Thanks for those insights, Petra. A few words on the seasonal variation you alluded
to in your post. While many of the largest events in California have occurred in April,
this is not necessarily true in recent years for moderate earthquakes. Following
is a listing of the number of non-aftershocks of Ml>=4.5 per month since 1980 in
the region of California:

MONTH # events Ml>=4.5

JAN 52
FEB 29
MAR 27
APR 33
MAY 39
JUN 49
JUL 57
AUG 40
SEP 46
OCT 49
NOV 37
DEC 45

As you can see, February is the least active month currently with less than
half the activity of June and July. There is an apparent seasonal fluctuation with
the lowest seismicity in late winter through Spring - February through May and
the highest seismicity in the summer and early winter months. Perhaps
random noisiness, but it would indicate that seismicity should be getting
stronger over the next several months if this trend continues this year (as
suggested in Petra's post).