Slip and Seismicity along Major Fault Zones

If we consider the rate of seismicity of all magnitudes (which is dominated by small earthquakes) in southern California and the slip rates of the faults most closely related to those earthquakes, there seems to be little correspondence between the two. Though some high-slip faults show a lot of microseismic activity, you'll note that others do not. For example, notice on the map at right (click to enlarge it, if you need to) that the amount of historic seismicity along certain stretches of the infamous San Andreas fault zone (in red) is minimal, even non-existent. Conversely, you can see that while many of the faults with low slip rates display very little activity, some areas of the map are densely covered with blue and purple pixels that each represent the epicenter of an earthquake. Though this view is somewhat biased in places, due to large aftershock sequences on faults with low slip rates (e.g. Landers), it is nonetheless fair to say that there isn't a strict correlation between the two rates.

For large earthquakes (M > 6), you can see more correlation (use the mouse to switch maps, at right). The San Jacinto and Imperial fault zones both have a high slip rate and a high rate of large ruptures -- but two other fault zones with high slip rates, the San Andreas and the Garlock, are noticeably quiet, with only a few exceptions. Because these faults are slipping quickly, and will eventually release a lot of strain energy, their silence for the past 100 years should suggest to you that these faults rupture primarily in very large, infrequent events.

What do these two large fault zones have in common that might cause their apparent quiescence? According to geologic studies, these are two of the oldest active fault zones in southern California. They possess a high degree of maturity, meaning that they are firmly established and well-developed structures. It is possible, then, that fault maturity may influence certain properties of a fault, thereby decreasing its seismicity rate.