At 6:48 a.m. local time, an earthquake shook the floor of the Pacific Ocean. Seismometers detected a magnitude 7.9 quake and alerted the Pacific Tsunami Warning Center in Hawaii.

By 7:03, the center had sent out a warning bulletin. "An earthquake of this size has the potential to generate a destructive tsunami," it read. "Authorities should take appropriate action."

Minutes later, the center's pressure sensors anchored to the ocean floor felt the pressure wave of the tsunami pass and transmitted this information via buoys on the surface. Calculations confirmed waves up to a meter and a half higher than sea level, four meters trough-to-crest.

"Sea level readings indicate a tsunami was generated," read a new warning bulletin.

But even before the first warning was issued, it was too late for the inhabitants of the island American Samoa, which had been struck by the tsunami five minutes after the first of the tremors. Preliminary reports indicate that at least 100 people were killed by the waves of water, which struck Samoa, American Samoa, and Tonga.

On Tuesday, a section of the seafloor along the boundary of the Pacific and Australian tectonic plates shifted directly upward, creating the tsunami. The uplift was caused by a "shallow" earthquake that shook for two to three minutes and was upgraded to a magnitude of 8.3 by the end of the day. It occurred about 20 miles below the surface and centered about 120 miles south of the islands of Samoa.

This article is provided free for media use by Inside Science News Service.