Errors in extension measurements from planar faults observed on seismic reflection lines

Abstract

Abstract The preferred model for the extension of brittle crust involves the rotation of planar fault blocks. We show that in general the distortion at the ends of the blocks does not affect the measurement of extension. The horizontal displacement on a normal fault, the heave, is observed with little distortion on a seismic reflection time‐section. It can be used to estimate the amount of extension. We demonstrate that the sum of the heaves is not equal to the actual elongation if the blocks have rotated. However, the error in the extension factor, β, introduced by equating elongation with the sum of the heaves is small. It increases with the amount of rotation from 0 for no rotation to 13% for the maximum observed angle of rotation of 30o. We compare this value with the practical error introduced by uncertainties in seismic velocities when the elongation is measured from a depth‐converted seismic section. This latter error is significantly smaller being approximately 5% for an error in velocity of 20% when the rotation angle is less than 30o.