All this microseismicity started early in September. At the time several Pasadena
seismologists put out a memo suggesting this might be a precursory pattern leading
up to a larger quake. Part of the reason for changing the speed and accuracy of the
listing was to try to catch any immediate precursory activity. There is not really
an increased level of reporting according to senior staff at Pasadena, but a real
increase in microearthquakes. Following is a listing of the number of events listed
in the SCSN catalog for the L.A. region (33.7N-35N; 117.8-119.4W) for each month of
the year 2001. The first is the month of 2001 the second the number of events
recorded during that month.

MO.YEAR #E RATE
01/2001 64 (2.06 events per day)
02/2001 68 (2.42 events per day)
03/2001 44 (1.46 events per day)
04/2001 51 (1.64 events per day)
05/2001 65 (2.09 events per day)
06/2001 41 (1.36 events per day)
07/2001 36 (1.16 events per day)
08/2001 36 (1.16 events per day)
09/2001 84 (3.40 events per day) Through September 25, 2001

The rate of activity in the region has nearly tripled over that of August and July
and is more than double that of June and 1.5X that of the other months.
It is hard to say what this means, more stress in the area, a large event on
the way, or normal variation in background seismicity?
An interesting thing to see is how the monthly rate of seismicity in the region
varied before the Northridge earthquake on January 16, 1994 and compare this
with how the monthly rate is varying this year. Following are the monthly totals
of events in the same region as above during the period Jan 1993 thorough
the Northridge earthquake on Jan 16, 1994:

01/1993 39 (1.25 events per day)
02/1993 38 (1.35 events per day)
03/1993 65 (2.09 events per day)
04/1993 47 (1.56 events per day)
05/1993 65 (2.09 events per day)
06/1993 70 (2.33 events per day)
07/1993 62 (2.00 events per day)
08/1993 80 (2.58 events per day)
09/1993 78 (2.56 events per day)
10/1993 70 (2.25 events per day)
11/1993 70 (2.33 events per day)
12/1993 62 (2.00 events per day)
01/1994 46 (3.06 events per day) Through January 15, 1994

The daily rate jumped by about 1.5 times that of the previous month in the
15 days before the Northridge earthquake and was higher than at any time
during the previous year. This is not dissimilar to what is currently happening
in the area, except the rate increase is larger and stronger this at this time
than it was in January, 1994.