Southern California Earthquake Data Center

Earthquake Information

2000 Monthly Seismicity

January 2000 MAP GRAPH
    Though the backlog of data from previous months has not yet been fully processed, all the earthquakes recorded in January 2000 have now been analyzed, and are available for viewing in this format. The backlog, of course, stems from the huge number of aftershocks produced by the M 7.1 Hector Mine earthquake of October 16, 1999. Aftershocks from this earthquake account for at least 1100 of the 2138 earthquakes recorded across southern California in January 2000. Note the obvious, linear set of epicenters that mark the Hector Mine rupture on the map, and in the animation. Do you also see a similar but weaker set of epicenters to the lower left (southwest) of the Hector Mine trace? This is the Landers aftershock series, still going, well over 7 years after the mainshock. Though Landers was a significantly larger earthquake, this still suggests that we can reasonably expect to see Hector Mine aftershocks continue for years to come.

    As this month's graph shows, January 2000 was a fairly uniform month in terms of the distribution of seismicity over time. The average rate is, of course, much higher than in months prior to October 1999, but the magnitude of the largest earthquake recorded was a surprisingly low M 3.6. There were actually two M 3.6 earthquakes this month: one was a Hector Mine aftershock on the 27th, and the other occurred near Walker Pass (north of Mojave) on the 26th.

    For an interesting exercise in observing the changing face of southern California's earthquake distribution, use the Monthly Seismicity Viewer (or the side-control version) to compare the still map image of January 2000 with that of September 1999, the last month before the Hector Mine earthquake. Can you see how great a difference that earthquake has made in the overall picture of seismicity in southern California?

February 2000 MAP GRAPH
    Though the average rate of seismicity fell from the previous month, February 2000 was much more exciting in terms of the size of the largest earthquakes recorded. Five of the 1563 earthquakes that occurred in the region measured magnitude 4 or greater. The first, a M 4.4 Hector Mine aftershock, struck early on Valentine's Day, and its own aftershocks produced a small peak in the daily rate, visible on the graph of events per day. On the 21st, a M 4.5 earthquake centered beneath Loma Linda (an area that experiences frequent minor earthquakes) shook the region but caused no damage. Another magnitude 4 Hector Mine aftershock happened on the 24th; it shows up clearly in the animation. Finally, a flurry of activity in the Coso region spawned a M 4.2 earthquake on the 28th, and a M 4.1 earthquake on Leap Day 2000. Though decreasing in strength, the Hector Mine aftershock sequence continues to contribute a majority of the seismic activity recorded in southern California; over 800 of this month's recorded earthquakes were Hector Mine aftershocks.

March 2000 MAP GRAPH
    A total of 714 earthquakes1626 earthquakes were recorded in the region during March 2000. Though more than half of those were aftershocks of the Hector Mine earthquake, none of the four earthquakes measuring magnitude 4 or greater were part of that aftershock sequence. The largest earthquake in southern California this month was a M 4.3 event on March 28 in the Coso area. It initiated a minor swarm of earthquakes that is responsible for the obvious peak in activity at the end of the month, visible on the graph of events per day. The tremor that drew the most attention, however, was a M 4.0 earthquake that occurred on the afternoon of March 6th in the Santa Ana Mountains, 7 miles southeast of Yorba Linda. It was felt as far away as Pasadena, and produced numerous small aftershocks over the course of the month.

April 2000 MAP GRAPH

    May 2000 MAP GRAPH

      June 2000 MAP GRAPH

        July 2000 MAP GRAPH

          August 2000 MAP GRAPH

            September 2000 MAP GRAPH

              October 2000 MAP GRAPH
                The Southern California Seismic Network recorded & analyzed 1,387 earthquakes in October 2000. 216 of these were M2.0 & larger, 16 were M3.0 & larger, none were M4.0 & larger. The largest earthquake of the month was a ML3.9 on October 12, located 11 miles north of Fillmore. It was felt widely in the Simi Valley, Malibu & Fillmore areas, with a few reports from as far away as Santa Monica. There was also another ML3.9 on September 30 Greenwich time (October 1 local time), which was located in Mexico 43 miles south-southeast of Calexico.

              November 2000 MAP GRAPH
                The Southern California Seismic Network recorded & analyzed 1,140 earthquakes in November 2000. 198 of these were M2.0 & larger, nine were M3.0 & larger, none were M4.0 & larger. The month’s largest earthquake was ML3.6 on November 2, part of a swarm located in Mexico 22 miles south-southeast of Calexico. The ML3.6 was felt in Brawley & El Centro. Only slightly smaller was a ML3.5 Hector Mines aftershock on November 28, which was located 31 miles north of Joshua Tree. Because this earthquake was located in a remote part of the Mojave Desert, we got not reports that it was felt.

              December 2000 MAP GRAPH
                The Southern California Seismic Network recorded & analyzed 1,337 earthquakes in December 2000. 244 of these were M2.0 & larger, 29 were M3.0 & larger, four were M4.0 & larger. The month’s largest earthquake was a ML4.4 on December 23, located 5 miles west-southwest of Grapevine (at the bottom of the Grapevine Grade on Interstate 5). This quake was felt as far away as Santa Barbara & Porterville. There were 234 felt reports. The second largest was a ML4.1 on December 4, located in Mexico 33 miles south-southwest of Yuma AZ. There was no public interest in the U.S. The other M4.0+ events were: a ML4.0 on December 2, located 4 miles east of Big Bear City & a ML4.0 on December 26, located offshore 17 miles east-southeast of San Clemente Island. The Big Bear quake was widely felt, from San Diego to Lancaster, with a peak intensity of V in the epicentral area. There were 711 felt reports. There was also a ML3.4 on December 23 near Loma Linda that was fairly widely felt. There were 207 felt reports from as far as Diamond Bar & Hemet.


              Division of Geological and Planetary Sciences | California Institute of Technology