Which Surah Is Equal to half of the Quran?
Question

Which surah of Al-Quran when recited is equal to reciting 1/2 Al-Quran?
Praise be to Allah.Analyzing Hadiths about the Virtue of Surat Az-Zalzalah
There are many Hadiths which speak of the virtue of certain Surahs of the Quran, some of which are authentic, but many of them are inauthentic. That includes the Hadith which speaks of the virtue of Surat Az-Zalzalah and says that it is equivalent to half of the Quran. A number of Munkar (inauthentic) Hadiths were narrated concerning that. They are as follows:
Examining the First Hadith

It was narrated that Ibn `Abbas (may Allah be pleased with him) said: The Messenger of Allah (peace and blessings of Allah be upon him) said: “ `When the earth is shaken with its (final) earthquake’ (i.e., Surat Az-Zalzalah) is equivalent to half of the Quran.” (Narrated by At-Tirmidhi, 2894 and Al-Hakim, 1/754)

Its chain of narration includes Yaman ibn Al-Mughirah Al-`Anzi, concerning whom Al-Bukhari and Abu Hatim said: his Hadith is Munkar. Ibn Hibban said: his Hadith is very Munkar; he narrates Munkar Hadiths that have no basis and deserve to be ignored. (Tahdhib At-Tahdhib, 4/452)
Yaman is the only one who narrated this Hadith, alone among the companions of `Ata’, and he is not acceptable as a sole narrator of a Hadith, rather the fact that he is the sole narrator indicates that it is Munkar. And Allah knows best.
The majority of scholars are of the view that the Hadith is to be regarded as weak because of Yaman ibn Al-Mughirah, in contrast to those scholars who classed it as authentic. It was classed as inauthentic by At-Tirmidhi when he said, after quoting it: it is Gharib and we know it only from the Hadith of Yaman ibn Al-Mughirah. It was also classed as such by Ibn `Abd Al-Barr in At-Tamhid, where he said: It is one of the Hadiths of the Shaykhs, not one of the Hadiths of the imams. And it was classed as inauthentic by Ibn Hajar in Fat-h Al-Bari (8/687), by Adh-Dhahabi in Talkhis Al-Mustadrak, by Al-Manawi in Fayd Al-Qadir (1/367) and by As-Shawkani in Al-Fat-h Ar-Rabbani (12/5930). Al-Albani said in As-Silsilah Ad-Da`ifah (1342): it is Munkar.
Examining the Second Hadith

It was narrated from Anas ibn Malik (may Allah be pleased with him) that the Prophet (peace and blessings of Allah be upon him) said: “Whoever recites `When the earth is shaken with its (final) earthquake’, it will be equivalent to half of the Quran for him.” (Narrated by At-Tirmidhi, 2893)

Its chain of narration includes Al-Hasan ibn Salim Al-`Ajali. Al-Mizzi said: He is a Shaykh who is unknown and recited only one Hadith. Al-`Aqili said: He is unknown with regard to transmitting Hadith. Ibn Hibban said: he narrates from trustworthy narrators things that do not sound right. (Tahdhib At-Tahdhib, 1/396)
The scholars unanimously agreed that this Hadith is also to be regarded as inauthentic. It was classed as inauthentic by At-Tirmidhi when he said, after quoting it: It is Gharib. Al-`Aqili said: It is not known, as stated in Tahdhib At-Tahdhib (1/396). It was also classed as inauthentic by Ibn Hibban in Al-Majruhin (1/279), and Al-Bayhaqi in Shu`ab Al-Iman (2/497). Adh-Dhahabi said in Mizan Al-I`tidal (1/523): it is Munkar. Al-Albani said likewise in As-Silsilah Ad-Da`ifah (1342)
Examining the Third Hadith

It was narrated from Abu Hurayrah (may Allah be pleased with him) that the Prophet (peace and blessings of Allah be upon him) said: “Whoever recites `When the earth is shaken with its (final) earthquake’ in one night, it will be equivalent to half of the Quran for him.” (Narrated by Ibn As-Sunni in `Aml Al-Yawm wal-Laylah, 691)

Shaykh Al-Albani (may Allah have mercy on him) said in As-Silsilah Ad-Da`ifah (1342): It was narrated by Abu Umayah At-Tarsusi in Musnad Abu Hurayrah (2/195), but its chain of narration is very weak; it includes `Isa ibn Maymun Al-Madani, who was classed as inauthentic by a number of scholars. Abu Hatim and others said: His Hadith is to be rejected. (End quote)
Ibn Hajar (may Allah have mercy on him) said in Nata’ij Al-Afkar (3/268): Its chain of narration includes a narrator who is very weak. (End quote)
From the above it is clear that there is no authentic report from the Prophet (peace and blessings of Allah be upon him) about Surat Az-Zalzalah being equivalent to half of the Quran. But it seems that some of the imams of the Salaf followed this Hadith and, based on their own Ijtihad, stated that the important meanings that are contained in this Surah are equivalent to half of the principles that the Quran referred to.
It says in Musannaf `Abd Ar-Razzaq (3/372): It was narrated that Ma`mar said: I heard a man narrating that `When the earth is shaken with its (final) earthquake’ is equivalent to half of the Quran.
It was narrated from Ja`far that Hisham ibn Muslim said: I heard Bakr ibn `Abdullah Al-Muzani say: ‘When the earth is shaken with its (final) earthquake’ is half of the Quran.
Abu `Ubayd narrated a Mursal report from Al-Hasan Al-Basri: “ `When the earth is shaken with its (final) earthquake’ is equivalent to half of the Quran, and `By the (steeds) that run’ (Surat Al-`Adiyat) is equivalent to half of the Quran.”  (End quote from Al-Itqan by As-Suyuti, 2/413)
It was narrated that `Asim ibn Abu An-Nujud Al-Imam Al-Muqri’ (may Allah have mercy on him) said:
It was said that whoever recites ‘When the earth is shaken with its (final) earthquake’ it is as if he has recited half of the Quran.
Ibn Hajar said in Nata’ij Al-Afkar (3/270): its men are trustworthy.
Al-Manawi (may Allah have mercy on him) said in Fayd Al-Qadir (1/367):
Because the greatest objective of the Quran is to explain the creation and the resurrection, and ‘When the earth is shaken with its (final) earthquake’ refers only to the resurrection, therefore it is equivalent to half of the Quran. This was referred to by Al-Qadi.  (End quote)
This is the Ijtihad (the exertion of a Muslim jurist’S best intellectual effort to derive legal rulingsS from Islamic sources when clearcut texts are not available) of some of the Salaf, and it is not permissible to attribute it to the Prophet (peace and blessings of Allah be upon him). It does not mean that it is equivalent to half of the Quran in terms of reward.
For more details, please see the following answers: 90186, 75894, 2241, 132386, 10538, 114281.
And Allah knows best.