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Southern California Earthquake Data Center

Earthquake Information

2003 Monthly Seismicity




January 2003 MAP GRAPH
    The Southern California Seismic Network recorded & analyzed 1,192 earthquakes in January 2003. 248 of these were M2.0 & larger, 18 were M3.0 & larger, one was M4.0 & larger. The largest earthquake of the month was a ML4.5 on January 25, in the Tehachapi Mtns. 12 miles northeast of Arvin. This Arvin quake was part of an on-going swarm, which had 646 members between December 16, 2002 & March 9, 2003. The focal mechanism was thrust. 380 felt reports were generated, from 56 zip codes ranging from Gorman to Porterville & the Little Lake area. Two other widely felt earthquakes occurred in January: a M3.1 on January 18, located 3 miles north-northeast of Chatsworth (375 felt reports from 33 zip codes) & a M3.0 on January 22, located 7 miles west-southwest of the Los Angeles Airport (137 felt reports in 42 zip codes).

February 2003 MAP GRAPH
    The Southern California Seismic Network recorded & analyzed 1,330 earthquakes in February 2003. 257 of these were M2.0 & larger, 39 were M3.0 & larger, nine were M4.0 & larger & two were M5.0 & larger. The month~Rs largest earthquake was a Mw5.4 on February 22, located 3 miles north of Big Bear City. It had 500 aftershocks in the month of February, of which the largest was a ML4.6 on February 24. The Mw5.4 quake generated 7,532 felt reports from 570 zip codes, as far away as the Simi Valley & Yuma AZ. The peak intensity was Modified Mercalli VI, in the epicentral area.

March 2003 MAP GRAPH
    The Southern California Seismic Network recorded & analyzed 1,136 earthquakes in March 2003. 169 of these were M2.0 & larger, 14 were M3.0 & larger, one was M4.0 & larger. The month~Rs largest earthquake was a M4.6 on March 11, located 16 miles north-northwest of Twentynine Palms. This quake was a Hector Mines aftershock (of the Mw7.1 Hector Mines earthquake in October 1999). It was felt in all the High Desert communities & as far away as San Diego & Whittier. Peak intensity was Modified Mercalli IV. Also of interest was a ML3.6 on March 15, located 3 miles north-northeast of Big Bear City. This quake was an aftershock of the February 22 Mw5.4 earthquake. The month~Rs ~Smost popular~T earthquake was a ML2.9 on March 27, located 2 miles south of Encino. It generated 688 felt reports from throughout the San Fernando Valley & the West Side. The peak intensity was Modified Mercalli IV.

April 2003 MAP GRAPH
    The Southern California Seismic Network recorded & analyzed 797 earthquakes in April 2003. 125 of these were M2.0 & larger, nine were M3.0 & larger, none were M4.0 & larger. The earthquakes of interest this month were all ML3.3~Rs. There was one on April 10 that was located 3 miles north of Big Bear City. It was reported felt in the mountain communities. This quake was an aftershock of the February 22 Mw5.4 earthquake. Another ML3.3 occurred on April 13, 2 miles east-northeast of Obsidian Butte in the Imperial Valley. This quake was part of a small swarm of 82 events between April 10 & April 20. Another ML3.3 occurred on April 28, 4 miles east-southeast of Ocotillo Wells. It was reported felt in Borrego Springs. There were also ML3.3~Rs on April 8, 60 miles south-southwest of Calexico, and on April 25, 19 miles west of San Clemente Is. The ~Smost popular~T earthquake of the month was a M3.1, which occurred on April 14, located 1 mile north of Compton. There were 562 felt reports, ranging throughout the Los Angeles Basin. The peak intensity was Modified Mercalli IV.

May 2003 MAP GRAPH
    The Southern California Seismic Network recorded & analyzed 923 earthquakes in May 2003. 155 of these were M2.0 & larger, 11 were M3.0 & larger, one was M4.0 & larger. The largest earthquake was a M4.2 on May 23, located in the Imperial Valley 1 miles southwest of Brawley. This earthquake only generated 80 felt reports, but the peak intensity was Modified Mercalli V. The quake was a member of a swarm which started on May 21 & started tapering off on May 25 This swarm had 183 detected members. There was also a swarm near Obsidian Butte in the Imperial Valley, from May 14 through 20, the largest of which was a ML3.1 on May 17. The second largest was a M3.9 on May 14, located in the Little San Bernardino Mtns. 11 miles east-northeast of Indio. The Indio quake generated 101 felt reports, ranging from Palm Springs to Brawley. The peak intensity was Modified Mercalli III.

