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Location:
10km S of Idyllwild, CA
Period:
21 Dec 2016 05:16:20 - 23 Dec 2016 02:57:22 (1 day 21 hours 41 minutes)
Volcanoes in 100km radius:
None
Earthquakes:
34
61 swarms found nearby.
2002
S20021024.3(25.9km)
23 Oct
2 days 17 hours
38 earthquakes
2003
5 Mar
1 day 2 hours
35 earthquakes
2005
S20050612.1(24.9km)
12 Jun
26 days 19 hours
904 earthquakes
S20050820.1(28.4km)
19 Aug
3 days 23 hours
96 earthquakes
2009
25 Jul
1 day 19 hours
29 earthquakes
2010
S20100708.1(29.9km)
7 Jul
35 days 19 hours
1709 earthquakes
S20101203.2(27.7km)
2 Dec
10 days 0 hours
68 earthquakes
2011
1 Feb
1 day 18 hours
43 earthquakes
S20110416.1(24.1km)
15 Apr
7 days 6 hours
90 earthquakes
S20110727.1(26.0km)
26 Jul
4 days 1 hours
42 earthquakes
2013
S20130922.1(27.9km)
21 Sep
8 days 10 hours
91 earthquakes
2014
26 Jan
2 days 4 hours
38 earthquakes
18 Apr
8 days 7 hours
191 earthquakes
9 May
2 days 9 hours
44 earthquakes
2015
10 Feb
12 days 21 hours
168 earthquakes
S20150401.1(29.9km)
31 Mar
38 days 7 hours
543 earthquakes
2016
14 Jun
7 days 5 hours
80 earthquakes
S20160720.2(29.8km)
19 Jul
6 days 5 hours
68 earthquakes
S20161230.1(26.3km)
29 Dec
4 days 7 hours
48 earthquakes
2017
S20170503.1(12.1km)
2 May
5 days 5 hours
91 earthquakes
S20170510.1(11.3km)
9 May
15 days 6 hours
129 earthquakes
8 Jun
3 days 22 hours
61 earthquakes
S20170727.1(12.1km)
26 Jul
14 days 9 hours
192 earthquakes
S20170907.1(11.0km)
6 Sep
14 days 21 hours
245 earthquakes
S20170926.1(10.4km)
25 Sep
124 days 21 hours
2217 earthquakes
2018
S20180203.1(12.7km)
2 Feb
28 days 21 hours
426 earthquakes
S20180305.1(12.9km)
4 Mar
19 days 17 hours
203 earthquakes
S20180403.1(11.6km)
2 Apr
10 days 14 hours
197 earthquakes
S20180420.1(11.2km)
19 Apr
69 days 21 hours
1109 earthquakes
S20180527.1(12.7km)
26 May
4 days 20 hours
54 earthquakes
S20180630.1(12.4km)
29 Jun
41 days 14 hours
582 earthquakes
S20180811.1(12.1km)
11 Aug
209 days 15 hours
6032 earthquakes
2019
S20190328.1(13.4km)
27 Mar
5 days 20 hours
79 earthquakes
S20190426.1(12.3km)
25 Apr
1 day 22 hours
37 earthquakes
S20190905.1(23.7km)
4 Sep
4 days 3 hours
56 earthquakes
2020
8 Jan
11 days 9 hours
159 earthquakes
S20200122.1(10.6km)
21 Jan
3 days 12 hours
57 earthquakes
1 Feb
8 days 17 hours
147 earthquakes
S20200301.1(10.7km)
29 Feb
6 days 16 hours
67 earthquakes
25 May
7 days 18 hours
100 earthquakes
6 Jun
9 days 19 hours
109 earthquakes
2 Jul
3 days 16 hours
70 earthquakes
S20200708.1(11.1km)
7 Jul
26 days 8 hours
287 earthquakes
S20200926.2(10.2km)
25 Sep
3 days 10 hours
42 earthquakes
18 Dec
5 days 0 hours
66 earthquakes
2021
27 Mar
2 days 21 hours
33 earthquakes
26 Apr
5 days 5 hours
80 earthquakes
S20210609.1(11.8km)
8 Jun
5 days 16 hours
126 earthquakes
20 Sep
8 days 7 hours
216 earthquakes
S20211223.1(26.8km)
22 Dec
1 day 1 hours
46 earthquakes
S20211231.1(26.7km)
30 Dec
4 days 1 hours
63 earthquakes
2022
S20220109.1(26.6km)
8 Jan
18 hours
37 earthquakes
18 Apr
8 days 1 hours
76 earthquakes
1 Jul
3 days 23 hours
49 earthquakes
25 Oct
1 day 10 hours
35 earthquakes
2023
20 Feb
1 day 17 hours
25 earthquakes
S20230305.1(21.1km)
5 Mar
1 day 2 hours
25 earthquakes
S20230326.1(11.4km)
25 Mar
2 days 6 hours
38 earthquakes
2024
28 Aug
2 days 1 hours
31 earthquakes
2025
16 May
4 days 7 hours
45 earthquakes
28 Nov
7 days 2 hours
82 earthquakes
AI-generated article — for informational and entertainment purposes only. May contain inaccuracies. Full disclaimerFound an error?

Seismic Swarm S20161221.1 Near Idyllwild, California: Analysis and Regional Context

A seismic swarm designated S20161221.1 occurred 10 km south of Idyllwild in Riverside County, California. The sequence began at 05:16 on 21 December 2016 and concluded at 02:57 on 23 December 2016, spanning 45 hours and 41 minutes. During this period, 34 earthquakes were recorded, with magnitudes ranging from 0.2 to 2.4 and focal depths between 7 km and 16 km.

The swarm initiated with a magnitude 1.3 event at 14 km depth. Activity remained low until a cluster of events on the evening of 21 December, including two magnitude 2.4 earthquakes at 13 km depth occurring 21 seconds apart. Subsequent events showed a gradual decline in frequency and magnitude, ending with a magnitude 0.9 event at 7 km depth.

This swarm unfolded within the San Jacinto Fault Zone, a major component of the Pacific-North American plate boundary in Southern California. The zone accommodates right-lateral strike-slip motion and forms part of the broader San Andreas Fault system. The Peninsular Ranges, including the San Jacinto Mountains surrounding Idyllwild, consist primarily of Mesozoic granitic and metamorphic rocks overlying older basement complexes. Seismicity in the region arises from ongoing transpressional tectonics, with the San Jacinto Fault exhibiting slip rates of approximately 10–15 mm per year.

Earthquake swarms in this area typically reflect fluid migration or aseismic slip along fault segments rather than mainshock-aftershock sequences. Depths recorded in S20161221.1 align with the brittle-ductile transition zone in the upper crust of the Peninsular Ranges.

Historical data indicate 18 swarms have occurred in the vicinity since 1 January 2000. These include single events in 2002, 2003, and 2009; two each in 2005, 2010, 2015, and 2016; three each in 2011 and 2014; and one in 2013. Such recurrent swarms underscore the persistent low-level strain release along the San Jacinto Fault Zone.

References

  • United States Geological Survey Earthquake Catalog
  • California Geological Survey Fault Activity Map
  • Southern California Earthquake Data Center Swarm Database