The Ministry of Justice has issued the following guidelines:
Immigration control measures for extension of the period of stay and other related matters in accordance with the occurrence of the Tohoku Region Pacific Coast Earthquake Disaster (.pdf)
Bottom line: if you are a foreign resident of Tohoku and your visa is up for renewal, the Ministry is granting you a temporary extension until the end of August. This extension does not apply to your re-entry permit so, if you want to travel abroad, you will still need to ensure your permit is up-to-date.