Earthquakes can trigger aftershocks 1000 km away

New Scientist : Occasionally, a major earthquake can trigger additional quakes up to 1000 km from the epicenter of the first quake. How that happens is not clear because the strength of the seismic waves decreases the farther they travel. Now, Lucilla de Arcangelis of the Second University of Naples in Italy and her colleagues have a computer model that may provide one explanation. The model shows that the seismic waves from the original earthquake can create a lathering effect in the grains of dirt between tectonic plates. The lather reduces the friction between the plates, which increases the likelihood that they will slip. The model revealed that the frequency of the seismic waves was the only important variable. Even if the strength of the waves was very low, or they move in a direction opposite that of the fault’s slip, the waves could trigger an earthquake if their frequency matches the resonance frequency of the fault they’re hitting.