An Implosive Precursor to the Colombia Earthquake 1970 July 31

Abstract

A direct search has been made for the effects of the implosive precursor to the Colombia earthquake, 1970 July 31 proposed by Dziewonski & Gilbert from spectral studies. Theoretical seismograms, calculated for various models of the time‐dependence of the isotropic part of the moment tensor, have been compared with observed records for the period preceding the earthquake for 10 nearby WWSSN stations and the strain network at Ñaña, Peru. Whilst no significant correlation was found between observation and theory at any of the WWSSN stations, a correlation coefficient of more than 0.9 was found for the Ñaña record for precursor times around 200 s. Since this latter record is nearly free of instrumental effects the high correlation coefficient gives good direct evidence for the existence of the precursor. Copyright © 1976, Wiley Blackwell. All rights reserved