"It was a lot of swaying, you could hear the building creaking in it ... you could see the shades shaking back and forth," Castle said. "The people that I work with here in Tokyo, it was normal for them, but shortly after that it started to get scary the longer it went."
"We were just hanging out in Shibuya [Tokyo] today and walking down the street and all of a sudden felt like we were on a boat and looked up and tall buildings were going crazy, looking like they were going to tip over," said Kevin Williams, an American vacationing in Tokyo.
It's the strongest earthquake the world has seen since the 9.1 magnitude Indian Ocean quake in 2004, which triggered a massive tsunami. The combination of the quake and tsunami left 230,000 people dead.
The Japan quake hit at 2:46 p.m. local time in Japan, and lasted an astonishing five minutes. The devastating earthquake in Northridge, Calif., in 1994 lasted just six seconds.
"The bigger the earthquake, the larger the size of the fault that has to rupture to make it happen. You're seeing waves generated along a huge fault," California Institute of Technology seismologist Kate Hutton said.
At least 125 aftershocks -- some as large as 7.4 -- have struck Japan, reversing the path of rivers, washing away boats and cars, and leaving buildings shaking.
ABC News' Michael James and the Associated Press contributed to this report.