From this link:
...The Landers temblor, however, permanently changed the established view of earthquake sequences. In the minutes and days that followed the quake, a substantial number of smaller seismic events occurred well beyond its aftershock zone--as far away as the Lassen Peak area and at the Geysers, about sixty miles north of San Francisco. The largest of these distant events, with a magnitude of 5.4, struck in western Nevada twenty-two hours after the Landers event. Quickly dubbed remotely triggered earthquakes, these outlying seismic events seemed to be different beasts from anything seismologists had previously encountered...
...One possibility is that earthquakes--all earthquakes--represent nothing more than cracks in the earth that grow very, very slowly in response to the forces applied on them, until the cracking process accelerates into a runaway failure. Such a phenomenon would be analogous to cracking in rocks, a process that can be studied in detail in laboratory experiments. The final kick that initiates the runaway process might be infinitesimally small-perhaps nothing more than the final grain of sand landing atop the increasingly unstable sandpile. Or, as in the case of triggered events, it might be an abrupt kick, delivered by the shaking from a distant earthquake.
The idea of earthquakes as the culmination of runaway crack growth is not new. What is new is that we can now quantify the type of shaking that does, and does not, produce additional earthquakes at distant points. Preliminary results suggest that only quakes close to or above magnitude 7.0 will produce remotely triggered events. Seismologists are able to make rocks crack in the laboratory, but we have a very limited ability to test the conditions under which actual faults in the crust rupture. We now know that, at least once in a while, the ground beneath our feet performs its own experiments, giving us important new information about how earthquake ruptures occur. And some of these experiments have been available to us all along. The old data just had to wait for new eyes--eyes aided by years of accumulated scientific understanding.