June 2003 MAP GRAPH
    The Southern California Seismic Network recorded & analyzed 788 earthquakes in June 2003. 101 of these were M2.0 & larger, 12 were M3.0 & larger, one was M4.0 & larger. The largest earthquake was a M4.1 on June 25, offshore of Mexico, 42 miles south-southwest of San Ysidro. We did get reports that this quake was felt in Imperial Beach, Chula Vista & San Diego. The largest earthquake within a populated area was a ML3.6 on June 11, 1 miles west-southwest of Brawley, in the Imperial Valley. This quake was a member of a small cluster of about 10 members, the largest of which were the ML3.6 & a ML3.4 one minute later. These two quakes were reported felt in Brawley, Westmorland, El Centro & Imperial. The ~Smost popular~T earthquake of the month, generating 449 felt reports, was a M2.6 on June 18, which was located 2 miles northeast of Caltech, in Pasadena.

July 2003 MAP GRAPH
    The Southern California Seismic Network recorded & analyzed 777 earthquakes in July 2003. 72 of these were M2.0 & larger, five were M3.0 & larger, one was M4.0 & larger. The month's largest quake was a M4.2 Landers aftershock on July 14, located 20 miles northeast of Lucerne Valley. The focal mechanism was strike-slip, as expected for Landers aftershocks. It generated 163 felt reports, ranging from Wrightwood & Palmdale to Fort Irwin. The peak intensity reported (not close to the epicenter, which was in a very isolated area) was Modified Mercalli III.

August 2003 MAP GRAPH
    The Southern California Seismic Network recorded & analyzed 839 earthquakes in August 2003. 91 of these were M2.0 & larger, seven were M3.0 & larger, none were M4.0 & larger. The largest quake & the one that generated the most public interest was the M3.9 on August 26. This quake was located 5 miles west of Valencia. The Valencia quake generated 1,714 felt reports, ranging from Santa Barbara to Palmdale & Pasadena. Peak intensity was Modified Mercalli IV. This quake was a member of an on-going cluster of activity from August 22 to August 31, including 23 recorded events, the largest of which was a ML3.2 foreshock on August 25. There was also a ML3.7 quake on August 19, 8 miles south of Mt. Whitney in the Sierra Nevada. Due to the sparse population in the epicentral area, this quake generated significantly less interest. It was felt in Porterville, Lone Pine, Yosemite National Park & various other communities.

September 2003 MAP GRAPH
    September was a relatively quite month in our region. The Southern California Seismic Network recorded & analyzed 777 earthquakes. 58 of these were M2.0 & larger, five were M3.0 & larger, none was M4.0 & larger. The month's largest event took place on September 11 & was located in Mexico, 16 miles southeast of Calexico. The magnitude was ML3.7. This quake was felt in El Centro & other points in the southern Imperial Valley. There were 23 felt reports, with a peak intensity of Modified Mercalli III. The second largest quake was a ML3.6 on October 12, located 5 miles north-northwest of Simi Valley. This earthquake prompted 510 felt reports, from the San Fernando Valley westward to the Camarillo area. Peak intensity was also Modified Mercalli III.

October 2003 MAP GRAPH
    In October 2003, the Southern California Seismic Network recorded & analysed 1,064 earthquakes. 157 of these were M2.0 & larger, 21 were M3.0 and larger & only one was M4.0 and larger. The largest event took place on October 8 & was located in Mexico 35 miles south of Calexico. The magnitude was M4.3. There were two quakes that were widely felt, both M3.6. The first occurred on October 7 & was located 5 miles west-southwest of San Ysidro. There were 1,962 felt reports (on the Community Internet Intensity Map site), with a peak intensity of Modified Mercalli IV. The other occurred on October 29 & was located 3 miles west of Simi Valley. For this quake, there were 1,471 felt reports, also with a peak intensity of IV.

November 2003 MAP GRAPH

    December 2003 MAP GRAPH



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      Division of Geological and Planetary Sciences | California Institute of Technology