THE focal mechanisms of micro-aftershocks occurring 3.5 yr after the Inangahua (New Zealand) earthquake of May 23, 1968 (magnitude 7.1), differ drastically from those of the main shock and early aftershocks and are in apparent conflict with the known tectonic stress. This anomaly calls for an explanation as microearthquakes are widely regarded as reliable indicators of tectonic stress. A possible explanation is suggested by the dilatancy hypothesis, which has recently received much attention as a promising basis for earthquake prediction1–4.