Is Earthquake Triggering Driven by Small Earthquakes?

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 nontrivial scaling of the number of triggered earthquakes emerges from the fractal spatial distribution of seismicity.

Figure 1

Average rate of triggered earthquakes $n(M,t)$ as a function of time $t$ after the triggering earthquake (a) for different values of the magnitude $M$ of the triggering earthquake increasing from 3 to 7 with a step of 0.5 from bottom to top. The rate of aftershocks $K(M)$ as a function of $M$ is shown in panel (b) (circles) with the cumulative magnitude distribution (crosses). $K(M)$ is obtained by fitting each curve $n(M,t)$ by $K(M)/t^{0.9}$ in the range $0.01<t<365$ days for $M<6.5$. For large $M\ge 6.5$ mainshocks, there is a roll-off of the seismicity rate for small times after the mainshock due to the incompleteness of the catalog after large mainshocks, caused by the saturation of the seismic network. Therefore we measure $K(M)$ in the range $t>0.1$ day for $M=6.5$ and $t>0.3$ day for $M=7$.

Figure 2

Cumulative magnitude distribution of triggered earthquakes for different values of the mainshock magnitude between 3 (dark line, small circles) and 7 (gray line, large symbols) using the same time interval for the selection of aftershocks as in Fig. 1.