An early special-purpose seismometer consisted of a large , stationary pendulum , with a stylus on the bottom . As the earth started to move , the heavy mass of the pendulum had the inertia to stay still within the frame . The result is that the stylus scratched a pattern corresponding with the Earth 's movement . This type of strong-motion seismometer recorded upon a smoked glass ( glass with carbon soot ) . While not sensitive enough to detect distant earthquakes , this instrument could indicate the direction of the pressure waves and thus help find the epicenter of a local quake . Such instruments were useful in the analysis of the 1906 San Francisco earthquake . Further analysis was performed in the 1980s , using these early recordings , enabling a more precise determination of the initial fault break location in Marin county and its subsequent progression , mostly to the south
1906 San Francisco earthquake
1906 San Francisco earthquake
Seismometer