Seismometer records ground displacement or acceleration
A geophone (type of seismometer) contains a coil of wire, wrapped on a mass, surrounded by a magnet. As the Earth moves we assume the mass stays still due to inertia and this changing magnetic field will produce a current in the coil of wire. This current is measured as a voltage and relates to the rate of change of displacement of the Earth with respect to time or:

$$
v \propto \frac{dx}{dt}
$$

Where v is voltage, and $\frac{dx}{dt}$ is the velocity of ground motion. Correct me if I'm wrong but the difference in seismometers eg. MEMS accelerometers (acceleration) vs geophone (velocity) is their frequency response.

https://www.crewes.org/ResearchLinks/GraduateTheses/2008/Hons-MSc-2008.pdf