NEW : Japan 's Environment Agency issues an alert . Okinawa resident Eric Shepherd said his grandmother-in-law described it as the strongest quake she had felt in her 90 years on the island . ` NEW : ` ` There may be slight sea level rise from now on , '' agency says . Storm centered about 6 miles deep , 20 miles east of Okinawa .
NEW : Japan 's Environment Agency issues an alert . NEW : ` ` There may be slight sea level rise from now on , '' agency says . Storm centered about 6 miles deep , 20 miles east of Okinawa .