LONDON (Reuters) - Bitcoin tumbled 18 percent on Tuesday to a four-week trough close to $11,000, after reports that a ban on trading of cryptocurrencies in South Korea was still an option drove fears grew of a wider regulatory crackdown. Bitcoin’s slide triggered a massive selloff across the broader cryptocurrency market, with biggest rival Ethereum down 23 percent on the day, according to trade website Coinmarketcap, and the next-biggest, Ripple, plunging 33 percent.