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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 W of Templeton, California
Period:
4 Apr 2004 03:35:10 - 22 Apr 2004 11:36:30 (18 days 8 hours 1 minute)
Volcanoes in 100km radius:
None
Earthquakes:
226
18 swarms found nearby.
2003
S20031222.1(29.1km)
22 Dec
37 days 3 hours
2456 earthquakes
S20031222.2(26.9km)
22 Dec
15 days 17 hours
643 earthquakes
22 Dec
1 day 0 hours
127 earthquakes
S20031224.1(27.2km)
23 Dec
2 days 19 hours
170 earthquakes
31 Dec
2 days 2 hours
67 earthquakes
2004
S20040201.1(25.5km)
31 Jan
9 days 8 hours
199 earthquakes
1 Feb
1 day 12 hours
34 earthquakes
S20040205.1(26.0km)
4 Feb
2 days 23 hours
57 earthquakes
1 Mar
24 days 6 hours
484 earthquakes
S20040303.1(27.5km)
3 Mar
8 days 3 hours
172 earthquakes
16 Mar
7 days 18 hours
281 earthquakes
S20040425.1(28.8km)
25 Apr
19 days 16 hours
216 earthquakes
S20040528.1(29.9km)
27 May
2 days 11 hours
49 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(14.1km)
20 Jun
4 days 17 hours
84 earthquakes
AI-generated article — for informational and entertainment purposes only. May contain inaccuracies. Full disclaimerFound an error?

Seismic Swarm S20040405.1: Analysis of Activity Near Templeton, California

Seismic swarm S20040405.1 occurred 10 km west of Templeton in San Luis Obispo County, California. The sequence began at 03:35 on 4 April 2004 and concluded at 11:36 on 22 April 2004, spanning 440 hours and 1 minute. During this period, 226 earthquakes were recorded.

The first 100 events displayed consistent shallow focal depths, predominantly between 3 km and 8 km. Magnitudes remained low, with the majority falling between 0.5 and 2.1; the peak magnitude reached 3.1 at 13:45 on 4 April. Early activity featured rapid succession of events, including multiple occurrences within minutes on 4 April. Subsequent days showed sustained but gradually declining frequency, with notable clusters around 8 April and 9 April. Depths occasionally approached the surface at 0–1 km or extended to 9 km, yet most remained in the mid-crustal range typical of the local fault network.

This swarm aligns with the tectonic setting of central coastal California, where the Pacific–North American plate boundary produces distributed deformation along strike-slip and oblique-slip faults. The area lies near the Rinconada and Hosgri fault systems, which accommodate right-lateral motion and contribute to regional seismicity. Historical records indicate recurrent swarm activity, with 11 documented swarms since 1 January 2000. Prior episodes include five swarms in 2003 and six in 2004, underscoring episodic clustering rather than isolated mainshock-aftershock sequences.

Such swarms typically arise from fluid migration or aseismic slip transients that trigger numerous small events without producing a dominant rupture. Depths consistently under 10 km reflect brittle failure within the upper crust, consistent with the regional geology of Mesozoic basement rocks overlain by Cenozoic sedimentary units. No surface rupture or significant damage was associated with this sequence.

Continued monitoring of the central coast remains essential given the proximity to major population centers and infrastructure. The 2004 swarm provides a clear example of swarm behavior that informs probabilistic seismic hazard assessments for the region.

References

United States Geological Survey Earthquake Catalog
California Geological Survey Fault Database
SeismoSight internal swarm classification records