A tremor's long reach
Until 1992, seismologists generally believed that aftershocks only occurred near the main earthquake's rupture zone. That understanding changed on June 28, 1992, when the magnitude 7.3 Landers earthquake struck Southern California's Mojave Desert. In the hours and days that followed, seismic networks lit up with small earthquakes across the western United States. These were not conventional aftershocks. Quakes were detected as far as 1,250 kilometers away at locations like Yellowstone National Park in Wyoming. The Landers event showed the first clear evidence that the seismic waves from a large earthquake could trigger distinct, new earthquakes at immense distances.
The phenomenon, known as dynamic triggering, is caused by the passage of surface waves from the mainshock. These slow-moving, powerful waves, primarily Love and Rayleigh waves, travel along the Earth's surface much like ripples on a pond. As they propagate, they cause temporary, oscillating stress changes in the rock they pass through. If a distant fault is already tectonically stressed and close to failure, the slight nudge from these passing waves can be enough to push it over the edge, causing it to slip and generate a new earthquake.
Global reverberations
The 1992 Landers event was not an isolated case. On November 3, 2002, the magnitude 7.9 Denali Fault earthquake in Alaska provided an even more dramatic example. Its surface waves traveled over 3,100 kilometers and triggered swarms of small earthquakes in geothermal areas and volcanic fields across the western U.S., including Mount Rainier in Washington and the Geysers geothermal field in California. The dynamic stresses from the Denali quake's waves were measured to be as high as 0.22 megapascals in the Yellowstone region, enough to disturb the area's plumbing.
The distances can be even greater. The magnitude 9.2 Sumatra-Andaman earthquake on December 26, 2004, which caused a devastating tsunami, also sent seismic waves around the planet. These waves triggered tiny, non-volcanic tremors deep within California's San Andreas Fault, nearly 9,000 miles (about 14,500 kilometers) away. This showed that even faults on the opposite side of the Earth can be affected.
Areas with high fluid pressure, such as geothermal fields and volcanic regions, are particularly susceptible to remote triggering. The passing seismic waves are thought to agitate these subsurface fluids, changing pore pressures and essentially "lubricating" faults that are already near their breaking point.