Earthquakes are caused by the sudden release of accumulating strain in the Earth's upper crust or buried fragments thereof. This means that rocks must be breaking for an event to be detected by human beings. However, deep in the Earth's interior, rocks are no longer brittle. With overlying rock pressure and increased heat, they become plastic, so they cannot break.
Q: Is it possible that earthquake can happen at the inside of the earth? Like Mantle or outer core, inner core?
- Sarah G
A: Many things may be theoretically possible, but once you get below the point of plasticity in the Earth's crust there will be no rock-breaking earthquakes. This aseismic depth - where rocks deform plastically - varies as a function of temperature, pressure, and rock constituents. I believe the deepest earthquakes ever recorded are at around 800 km. These happen along subducted, down-going oceanic crust that is still dragging some of its cooler thermal regime with it.