Why did the tsunami in Samoan islands happen?

On September 29, 2009, a powerful earthquake centered just south of American Samoa's Tutuila Island was the source of widespread damage and numerous fatalities along the southern shores of all islands within American Samoa, and along the northwestern coastline of Upolu in nearby Western Samoa (now officially Samoa).

- The quake produced a tsunami; its maximum run-up along the southeastern edge of Tutuila is estimated to have exceeded 174ft, making it the largest ever instrumentally recorded in the Southern Hemisphere and one of the deadliest tsunamis on record

- The shallow Mw 8.1 earthquake occurred at 06:48 in the morning at a very shallow crustal depth beneath the shallow submarine trench north of the American Samoa Islands

- Initial aftershocks (all greater than M 6.2) occurred immediately, with additional strong shocks ( M > 6) registered for three weeks