The 2009 Samoa earthquake had a moment magnitude of 8.1. It struck on 29 September 2009 in the southern Pacific Ocean near the Tonga–Kermadec subduction zone and generated a deadly tsunami.
The event began with a magnitude 8.1 normal-faulting earthquake in an extensional setting on the outer rise of the subduction zone. Within about two minutes, two magnitude 7.8 earthquakes occurred on the subduction-zone interface. Together, the sequence is sometimes described as a doublet earthquake, but the principal earthquake's listed magnitude is 8.1.
The resulting tsunami caused severe damage in Samoa, American Samoa, and Tonga. Waves reached as high as about 14 meters on parts of the Samoan coast, and at least 189 people died across the affected region. The event was the largest earthquake recorded in 2009.
The magnitude should not be confused with tsunami height or earthquake intensity. Magnitude measures the earthquake's released energy, while intensity describes shaking at a particular location and run-up measures how far tsunami water rises on land.