located on firm ground at a distance of exactly 100 km (62 mi) from the epicenter
of the earthquake. Charts and tables are available to adjust the maximum
trace amplitude for the usual case where the seismograph is not located exactly
100 km (62 mi) from the epicenter.
Ao 0.001 mm. The zero of the local magnitude scale was arbitrarily fixed as an
amplitude of 0.001 mm, which corresponded to the smallest earthquakes then
being recorded.