Looking at the graph of events per
day for January 1999, it's impossible to ignore the peak on the
19th day of the month, which carries over to a lesser extent to the
20th. This jump in the seismicity rate was caused primarily by
a swarm of earthquakes in the Coso area, and the count remained
elevated the next day with the help of a smaller swarm offshore
from Santa Barbara. Together, these two swarms were responsible for
over 100 of the month's total of
It was an entirely different swarm that produced the largest event of the
month, however. That earthquake, a magnitude 4.4, occurred on the 13th
near the Mexican border, and was the largest of a moderately active
swarm which continued from the latter half of
With only 683 earthquakes over 28 days, the
seismicity rate for February 1999 was notably lower than the previous month.
The graph of events per day has no
obvious peak, as did that of January 1999. Yet in spite of these
rather dull features, there was some excitement brought on by
two earthquakes greater than magnitude 4. The first and larger was
a M 4.3 quake on the 18th, located along the Mexican border, just to
the southwest of the largest event in January. It was followed almost
immediately by a M 3.5 aftershock, both of which are obvious in the animation.
The other large event was a solitary M 4.1 shock along the San Andreas
fault zone near Parkfield on the 26th.
The seismicity rate in March 1999 was even lower than the previous month,
with only 714 earthquakes recorded in southern
California over those 31 days. There wasn't even much variation in the
rate -- the graph of events per day for this
month can adequately be described as "lackluster". Even the largest
earthquake of the month failed to bring much excitement; it was a repeat
of the largest earthquake of February 1999, a magnitude 4.3 event
on the 13th, along the same stretch of the Mexican border.
If you look at the graph of events
per day for April 1999, you can see how the average seismicity
rate increases around the 10th of the month, after a somewhat
slower start. This small jump doesn't correspond to any particular
event, however. The two largest earthquakes in the area
were both located just south of the Mexican border, near the
epicenters of the largest events from the three previous months.
The larger of the two was a magnitude 4.2 earthquake on the 18th;
the smaller had a magnitude of 4.0 and occurred on the 6th of
the month. Despite the slight increase in the seismicity rate,
there were still only 739 earthquakes
recorded during April 1999.
The seismological highlight of May 1999 was a magnitude 4.9
aftershock of the M 7.3 Landers earthquake
of 1992, located in the desert mountains northwest of Indio,
just south of Yucca Valley, and very close to the epicenter of
the original Landers mainshock. Due to the fact that frame "days"
are divided at 12:00 midnight, Pacific Standard Time, this large
aftershock appears on the frame for May 13, even though it occurred
at 12:54 am, Pacific Daylight Time, on the 14th.
This aftershock spawned numerous aftershocks of its own --
over 150 were recorded in just the first 24 hours. Nineteen of those
were magnitude 3 or greater, and two reached magnitude 4.
One glance at the graph of events
per day shows this obvious spike in activity.
Thanks primarily to this aftershock sequence, there were a
total of 1017 earthquakes
recorded in southern California this month.
With only 771 earthquakes recorded
in the area, June 1999 was a fairly slow month, seismically speaking.
This is certainly evident on the graph
of events per day -- not one single day had a total count of 40
or more earthquakes. This month began interestingly enough;
the largest earthquake, a magnitude 4.9 jolt centered 25 miles
southeast of Calexico, occurred on the 1st, as did a pair of
M 3.3 earthquakes near the Nevada border, an area of relatively
low seismicity. The event that may have generated the most public
interest was a M 3.8 earthquake on the morning of the 29th, located
about 3 miles south-southwest of the Los Angeles Civic Center.
This quake was felt throughout the Los Angeles Basin.
The monthly total of recorded events for July 1999
was 927 earthquakes, up significantly
from the previous month. Three earthquakes of magnitude 4 (more
specifically, their aftershocks) contributed to this increased rate.
The first and largest was a magnitude 4.4 quake near Lake Isabella on
the 11th of July. The spike of activity that resulted from this
earthquake's aftershock sequence is clearly visible on the
graph of events per day. On the
19th, an earthquake of magnitude 4.2 struck in the vicinity of Anza.
And finally, on the 22th, the area near Valencia was shaken by a
magnitude 4.0 tremor.
As you can see on the graph of events
per day, August 1999 was not a particularly active month.
The seismicity rate in this part of California dipped markedly
from the previous month's total, with only 759
earthquakes were recorded in the area. Not one of those was
greater than magnitude 3.5, though some larger earthquakes did occur
not too far outside of the area covered by this map (in western Nevada,
and in Baja California, south of Yuma, AZ). Within the map
area, two earthquakes tied for the title of the largest event of the
month. The first was a magnitude 3.5 jolt centered just south of
Mission Viejo on the evening of the 9th, and was felt throughout much
of Orange County. The other M 3.5 earthquake was located near the
western shore of the Salton Sea, early in the morning on the 10th,
and consequently generated much less public interest!
With a total of 867 earthquakes
recorded across the region, September 1999 was a fairly typical
month, seismically speaking, in southern California.
The graph of events per day
reveals only moderate variations, with a small peak on the
15th of the month related to a minor swarm in the Coso area.
The largest earthquake of the month was a magnitude 4.8 tremor
which struck south of the border on the 10th. A magnitude 4.2
earthquake shook the Big Bear area on the 19th.