  Upon employing the analysis in a new time domain, termed natural time, it has
been recently demonstrated that a remarkable change of seismicity emerges
before major mainshocks in California. What constitutes this change is that the
fluctuations of the order parameter of seismicity exhibit a clearly detectable
minimum. This is identified by using a natural time window sliding event by
event through the time series of the earthquakes in a wide area and comprising
a number of events that would occur on the average within a few months or so.
Here, we suggest a method to estimate the epicentral area of an impending
mainshock by an additional study of this minimum using an area window sliding
through the wide area. We find that when this area window surrounds (or is
adjacent to) the future epicentral area, the minimum of the order parameter
fluctuations in this area appears at a date very close to the one at which the
minimum is observed in the wide area. The method is applied here to major
earthquakes that occurred in California during the recent decades including the
largest one, i.e., the 1992 Landers earthquake.
