  Using a catalog of seismicity for Southern California, we measure how the
number of triggered earthquakes increases with the earthquake magnitude. The
trade-off between this relation and the distribution of earthquake magnitudes
controls the relative role of small compared to large earthquakes. We show that
seismicity triggering is driven by the smallest earthquakes, which trigger
fewer events than larger earthquakes, but which are much more numerous. We
propose that the non-trivial scaling of the number of triggered earthquakes
emerges from the fractal spatial distribution of seismicity.
