Various masaail of Jamaat Salaah and Imaamat

Should the Imaam terminate the salaah in the case of an earthquake?

Q: If an Imam is leading an obligatory prayer in the Masjid and an earthquake strikes severely, under the Shariah, should he not suspend that salah and ensure for human safety and not show heroism? Taking the cue from the ruling on rains and harsh cold weather and the Prophet (sallallahu alaihi wasallam)'s command to stay at home and pray, is earthquake not a bigger calamity? 

Completing the salaah individually if the imaam performs 3 rakaats

Q: Our imaam offered 3 rakahs instead of 4 rakahs in asr farz Salah and ended the salah. No one interrupted him for his mistake and all the people ended offering 3 rakahs, then people said it was only 3 rakahs completed so the imam repeated the 4 rakahs again and every body prayed 4 rakahs again.

My question is if a person is behind a imam in the above case and is aware that the imam has made a mistake can he individually continue the same Salah and complete it ?