A total of 926 earthquakes were recorded
in this part of southern California in the first month of 1998.
The largest of these quakes was a magnitude 4.3 right-lateral
strike-slip event on the afternoon of January 5. This earthquake,
located in the Chino Hills, was near enough to major population centers
to gain considerable public notice, but little damage was done. The
flurry of activity along the Brawley Seismic Zone continued from the
Only 679 earthquakes appear in this animation
of February 1998. Though a magnitude 4.9 earthquake rocked Baja California
on the 18th of the month, this event and its associated swarm are just a bit
too far south to be shown on this map. The largest earthquake visible in this
animation is a magnitude 3.5 event which occurred on February 3 in the
San Bernardino Mountains northwest of Yucaipa.
As the counter on the full-month image will show you,
2094 earthquakes are visible in this
animation -- in large part due to the inclusion of the Coso area.
The largest event of March 1998 was a magnitude 5.2 earthquake in
the Coso area on the night of the 5th. It was followed by a magnitude
5.0 quake the next day. Both quakes generated numerous aftershocks, which
decayed in frequency over the course of the month (note the counter, which
shows this decay beautifully).
The largest earthquake outside the Coso area in March was a magnitude
4.5 earthquake near Redlands, at the intersection of the
Crafton Hills fault zone and the
San Jacinto fault zone, on the morning of
the 11th. This quake spawned aftershocks of its own, but the largest
only reached magnitude 2.2.
Of the 1151 earthquakes in this
animation, the largest is a magnitude 3.8 event which occurred
beneath Alhambra (10 miles ENE of downtown Los Angeles) on the
evening of April 25. There were no damage reports, but because
this tremor was felt throughout much of Los Angeles, it generated
considerable public interest, relative to its size. If you watch the
counter in the upper right corner of the animation, you'll note that
the seismicity rate stayed reasonably constant throughout the month.
The very first frame of this animation contains the largest earthquake
which occurred in southern California during May 1998.
On the afternoon of May 1st, this magnitude 3.8
Northridge aftershock shook the Simi Valley area, and was also felt in the western part of the
San Fernando Valley. The next largest earthquake was a magnitude 3.5
event in the Coso area on May 10, and of the
890 earthquakes in southern
California this month, no others exceeded magnitude 3.1.
If you look at the graph of
earthquakes per day for June 1998, you'll note that the peak is
quite obvious, and falls on June 8. This day was marked by an earthquake
swarm in the Coso area, near the northern boundary of the map. One of the
tremors generated during this swarm reached magnitude 3.9, putting
it into a three-way tie for the title of the largest earthquake in this
region in June. The other two magnitude 3.9 earthquakes were a
Northridge aftershock near Chatsworth around
noon on the 17th, and a mid-morning earthquake southeast of the Salton Sea,
near the Superstition Hills fault, on the 26th.
There were a total of 851 earthquakes
recorded in this area during the course of the month.
With 885 earthquakes in this area for the
month of July 1998, the seismicity rate continued at a pace comparable
to that of the previous two months. As the
graph of daily totals reveals, there
wasn't a lot of variation within the month of July, either. Still, this
month did produce one earthquake above magnitude 4 -- a M 4.1 strike-slip
event near Ocotillo Wells on the afternoon of the 10th, felt
in Palm Springs and San Diego. Also recorded by the Southern California
Seismographic Network this month was a magnitude 4.8 earthquake near
the northern end of Death Valley. Unfortunately, its location was about
75 km north of the northern edge of this map.
August 1998 brought two earthquakes larger than magnitude 4 to the
mountains just north and east of the Los Angeles metropolitan area.
The larger, a magnitude 4.8 strike-slip event near Angelus Oaks on the
morning of the
16th, was followed by a notable aftershock sequence that produced several
tremors in the magnitude 3 range and lasted until the end of the month.
The other was a magnitude 4.4 earthquake near Wrightwood on the afternoon
of the 20th. It occurred on a small thrust fault very close to the
San Andreas fault zone. When watching
the animation, notice how its aftershock sequence is much weaker.
The overall seismicity rate for this area remained remarkably stable
for the fourth straight month -- August gave us a total of
883 earthquakes, for a daily average
of just under 28.5 earthquakes recorded per day, compared to May's average
of 28.7, June's average of 28.4, and July's average of 28.6.
Because the mean (as seen of the graph of daily
totals) for each month is rounded to the nearest whole number, that number
has apparently varied between 28 and 29 over the past four months, even
though the actual variation in the average seismicity rate has been less
than 0.4 earthquakes per day!
September 1998 brought no earthquakes larger than magnitude 3.4 to
southern California. The two largest tremors in the area during
this month were two M 3.4 events near Parkfield on the 16th.
Ignoring this lack of sizable earthquakes, there was something
of note about the seismicity of September 1998 --
854 earthquakes were recorded over these
30 days, meaning that the rate of seismicity was 28.5 earthquakes per
day -- the fifth straight month with roughly this rate!
The graph shows that the daily count
does vary considerably, despite the consistent monthly average.
A total of 937 earthquakes were
recorded in southern California during October 1998. Yielding an
average of just over 30 earthquakes per day, this broke a five-month
trend in the monthly seismicity rate.
One reason for this increase was a magnitude 4.9 earthquake
northeast of Big Bear on the 26th. Its numerous subsequent aftershocks
drove up the seismicity rate temporarily, as is visible on the
graph of daily counts. Also contributing
to the higher count were a magnitude 4.7 earthquake in the southern San
Bernardino mountains on the 1st of October (followed by several aftershocks)
and a swarm of earthquakes in Baja California during mid-month.
The seismicity rate in southern California fell markedly in November 1998,
with respect to the previous months. The graph
shows an average of just 25.5 earthquakes per day, the lowest rate
since February 1998. Note that the
766 earthquakes recorded were
spread out relatively evenly
over the course of the month, as well. A lack of
major swarms or aftershock sequences kept the peak count for any single
day down to an unimpressive total of 43. The largest earthquake in
November 1998 was a magnitude 3.9 event near the Salton Sea on the 2nd
of the month. This earthquake was followed by a few small aftershocks.
Also notable was a magnitude 3.6 earthquake far offshore,
south of San Clemente Island, on the 10th. No aftershocks of this event
were recorded, but that may be due in part to its location on the edge
of our seismic network.
December 1998 was in many ways a seismic repeat of the previous
month in southern California. In those 31 days, a total of
793 earthquakes were recorded --
a daily average almost identical to that of November 1998. As the
graph shows, the peak daily total
was 46, very similar to the prior month's most active day.
Note, however, that there is an asymmetry to the
graph; the second half
of the month is more active than the first. One other
interesting note: three of the four largest earthquakes this month
occurred offshore. This includes the largest event, a magnitude
3.9 quake on the 18th, located north of Santa Barbara Island.
|End of Year 1998 Summary