An instrument that simply measures earth waves is a

Seismograph

Explanation:

A seismometer is the internal part of the seismograph, which may be a pendulum or a mass mounted on a spring; however, it is often used synonymously with "seismograph". Seismographs are instruments used to record the motion of the ground during an earthquake.

Seismograph

Explanation:

Seismograph, instrument that makes a record of seismic waves caused by an earthquake, explosion, or other Earth-shaking phenomenon.

electric field is a function of 1/r^2

yes, the force per unit charge for a point charge is inverse square law.