Seismic activity at a point of the surface of the earth up to a given time has to be computed by the total energy per unit area at the point considered in consequence of the earthquakes up to that time. The definition of seismic activity of a region can be developed from that. Seismic activity in a given time-interval results from earthquakes up to the upper limit of that interval. In consequence of a suitable valuation seismic activity is always convergent. For sufficiently large time-intervals a formal relationship between seismic activity and seismicity can be constructed.