What is "modern seismometer"
Hopefully, the reader would not be satisfied with such a short
description. See the following pages for a very thorough
review of seismometers and seismic observations in general:
You may also check the following links
The principle of a "modern seismometer" is still the same:
a mass on a spring. What makes a seismometer "modern", is
the careful mechanical design, accurate method of detection
of the test mass motion (coil in magnetic field, sometime
capacitive method), and, in some wideband sensors,
feedback applied to the test mass which allowing to avoid
the resonance of the mass and to extend sensitivity
to lower frequency range.
Seismic Sensors and their Calibration, by Erhard Wielandt,
Institute of Geophysics, University of Stuttgart