Quake prediction has proved notoriously difficult. But a special type of jolt, triggered by underground water pressure, has been successfully predicted by scientists in India, according to a story posted online by the American Association for the Advancement of Science.

The prediction was made on May 12, based on a swarm of small temblors. The scientists said a magnitude 4.0 quake would occur with 15 days. A magnitude 4.2 quake struck May 21.

The work was led by seismologist Harsh Gupta at the National Geophysical Research Institute in India and is detailed this month in the Journal of the Geological Society of India, according to the AAAS story.

Prediction of other types of earthquake remains a rough science at best. One recent advancement in California is predicting the worst part of a quake—with warnings of a few seconds—just after the shaking begins.