitudes of the earthquakes observed, the stress on the main caldera fault and the seismic efficiency. The seismicity of the rift zone of the Mid‐Atlantic Ridge has many similarities with that of the Fernandina collapse and it is proposed that the median valley which is characteristic of slow‐spreading ridges is a zone of caldera collapse. No such collapse occurs along the axes of fastspreading ridges, so these are relatively aseismic. Copyright © 1974, Wiley Blackwell. All rights reserved