How do we know that some layers of the earth are solid and some liquid?

... The deepest drill hole ever completed is around 12 km deep, and it showed that rocks at that depth are still solid. Volcanoes erupting liquid lava on surface shows that there must be liquid somewhere in the Earth. Beyond 12 km in depth, we know very little about the Earth's internal composition and structure.

Answerdistinguish between the mercalli scale and the richter scale;