A method was devised to extract useful information about the earthquake source from the coda of local small earthquakes. The method is based on the assumption that the power spectrum of coda waves of a local earthquake is only a function of time measured from the earthquake origin time and is independent of distance and details of wave path to the station. Evidence supporting this assumption is presented using the data on aftershocks of the Parkfield earthquakes of June 28, 1966. A simple statistical model of the wave medium, which accounts for the observations on the coda, is proposed. By applying the method to many Parkfield aftershocks, the relation between the seismic moment Mo and local magnitude ML is determined as
log Mo (dyne cm) = 15.8 + 1.5ML
The size of a microearthquake with magnitude zero is estimated as 10m×10m.