The Rasul of Allah (saw) says:

“Idhaa zulzilat is equivalent to half of the Quran! Kul huwallahu ahad is equivalent to a third of the Quran. Kul ya ayyuhal kafiroom’ is equivalent to a quarter of the Quran.”^[1]

According to my understanding, this hadith means the Quran is based upon two primary themes:

 

1. There is no deity-god; we are here to realize the Unity and Oneness of Allah and to fulfill its requirement as best we can.
2. The necessity of engaging in certain practices to prepare for the life after death and the truth that the individual will face the consequences of every action transpired from them, whether good or bad, even an iota’s weight.

 

So, since chapter Zalzala mentioned above is the perfect summary of the second point above, it is equivalent to half of the Quran.