Seismometers are the most common instruments used to measure earthquakes, which are the result of seismic waves traveling through the Earth from a sudden release of energy. Also used are geophones (using voltage,) hydrophones (in water) and accelerometers. The seismometer is quite interesting, I found out as I was looking into the details.
An inertial mass is placed in a frame. The property of inertia is such that change in motion is resisted. The mass is placed in the frame using, for example, a spring. As the frame moves from the seismic waves, the frame moves as well, but the mass will stay stationary relative to the Earth as the spring oscillates, allowing it to do so. The movement of the inertial mass with respect to the frame is measured, which is then amplified using electronics. Electrostatic or magnetic forces are used to keep the mass stationary with respect to the frame (which naturally wants to be stationary with respect to the earth) and with knowing the force required to keep the mass stationary relative to the frame, they can then infer the forces acting on the system that caused the displacement to begin with.
Initially, I didn’t know how these work so like I said, I had to look into it. What I couldn’t quite clarify is why the compression of the spring isn’t enough to infer the forces, perhaps electromagnetic forces can be measured more accurately.
