Anomalous focal mechanisms: tensile crack formation on an accreting plate boundary

Abstract

Conventional double-couple solutions do not satisfactorily account for many small magnitude earthquakes in the Hengill geothermal area in the Neovolcanic Zone of Iceland. The far-field radiation pattern is predominantly compressional but a few dilatational arrivals have been recorded. We prefer the interpretation that these events are due to tensile crack formation at a shallow (1–7 km) depth within a cooling intrusive body. Volume considerations indicate that initial fracturing of the rock is seismic but that subsequent widening proceeds aseismically. Other reports of anomalous radiation patterns from earthquakes on the Mid-Atlantic Ridge describe events of a similar type, that is, exhibiting predominantly compressional radiation patterns and a reduced dilatational field1–4.