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M5.1 (SCSN), Mw 4.7 (SCSN)

15.2 km (9.4 miles)

Moderate M5.1 earthquake strikes Southern California

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SCSN-TriNet Outage

The Southern California Seismic Network experienced a major, single point failure of one of its internet routers early yesterday evening that crippled internal and external data access and processing, including the entire TriNet Recent Earthquake web pages, from about 10 p.m. on Tuesday night till about mid Wednesday morning.

Unfortunately, the Anza earthquake occurred at 11:56 p.m. on Tuesday night, a couple hours after the failure, frustrating not only the users of the system but the SCSN engineers as well.

Patches and fixes have temporarily enabled the system and work continues to repair the situation.

Background Information

This was the first tremor to register in the M5 range in Southern California since the Big Bear M5.1 earthquake on February 10 earlier this year. It follows an ongoing sequence in the Los Angeles Basin which have greatly raised the concerns of the public. Overall, however, seismic activity in Southern California has been relaxed, posting between 35-50 M2+ events for the last few months tectonic action well within typical background levels.

San Jacinto Fault Zone

The quake occurred along the San Jacinto fault zone (SJFZ), one of the major tectonic features in Southern California. It strikes northwest, parallel with the San Andreas Fault, and extends for more the 140 miles from Imperial Valley in the south to San Bernardino in north.

The surface trace of the SJFZ is complex and composed of several discontinuous splays and strands that sometimes form a zone 4-6 miles wide. They are five main segments, San Bernardino Valley, San Jacinto Valley, Anza, Borrego Mountain and Superstition Hills. The fault shows predominantly right-lateral strike-slip (sideways) movement but some portions show vertical displacements.

The San Jacinto fault zone (SJFZ) is by far the most active fault in Southern California at all magnitude ranges less than M7. It has produced at least nine events registering M6.0 or stronger since 1890 and four since 1932. The last two large (M6+) temblors on the SJFZ were the Borrego Mountain M6.4 earthquake on April 9, 1968 and the Superstition Hills M6.6 earthquake on November 24, 1987.

Anza Fault Segment

The October 31, 2001 Anza M5.1 earthquake occurred along the Anza segment, the only segment not to rupture with a large M6+ earthquake in historic times, thus creating a notable seismic gap.

Including this latest event, the Anza fault segment has produced 20 quakes registering M4.0 or stronger since 1932, four of which have measured in the M5 range (see list and graph). The strongest quake measured M5.8 and occurred on April 28, 1969. The other to M5s were a M5.1 on September 23, 1953, and a M5.5 on February 25, 1980.

Activity registering M3.0 or stronger on the Anza fault segment from 1932 to 1966 was fairly relaxed and ranged from 0-4 quakes per year. Only once did the annual tally of M3+ events hit the 5 quake per week mark. But activity changed in the next 15 years, posting nine years above the 5 quake per week level. The most robust year was 1980 when 12 quakes were recorded. Most of these were aftershocks to the M5.5 temblor on February 25.

Following 1982, seismicity relaxed significantly for the next 18 years and posted only on year during which seismicity reached the 5 quake per week mark. Interestingly, the M4+ activity dropped of as well, releasing only two quakes in the M4 range, one in 1989 and the other in 1999.

Seismicity on the Anza segment suddenly reversed its quiescence in 2001 and as of this date, has produced 11 quakes measuring M3.0 or stronger. Considering none of the events were aftershocks to another event, this is clearly the most active year for this fault segment in historic times.

Additional Seismic information links:

Seismograms from nearby stations:
Mainstream media news reports:
Report update: 11:00:54 AM, Thursday, November 1, 2001

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