How big does an earthquake need to be to trigger the seismograph?
The seismograph will trigger if the acceleration is greater than 0.25% of the earth’s gravity on the horizontal sensors. The plot below shows that the seismograph should trigger for earthquakes of magnitude(M)>4 if within 60 km (37 miles) and M>5 at twice that distance. It is common for the seismograph to trigger on quakes as small as magnitude 3 within distances of a few 10’s of km.
