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Southern California Earthquake Data Center

Earthquake Information

1998 Monthly Seismicity

January 1998 MAP GRAPH
    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 previous month.

February 1998 MAP GRAPH
    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.

March 1998 MAP GRAPH
    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.

April 1998 MAP GRAPH
    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.

May 1998 MAP GRAPH
    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.

June 1998 MAP GRAPH
    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.

July 1998 MAP GRAPH
    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 MAP GRAPH
    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 MAP GRAPH
    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.

October 1998 MAP GRAPH
    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.

November 1998 MAP GRAPH
    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 MAP GRAPH
    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 MAP GRAPH


Division of Geological and Planetary Sciences | California Institute of Technology