What is the meaning of "ha'Mabit la'Aretz va'Tir'ad"?
Radak: This is a metaphor for eradication of Resha'im. It is as if the land trembles, and the mountains are burning ("be'Harim v'Ye'eshanu") due to Hashem's anger. The strong horn of Resha'im will be wiped out - "Ki Esh Kadchah v'Api? va'Telahet Mosedei Harim" (Devarim 32:22).
Radak (31): If He wants, He can destroy everything and they will revert to nothingness.
Radak (35, citing Ibn Ezra): He suspends the earth on nothingness; He bears it with His strength, and never falters. If He wanted, it would tremble.
Malbim: Also when a desire is aroused in Hashem to destroy the world, He looks at the land, and it trembles from fear of His punishment.
Why does it say "Yiga be'Harim v'Ye'eshanu"?
Rashi: So it was at Sinai - "v''Har Sinai Ashan Kulo" (Shemos 19:18).
Radak: Since before it said "ha'Mabit", this teaches that Hashem's 'touching' is His looking and Hashgachah.
Radak (35, citing Ibn Ezra): If He wanted, the mountains would burn.
Malbim: The mountains smoke via the noise of the land, through the fire to destroy cities and countries. Then, Hashem's eye is to those who fear Him, and He looks to destroy only the Resha'im.
