Here is some data on the seasonal occurrence of large
earthquakes in California in response to a post yesterday
from Todd. There does seem to be a slight increase in activity
during the autumn months. During winter (Jan-Mar) there have
been 17; spring (Apr-Jun) 23; summer (Jul-Sept) 20 and autumn
(Oct-Dec) 30 events. This is about a 50% increase in the numbers
of events of Mw>=6.5 during the autumn months over the rest
of the year. It is hard to say if this is statistically
significant since this is a small sample of only 90 earthquakes.
On the other hand, the region off the coast of Northern California
appears to be preferentially hit during late August and early September
when 6 events have occurred in the 21 day period between August 20
and September 10, a period we are currently in.
The most active days have been February 9 and December 21
(winter solstice) with 3 earthquakes on each of these days.
The Bay area got hit extensively during late March and
early April in the years prior to the 1906 quake. There was
a paper published several years ago which pointed this out
and suggested that the winter rains were somehow responsible.
However, three sizable quakes have also occurred in the
Bay ara in October, so this may be a questionable correlation.

Following is a listing of all earthquakes of Mw>=6.5
in the western U.S. sorted by date of the year with
aftershocks removed. There is no obvious seasonal variation
however some days seem more seismic than others. For example
3 large earthquakes have occurred on Feb. 9.

January 5
February 5
March 7
April 11
May 8
June 4
July 11
August 7
September 2
October 8
November 12
December 12

