Why Did the Stock Index Drop After the Japan Earthquake and Tsunami 2011?

I know that the stock index fell in many countries after the Earthquake, however I just don't understand why? Could someone help me out? Or at least sort of explain how the Stock Index works?

What is a VPN?

VPN is (Virtual Private Network) were first used by companies to enable their employees to securely access internal systems such as email remotely (e.g. from home or while on business trips). Today they are increasingly being used for personal use by individuals to protect their privacy while online in public places (e.g. when using the wi-fi connection in a cafe) or in a country where the internet is censored / blocked (e.g. China, Saudi Arabia…).