Why does it say "v'Nastem Gei Harai"?
Targum Yonasan: I will seal the canyon of the mountains. Rashi ? there are mountains around Yerushalayim. There is a canyon between the mountain in the north and Har ha'Zeisim, and similarly in the south. When He will draw half of Har ha'Zeisim towards the mountain in the north, this will seal the canyon between them, and similarly in the south.
Radak: When the mountain will split, they will flee to the canyon due to the noise. Harai is like Harim. Also "v'Kara Lo Chalonai" (Yirmeyah 22:14) is like Chalonim; "va'Chashufei Shes" (Yeshayah 20:4) is like va'Chashufim. Malbim ? there is no danger of another earthquake where there already was one.
Radak citing Targum Yonasan 1 : You will be gathered. Radak ? it seems that he holds that it says v'Nistam, like the grammatical form of "Nichtam Avonech" (Yirmeyahu 2:22). Some Seforim are like this 2 . After Har ha'Zeisim will split, it will close after an hour, hours, day or days. This will be a greater wonder than splitting! When a earthquake occurs, the gap is not sealed afterwards. Also when Korach and his congregation 3 were swallowed, the land sealed after it opened ? "va'Techas Aleihem ha'Aretz" (Bamidbar 16:33).
Malbim: Some texts say v'Nistam. When the mountain splits, it will open until the Tehom (depth) ? from there comes the Ra'ash that breaks mountains. It will seal immediately, and there will be a valley there.
What is the meaning of "Ki Yagi'a Gei Harim El Atzal"?
Rashi: The height of canyon between the mountains will reach the height of the mountains on each side. So Menachem explained "ume'Atzileha Kerasicha" - it is an expression of height. Radak - it is like "v'El Atzilei Bnei Yisrael" (Shemos 24:11). It says Harim, even though it is only one (Har), because it split and became two.
Radak: The canyon between the mountains will reach Atzel (a place). I.e. the valley will extend until there, and they will flee to there, thinking to be saved there. Malbim - Atzal is 45 Mil south of where Yerushalayim is today.
Why will you flee?
Rashi: It will be due to seeing this great wonder.
What was the "Ra'ash"?
Rashi: "Va'Yanu'u Amos ha'Sipim" (Yeshayah 6:4), on the day that Uziyah became a Metzora. Radak - also "Shenasayim Lifnei ha'Ra'ash" (Amos 1:1) refers to it. (That was long ago!) You will flee like your fathers fled. The same applies to "v'Osanu Hotzi mi'Sham (Devarim 6:23).
Malbim: Then the Ra'ash was due to Hashem's Kin'ah for His honor, and to [punish] one who disputes the Kehunah of Aharon [and his seed]. So in the future He will have Kin'ah against those who dispute the honor of Yisrael's creed and their Kedushah.
Who are "Kol Kedoshim"?
Rashi: They are angels. Radak - this is like "Ki Mal'achav Yetzaveh Lach" (Tehilim 91:11). It is as if it says v'Chol Kedoshim. Also "Shemesh Yare'ach" (Chabakuk 3:11, it is as if it says v'Yare'ach).
Radak citing Rambam: They are Nevi'im. Our Parashah refers to the good promise that Hashem promised via the Nevi'im.
To whom does "Imach" refer?
Radak: It refers to Yerushalayim, like "b'Kirbech" at the beginning of the Parashah (1).
Radak citing Targum Yonasan: It is with Him.
Malbim: It is with Yisrael. In the days of Uziyah, the Ra'ash was due to anger against Yisrael; Yeshayah saw the Bayis full of smoke. In the future, it will not be - all the Kedoshim will be with you (Yisrael), to guard you and help you.
