When looking at the seismicity rate along the San Andreas and Garlock fault zones, focus on those areas that stand out as the most active, especially on the map of all magnitudes (since there have been no M > 6 earthquakes on the Garlock fault zone since 1892). Then compare these locations with a map of fault traces. Do you notice any correlations?
With the obvious exception of the Parkfield area (to be addressed later), you
should find that most of the areas of relatively high seismicity along
these two mature fault zones coincide with an area of complexity --
where the fault zone looks less like a single planar feature and is more
of an intricate set of fault steps, bends, and parallel strands.
Conversely, the quietest places tend to be those with a simple geometry.
If it's not evident why that makes sense, imagine you have two fault block models of the same size, like those at left. If one has a single planar fault running down the middle, and the other a single yet complex zone of faulting comprised of several different interacting segments, which block model will experience more areas of shifting when they are each strained the same amount? And on which block model do you think a single, continuous rupture (from one end to the other) is most likely to occur?
Complexity like this may occur at a natural obstacle to a more simplified
fault geometry (e.g. a major fault bend or step). It may also
result from the interaction of two different fault zones, as the slip
of each serves to distort the other. In either case, a fault may act to
"correct" itself by breaking a new, straighter segment. In this way,
a little "youth" can be injected into an otherwise mature zone of faulting,
and this can give rise to a locally higher rate of seismicity.
Indeed, this can raise the rate dramatically; such areas where faults
are forming are often the site of earthquake swarms.