MO DA HR MN SC YEAR LAT N LONG W DEP MAG

1 1 9 56 44 1965 36.400 -122.500 14 7.0 Central California
1 9 10 13 0 1857 35.000 -119.000 0 8.3 Fort Tejon, CA
1 17 12 30 55 1994 34.213 -118.537 18 7.0 Northridge, CA
1 22 9 4 18 1923 40.800 -124.050 0 7.2 Off the Coast of No. Cal.
1 31 13 17 22 1922 40.870 -125.350 0 7.6 Off the Coast of No. Cal.
2 9 9 44 8 1941 40.500 -125.367 0 6.6 Off the Coast of No. Cal.
2 9 14 8 7 1971 34.411 -118.401 8 6.7 San Fernando, CA
2 9 14 32 38 1956 31.750 -115.916 16 6.8 Baja California
2 19 4 3 15 1995 40.611 -125.898 18 6.7 Off the Coast of No. Cal.
2 24 7 20 0 1892 32.599 -116.300 0 6.9 Baja California
3 10 11 21 20 1922 35.750 -120.250 43 6.5 Lompoc CA
3 12 15 5 48 1934 41.700 -112.800 0 6.6 No. Utah
3 15 18 45 0 1860 39.000 -120.000 0 7.0 Northern California
3 22 22 12 0 1902 42.000 -130.000 0 6.8 Off the coast of Oregon
3 26 2 30 0 1872 36.500 -118.000 0 8.5 Long Valley, CA
3 31 7 43 0 1898 38.200 -122.400 0 6.5 San Francisco area
4 9 2 28 59 1968 33.183 -116.116 11 7.1 Borrego Springs, CA
4 11 19 0 0 1872 37.500 -118.500 0 6.9 Long Valley, CA
4 15 7 7 0 1898 39.200 -123.800 0 6.9 San Francisco area
4 16 13 40 0 1899 40.500 -125.500 0 7.0 Off the Coast of No. Cal.
4 18 13 12 0 1906 38.000 -123.000 60 8.3 San Francisco area
4 19 10 50 0 1892 38.500 -122.000 0 6.8 San Francisco area
4 21 22 32 25 1918 33.750 -117.000 0 6.8 Yucaipa, CA
4 23 4 50 23 1992 33.961 -116.318 12 6.7 Joshua Tree, CA
4 24 21 14 57 1984 37.300 -121.600 0 6.5 Morgan Hill, CA
4 25 18 6 4 1992 40.368 -124.316 15 7.3 Off the Coast of No. Cal.
4 29 10 49 27 1954 29.500 -112.500 0 7.4 Baja California
5 1 21 53 34 1939 40.500 -124.633 0 6.5 Off the Coast of No. Cal.
5 2 13 14 47 1939 29.500 -113.800 16 6.8 Baja California
5 2 23 42 38 1983 36.219 -120.317 9 6.7 Coalinga, CA
5 3 21 13 0 1887 31.000 -109.000 0 7.0 Baja California
5 6 12 9 0 1915 39.500 -126.500 0 6.8 Off the Coast of No. Cal.
5 9 4 25 0 1878 40.000 -124.500 0 6.5 Off the Coast of No. Cal.
5 19 4 36 41 1940 32.733 -115.500 16 7.1 Baja California
5 25 19 44 51 1980 37.569 -118.820 15 6.7 Mammoth Lakes, CA
6 0 0 0 0 1838 37.600 -122.400 0 7.0 San Francisco area
6 10 4 32 24 1917 44.000 -129.000 0 6.5 Off the coast of Washington
6 10 15 30 0 1836 37.800 -122.200 0 6.8 San Francisco area
6 28 11 57 35 1992 34.200 -116.500 13 7.8 Landers, CA
7 1 22 0 60 1911 37.250 -121.750 0 6.6 E. of San Francisco
7 6 22 7 41 1954 39.400 -118.500 33 6.5 Dixie Valley, NV
7 6 22 49 52 1934 41.250 -125.420 0 6.5 Off the Coast of No. Cal.
7 7 16 15 51 1932 29.000 -113.000 0 6.8 Baja California
7 8 9 20 45 1986 33.998 -116.615 6 6.9 Palm Springs, CA
7 8 9 37 27 1975 29.459 -113.346 33 6.5 Baja California
7 13 2 50 9 1991 42.269 -126.304 9 6.7 Off the Coast of Oregon
7 15 0 23 60 1918 41.000 -125.000 0 6.5 Off the Coast of No. Cal.
7 21 11 52 14 1952 35.000 -119.016 16 7.8 Kern County Bakersfield, CA
7 21 14 42 27 1986 37.537 -118.447 9 6.5 Bishop, CA
7 22 20 32 0 1899 34.300 -117.400 0 6.5 Lake Arrowhead CA
8 5 1 31 36 1910 42.000 -127.000 0 6.8 Off the Coast of No. Cal.
8 17 19 29 41 1991 40.287 -124.238 8 6.6 Off the Coast of No. Cal.
8 18 6 37 15 1959 44.833 -111.083 0 7.7 Hebgen Lake, MT
8 20 15 25 4 1952 43.250 -126.500 0 7.1 Off the Coast of No. Cal.
8 22 5 28 18 1914 44.000 -129.000 0 6.8 Off the Coast of No. Cal.
8 23 15 32 40 1955 43.500 -128.000 0 6.5 Off the Coast of No. Cal.
8 24 5 51 32 1954 39.583 -118.450 10 6.8 Dixie Valley, NV
9 1 15 15 49 1994 40.444 -126.331 15 6.8 Off the Coast of No. Cal.
9 10 3 14 7 1984 40.389 -127.151 0 6.7 Off the Coast of No. Cal.
10 3 6 52 48 1915 40.500 -117.500 60 7.8 Mt. Lassen CA
10 8 20 46 0 1865 37.200 -121.900 0 6.5 San Francisco area
10 15 23 16 53 1979 32.614 -115.318 12 6.8 Imperial Valley California
10 16 14 57 18 1907 28.000 -112.500 0 7.7 Baja California
10 18 0 4 15 1989 37.160 -121.680 18 7.2 Loma Prieta, CA
10 21 15 53 0 1868 37.700 -122.100 0 6.8 San Francisco, CA
10 21 16 22 13 1942 32.966 -116.000 16 6.5 Baja California
10 28 14 6 7 1983 43.967 -113.899 16 7.3 Borah Peak, ID
11 4 8 37 0 1908 36.000 -117.000 0 6.5 Coso Junction area, CA
11 4 13 50 0 1927 34.500 -121.500 0 7.5 Off coast of So. Cal.
11 8 10 27 34 1980 41.117 -124.253 19 7.3 Off the Coast of No. Cal.
11 9 0 0 0 1852 33.000 -114.500 0 7.0 So. of Yuma, AZ
11 14 4 39 0 1901 38.769 -112.084 0 7.0 Central Utah
11 20 7 9 47 1949 28.000 -112.000 0 6.8 Baja California
11 21 0 13 42 1915 32.000 -115.000 0 7.1 Baja California
11 23 5 0 0 1873 42.000 -124.200 0 7.0 Off the Coast of No. Cal.
11 24 1 54 14 1987 33.083 -115.775 5 6.5 Superstition Hills, CA
11 25 11 16 35 1954 40.267 -125.633 10 6.7 Off the Coast of No. Cal.
11 26 11 19 25 1976 41.289 -125.709 15 6.8 Off the Coast of No. Cal.
11 29 20 0 0 1852 32.500 -115.000 0 6.6 Imperial Valley
12 4 23 43 17 1948 33.933 -116.383 16 6.5 Joshua Tree
12 8 15 0 0 1812 33.700 -117.900 0 6.8 So. of Los Angeles
12 12 23 10 0 1902 29.000 -114.000 60 7.8 Baja California
12 16 11 11 34 1954 39.670 -117.870 0 7.3 Dixie Valley, NV
12 21 6 10 0 1932 38.733 -117.817 60 7.2 Northern Nevada
12 21 19 0 0 1812 34.200 -119.700 0 7.2 Santa Barbara, CA
12 21 19 56 29 1954 40.817 -124.083 10 6.6 Off the Coast of No. Cal.
12 25 12 25 0 1899 33.800 -117.000 0 6.7 Yucaipa, CA
12 27 1 30 0 1869 39.500 -120.000 0 6.7 Portola, CA
12 30 13 52 14 1934 32.250 -115.500 16 6.5 Imperial Valley, CA
12 31 12 20 0 1915 41.000 -126.000 0 6.5 Off the Coast of No. Cal.
12 31 18 45 0 1934 32.000 -114.750 16 7.1 Baja California