A powerful earthquake struck western Japan this afternoon, killing an estimated 75 people, injuring hundreds and trapping hundreds more in collapsed buildings. The quake, which measured 7.2 on the Richter scale and was followed by at least 17 aftershocks, hardest in the major port city of Kobe, where fires burned out of control, trains derailed and a major elevated expressway toppled, spilling about 50 vehicles onto the street below. The shaking, which began at 3:46 p.m. EST (5:46 a.m. in Japan) and lasted about 20 seconds, was also felt strongly 22 miles away in Osaka, Japan's second-largest city. Nearly 200 people were believed buried in rubble in Ashiya, a posh residential district between Kobe and Osaka. Japan's Cabinet was scheduled to hold an emergency meeting on the disaster tonight.