The Earth's core is so hot that the rocks are always boiling. If you pick up a boiling kettle, we can feel its vibration. Now suppose we are talking about the boiling of several million iron and nickel. That is, a powerful earthquake is always occurring and this vibration is repelled by the mountains. If there were no mountains, this earthquake would always be felt on Earth. We have a verse in the Quran that says:
An-Nahl
وَأَلۡقَىٰ فِي ٱلۡأَرۡضِ رَوَٰسِيَ أَن تَمِيدَ بِكُمۡ وَأَنۡهَٰرٗا وَسُبُلٗا لَّعَلَّكُمۡ تَهۡتَدُونَ (15)
And He has placed into the earth firm mountains, so it does not tremor with you, and there are rivers and pathways so that you may find your way,
 
Az-Zalzalah
إِذَا زُلۡزِلَتِ ٱلۡأَرۡضُ زِلۡزَالَهَا (1)
When the earth is shaken with a mighty quake, 
 
لَهَا  " " means for itself.​​​​​​​