Southern California Earthquake Data Center

Earthquake Information

1999 Monthly Seismicity

January 1999 MAP GRAPH
    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 926 earthquakes. 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 December 1998.

February 1999 MAP GRAPH
    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.

March 1999 MAP GRAPH
    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.

April 1999 MAP GRAPH
    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.

May 1999 MAP GRAPH
    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.

June 1999 MAP GRAPH
    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.

July 1999 MAP GRAPH
    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.

August 1999 MAP GRAPH
    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!

September 1999 MAP GRAPH
    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.

October 1999 MAP GRAPH

    November 1999 MAP GRAPH

      December 1999 MAP GRAPH


        Division of Geological and Planetary Sciences | California Institute of Technology