Earthquakes with anomalous non-double-couple (non-DC) mechanisms have been observed at many geothermal and volcanic areas, and possible physical explanations have been offered for them that involve tensile fracture caused by high fluid pressure or thermal contraction due to cooling. Nevertheless, the possible biasing effects of wave propagation in structurally complicated volcanic regions has made the identification of non-DC mechanisms as true source phenomena uncertain.
Non-DC earthquakes are common at the Hengill-Grensdalur volcanic complex in southwestern Iceland. Of 178 microearthquakes recorded by a 23-station network in the summer of 1981, 84, or 47%, have non-DC mechanisms, based on conventional interpretation of P-wave first motions using a one-dimensional crustal model. In this paper, we re-analyze the best constrained of these events by numerically tracing rays in a three-dimensional tomographically derived model of the crust. Computed positions of rays on the focal spheres change by as much as 35°, with the largest changes being caused by revision of focal depths. The number of earthquakes with clearly non-DC mechanisms remains large, however (56 out of 131 or 43%); the anomalous mechanisms are not artifacts of the mis-mapping of rays onto the focal sphere.