The paragraph is followed by an incomplete phrase or sentence which summarizes the main idea of the paragraph. Choose the missing words of the phrase or sentence.

"New simulations of earthquakes on California's active faults are providing considerable insight, showing that earthquakes tend to "cluster" in space and time due to their interactions: that is, an earthquake on one fault section can turn them on or off on nearby fault sections, depending on the relative orientation of the faults. 

Earthquake activity （） determined by fault system geometry.