Here are some details on Earthquakes within 30 km of today's Ml 2.8 near Parkfield.
Total events of Ml>=2.5 since 1850 : 883
Total events of Ml>=2.8 since 1850 : 576

Total events of Ml>=2.8 in 2000 and 2001 - 8 (four each year)

Number of times when an mainshock of 2.8<= M<=3.2 was followed by a larger earthquake
within one week (7 days) : 9 of 576 (1.5% of the time an event of 2.8<=M<=3.2
is a foreshock to something larger).

Number of mainshocks since 1964 of Ml>=4.9: 7
(1964 is usually considered the beginning of good seismic records)
Number of mainshocks since 1964 which were preceded within 3 days by a small
foreshock: 7

So, here are some generalizations we can make regarding the possibility that
this Ml 2.8 is a foreshock to something larger:

1) The probability is less than 0.2 that a larger mainshock will follow in the next
7 days.
2) If this is a foreshock, the mainshock should occur within 3 days.
3) In no case are mainshocks preceded by foreshocks between two weeks and
three days before the mainshock.
4) In all cases since 1964 (100% chance) the mainshocks have been preceded
by at least one foreshock of Ml<3.2 and Ml>=2.8 no more than 3 days before
the mainshock occurrence.

Following is the data on the Parkfield mainshocks and foreshocks from our data files
:
Listing of Parkfield Events of Ml>=4.9 and associated
foreshocks (if any):

MODAHRMNSCYEAR LAT LONG DEP ML #RECORDS

2 2 0 11 0 1881 36.000 -120.500 0 5.9 0
3 3 7 45 0 1901 36.000 -120.500 0 5.5 0
3 10 11 21 20 1922 35.750 -120.250 43 6.5 0
8 18 5 12 15 1922 35.750 -120.330 0 5.0 0
12 27 9 19 0 1926 36.170 -120.330 0 5.0 0

6 5 11 47 0 1934 35.800 -120.333 0 3.0 0
6 5 13 46 0 1934 35.800 -120.333 0 3.0 0
6 5 21 48 0 1934 35.800 -120.333 0 5.0 0
6 5 23 30 0 1934 35.800 -120.330 0 4.3 0
6 6 0 55 0 1934 35.800 -120.333 0 3.0 0
6 6 16 40 0 1934 35.800 -120.330 0 3.5 0
6 6 22 40 0 1934 35.800 -120.333 0 3.5 0
6 8 4 30 26 1934 35.800 -120.330 0 5.0 0
6 8 4 47 0 1934 35.800 -120.333 0 6.0 0

12 24 16 26 0 1934 35.933 -120.483 0 5.0 0

12 28 12 15 38 1939 35.800 -120.333 0 5.0 0

11 16 3 23 9 1956 35.950 -120.467 0 5.0 0

6 28 1 0 34 1966 35.833 -120.253 0 3.5 0
6 28 4 8 58 1966 35.777 -120.294 0 5.1 0
6 28 4 26 14 1966 35.916 -120.533 19 5.6 0
6 29 19 53 26 1966 35.950 -120.530 0 5.0 94

9 7 3 53 26 1975 35.721 -120.308 10 3.1 12
9 10 13 22 44 1975 35.789 -120.352 8 3.3 10
9 13 21 20 59 1975 35.997 -120.546 11 5.1 25

1 14 11 1 6 1976 36.081 -120.198 8 3.0 0
1 14 21 39 13 1976 36.079 -120.224 7 3.1 32
1 14 21 40 44 1976 36.076 -120.246 6 2.5 0
1 14 21 43 46 1976 36.078 -120.221 2 2.8 0
1 14 21 43 59 1976 36.007 -120.272 11 5.1 64

2 14 5 54 17 1987 35.973 -120.698 6 3.1 0
2 14 7 26 52 1987 36.148 -120.335 13 5.3 102

11 13 21 14 6 1993 35.951 -120.498 11 3.1 46
11 14 0 11 14 1993 35.951 -120.507 12 2.5 22
11 14 12 25 35 1993 35.953 -120.494 12 4.9 113

12 12 5 50 1 1994 35.943 -120.487 16 3.0 53
12 19 11 36 29 1994 36.025 -120.561 6 2.5 10
12 20 10 27 48 1994 35.912 -120.476 14 5.0 98