ABSTRACTIf earthquakes are recorded on your water‐level charts, you can make your own estimate of earthquake magnitudes! As an aid in doing this, this paper presents two graphs: one to calculate distance from well to earthquake epicenter; the other, a nomogram from which to read hydroseismic magnitude plus log C. Examples given show (1) direct computation of ms+ log C, (2) use of aftershocks to estimate total water‐level fluctuation caused by a major earthquake (when the recorder drum has spun around or when reversals cover up the record), and (3) how to determine if aftershocks theoretically should have been recorded.Hydroseismic magnitudeis the surface‐wave magnitude (Ms) calculated from seismically induced water‐level fluctuations in wells. As yet it must be computed indirectly from ms+ log C but because log C is a variable the result is only approximate. Log C is believed to represent a resultant controlled by magnification, period of the surface waves, instrumental frictions, well lag, and resonance if the well is unde