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Note:This page contains AI-generated content for informational and entertainment purposes only. It may contain inaccuracies. Raw event data is from USGS and EMSC. All statistics, lists, and derived information are generated by this site. Full disclaimerFound an error?
Location:
10 km NNE of Cambria, California
Period:
31 Dec 2003 15:52:49 - 2 Jan 2004 17:57:42 (2 days 2 hours 4 minutes)
Volcanoes in 100km radius:
None
Earthquakes:
67
16 swarms found nearby.
2003
S20031222.2(28.1km)
22 Dec
15 days 17 hours
643 earthquakes
22 Dec
1 day 0 hours
127 earthquakes
S20031224.1(28.4km)
23 Dec
2 days 19 hours
170 earthquakes
2004
S20040201.1(26.3km)
31 Jan
9 days 8 hours
199 earthquakes
1 Feb
1 day 12 hours
34 earthquakes
S20040205.1(27.1km)
4 Feb
2 days 23 hours
57 earthquakes
1 Mar
24 days 6 hours
484 earthquakes
S20040303.1(28.4km)
3 Mar
8 days 3 hours
172 earthquakes
16 Mar
7 days 18 hours
281 earthquakes
4 Apr
18 days 8 hours
226 earthquakes
S20040425.1(29.9km)
25 Apr
19 days 16 hours
216 earthquakes
26 Jul
10 days 1 hours
140 earthquakes
13 Oct
8 days 7 hours
97 earthquakes
2005
1 Oct
10 days 17 hours
151 earthquakes
2006
6 Apr
11 days 10 hours
151 earthquakes
2009
S20090620.2(14.8km)
20 Jun
4 days 17 hours
84 earthquakes
AI-generated article — for informational and entertainment purposes only. May contain inaccuracies. Full disclaimerFound an error?

Earthquake Swarm Near Cambria, California: December 2003–January 2004

An earthquake swarm designated S20040101.1 occurred approximately 10 km north-northeast of Cambria, California, between 15:52 on 31 December 2003 and 17:57 on 2 January 2004. During this 50-hour period, 67 events were recorded, providing a clear example of swarm behavior in the central California coastal region.

The sequence began with a magnitude 2.0 event at 5 km depth. Activity intensified on 1 January, featuring two of the largest shocks: a magnitude 3.7 at 5 km depth and a magnitude 3.6 at 8 km depth. Magnitudes across the swarm ranged primarily between 1.1 and 2.9, with the majority falling between 1.2 and 2.2. Focal depths remained shallow, concentrated between 2 km and 8 km, consistent with brittle failure in the upper crust.

Temporal clustering showed peak rates during the morning and afternoon of 1 January, followed by a gradual decline on 2 January. The final notable event reached magnitude 2.3 at 4 km depth. Such patterns—rapid onset, numerous small-to-moderate events without a dominant mainshock, and quick decay—are characteristic of fluid-driven or aseismic-slip-triggered swarms rather than classic foreshock–mainshock–aftershock sequences.

Cambria lies within the tectonically active central Coast Ranges, where the Pacific–North American plate boundary is accommodated by the San Andreas Fault system and subsidiary structures, including the Hosgri–San Gregorio fault zone. The region experiences distributed strike-slip and reverse faulting at rates of several millimeters per year. Historical seismicity includes moderate events along the coastal margin, with documented activity since the late nineteenth century. Since 1 January 2000, three swarms have been identified in the broader area; the December 2003–January 2004 sequence represents the first.

This swarm highlights the persistent low-level seismic hazard along California’s central coast. Continued monitoring by regional networks remains essential for distinguishing swarm activity from potential precursors to larger events.

References

  • SeismoSight internal swarm classification database
  • USGS Earthquake Catalog (historical regional data)