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Location:
10km S of Idyllwild, CA
Period:
20 Sep 2021 23:22:22 - 29 Sep 2021 06:44:51 (8 days 7 hours 22 minutes)
Volcanoes in 100km radius:
None
Earthquakes:
216
75 swarms found nearby.
2001
S20011031.1(27.0km)
31 Oct
10 days 17 hours
471 earthquakes
2002
S20021024.3(21.1km)
23 Oct
2 days 17 hours
38 earthquakes
2003
5 Mar
1 day 2 hours
35 earthquakes
2005
S20050612.1(21.5km)
12 Jun
26 days 19 hours
904 earthquakes
S20050820.1(23.1km)
19 Aug
3 days 23 hours
96 earthquakes
2009
25 Jul
1 day 19 hours
29 earthquakes
2010
S20100209.1(27.7km)
8 Feb
3 days 22 hours
44 earthquakes
S20100708.1(24.2km)
7 Jul
35 days 19 hours
1709 earthquakes
S20101203.2(22.8km)
2 Dec
10 days 0 hours
68 earthquakes
2011
1 Feb
1 day 18 hours
43 earthquakes
S20110319.1(28.1km)
18 Mar
1 day 23 hours
34 earthquakes
S20110416.1(18.7km)
15 Apr
7 days 6 hours
90 earthquakes
S20110727.1(21.6km)
26 Jul
4 days 1 hours
42 earthquakes
2012
S20120521.3(29.8km)
20 May
4 days 2 hours
45 earthquakes
S20120622.1(28.3km)
21 Jun
3 days 19 hours
39 earthquakes
S20120630.1(25.3km)
29 Jun
2 days 0 hours
31 earthquakes
2013
S20130311.1(27.7km)
10 Mar
72 days 2 hours
2118 earthquakes
S20130922.1(23.1km)
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
S20140808.1(26.8km)
7 Aug
1 day 13 hours
28 earthquakes
2015
10 Feb
12 days 21 hours
168 earthquakes
S20150401.1(24.8km)
31 Mar
38 days 7 hours
543 earthquakes
2016
S20160610.1(26.6km)
9 Jun
33 days 4 hours
1773 earthquakes
14 Jun
7 days 5 hours
80 earthquakes
S20160720.2(24.7km)
19 Jul
6 days 5 hours
68 earthquakes
S20161026.1(27.3km)
25 Oct
5 days 21 hours
73 earthquakes
21 Dec
1 day 21 hours
34 earthquakes
S20161230.1(21.1km)
29 Dec
4 days 7 hours
48 earthquakes
2017
S20170503.1(15.4km)
2 May
5 days 5 hours
91 earthquakes
S20170510.1(14.6km)
9 May
15 days 6 hours
129 earthquakes
8 Jun
3 days 22 hours
61 earthquakes
S20170727.1(16.0km)
26 Jul
14 days 9 hours
192 earthquakes
S20170907.1(14.6km)
6 Sep
14 days 21 hours
245 earthquakes
25 Sep
124 days 21 hours
2217 earthquakes
S20171110.1(29.9km)
9 Nov
2 days 20 hours
44 earthquakes
2018
2 Feb
28 days 21 hours
426 earthquakes
S20180305.1(12.4km)
4 Mar
19 days 17 hours
203 earthquakes
S20180403.1(14.8km)
2 Apr
10 days 14 hours
197 earthquakes
S20180420.1(15.4km)
19 Apr
69 days 21 hours
1109 earthquakes
26 May
4 days 20 hours
54 earthquakes
S20180630.1(15.0km)
29 Jun
41 days 14 hours
582 earthquakes
S20180811.1(14.1km)
11 Aug
209 days 15 hours
6032 earthquakes
2019
S20190328.1(17.4km)
27 Mar
5 days 20 hours
79 earthquakes
S20190426.1(14.5km)
25 Apr
1 day 22 hours
37 earthquakes
S20190905.1(28.5km)
4 Sep
4 days 3 hours
56 earthquakes
2020
8 Jan
11 days 9 hours
159 earthquakes
S20200122.1(14.5km)
21 Jan
3 days 12 hours
57 earthquakes
1 Feb
8 days 17 hours
147 earthquakes
S20200301.1(11.7km)
29 Feb
6 days 16 hours
67 earthquakes
S20200404.1(28.8km)
4 Apr
40 days 3 hours
2928 earthquakes
25 May
7 days 18 hours
100 earthquakes
S20200607.1(13.8km)
6 Jun
9 days 19 hours
109 earthquakes
S20200703.1(13.3km)
2 Jul
3 days 16 hours
70 earthquakes
S20200708.1(15.1km)
7 Jul
26 days 8 hours
287 earthquakes
25 Sep
3 days 10 hours
42 earthquakes
S20200926.1(29.1km)
25 Sep
2 days 2 hours
44 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(15.8km)
8 Jun
5 days 16 hours
126 earthquakes
2022
S20220113.1(28.9km)
13 Jan
4 days 5 hours
157 earthquakes
S20220419.1(10.1km)
18 Apr
8 days 1 hours
76 earthquakes
S20220519.2(25.1km)
18 May
1 day 8 hours
29 earthquakes
1 Jul
3 days 23 hours
49 earthquakes
S20220922.1(26.0km)
22 Sep
1 day 18 hours
31 earthquakes
S20220930.2(25.7km)
29 Sep
4 days 5 hours
69 earthquakes
25 Oct
1 day 10 hours
35 earthquakes
2023
20 Feb
1 day 17 hours
25 earthquakes
S20230305.1(24.7km)
5 Mar
1 day 2 hours
25 earthquakes
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 S20210921.1: Analysis of Activity Near Idyllwild, California

Seismic swarm S20210921.1 occurred 10 km south of Idyllwild in Riverside County, California. The sequence began at 23:22 UTC on 20 September 2021 and concluded at 06:44 UTC on 29 September 2021, spanning 199 hours and 22 minutes. During this interval, 216 earthquakes were recorded.

The first 100 events displayed predominantly low magnitudes, with the majority between 0.0 and 1.7. Notable outliers included a magnitude 3.4 event at 22:41 on 21 September and a magnitude 3.2 event at 22:57 the same day. Depths clustered tightly around 12–13 km, though occasional shallower readings near 5 km and deeper ones up to 16 km appeared. Activity intensified after the initial larger shocks, producing clusters of events within minutes of one another, consistent with swarm behavior rather than a classic mainshock-aftershock sequence.

Southern California lies at the boundary between the Pacific and North American tectonic plates. The Idyllwild area occupies the northern Peninsular Ranges, where distributed faulting accommodates right-lateral shear. Regional faults such as the San Jacinto and Elsinore systems contribute to elevated seismicity. Historical records indicate that earthquake swarms are a recurring feature in this portion of the plate boundary, reflecting fluid migration and stress redistribution within fractured crust.

Since 2000, 62 swarms have been documented in the vicinity. Annual counts vary, with notable increases in 2016 (6), 2017 (7), 2018 (7), and 2020 (12). The 2021 total reached three swarms by late September, underscoring ongoing tectonic adjustment.

The swarm’s shallow-to-moderate depths and small-magnitude character align with typical background seismicity observed along secondary faults in the Peninsular Ranges. No surface rupture or significant damage was associated with the sequence. Continued monitoring remains essential given the region’s proximity to major population centers and infrastructure.

  • USGS Earthquake Hazards Program
  • California Geological Survey
  • Southern California Seismic Network reports