Abstract: The magnitude of a seismic event can be estimated by measuring seismic energy, seismic moment, and relationship between the energy and seismic magnitude (M), etc. However, it is impossible to traditionally locate a microseismic event, including determining its magnitude, because its records are submerged in the background noise in most of cases. Comparing the microseismic magnitudes at different regions and with different causes, studying their mechanisms, we first extend the definition of the Richter magnitude to the microseismic range. Then, based on the scanning output of the minimum ratio of signal-to-noise in the available vector processing for microseismic monitoring, we estimate their equivalent Richter magnitude. The microseismic magnitudes induced by different causes are compared.