Why does it say that His feet will be on Har ha'Zeisim?
Radak: This is a Mashal. He will show a wonder on Har ha'Zeisim ? it will split.
Radak citing Rambam: Raglav is the reason, like "va'Yvarech Osecha l'Ragli" (Bereishis 30:30) ? due to me. Here, it means that Hashem will be the cause of the wonders; He does them.
Radak citing Targum Yonasan: His strength will be revealed at that time. Har ha'Zeisim is Al Pnei (near) Yerushalayim.
Malbim: There will be an earthquake. The verse depicts that Hashem's feet cause it. For spiritual beings, Regel is causing.
What is "mi'Chetzyo"?
Rashi: From the middle.
What is the meaning of "Mizrachah v'Yamah"?
Rashi: From east to west, like it concludes.
What is the meaning of "u'Mash Chatzi ha'Har Tzefanyah?"?
Rashi: The northern half will depart from its place and go [further] north. And so we find the southern half in the south. The canyon in between begins in the east and ends in the west. Radak ? there will be a great canyon in between. Mash is departing. This wonder is a sign that the Goyim who come against Yerushalayim will split, and fall here and there. Malbim ? this explains why in the future, Yerushalayim will be 45 Mil from the Mikdash (Yechezkel Perek 48). Other Nevi'im prophesized that Yerushalayim will remain in its place! Zecharyah resolves this. [The south half of] Har ha'Zeisim, which is part of Yerushalayim, will depart with 5000 Kanim (15 Mil, the future size of Yerushalayim) around it from its place. It will be 45 Mil from the northern half of the mountain.
