Location:

Period:

22 Sep 2016 15:56:05 - 23 Sep 2016 10:28:20 (18 hours 32 minutes)

Volcanoes in 100km radius:

None

Earthquakes:

14 swarms found nearby.

2000

PS20000629.1(156.7km)

28 Jun

1 day 0 hours

10 earthquakes

PS20000703.1(196.0km)

2 Jul

7 hours

5 earthquakes

2005

PS20050119.1(33.5km)

19 Jan

10 hours

6 earthquakes

PS20050121.1(52.7km)

20 Jan

1 day 3 hours

8 earthquakes

PS20050726.1(173.3km)

26 Jul

1 day 19 hours

17 earthquakes

PS20050729.1(177.9km)

29 Jul

16 hours

6 earthquakes

2008

PS20080507.1(168.6km)

7 May

17 hours

11 earthquakes

2011

PS20110311.2(88.7km)

11 Mar

21 hours

21 earthquakes

PS20110311.4(154.1km)

11 Mar

8 hours

57 earthquakes

PS20110311.5(151.0km)

11 Mar

7 hours

10 earthquakes

PS20110313.1(137.3km)

13 Mar

3 days 13 hours

14 earthquakes

PS20110317.1(148.4km)

16 Mar

1 day 16 hours

10 earthquakes

PS20110322.1(148.0km)

22 Mar

2 days 5 hours

14 earthquakes

2021

PS20210804.1(178.2km)

3 Aug

18 hours

5 earthquakes

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Seismic Swarm Offshore Katsuura, Japan: Analysis of the September 2016 Event

The September 2016 seismic swarm, classified internally as PS20160923.1, occurred approximately 143 km east-southeast of Katsuura on Japan’s Boso Peninsula. The sequence began at 15:56 UTC on 22 September 2016 and concluded at 10:28 UTC on 23 September 2016, spanning 18 hours and 32 minutes. During this interval, eight earthquakes were recorded, with magnitudes ranging from 4.9 to 6.2 and focal depths predominantly between 10 and 19 km.

The events unfolded as follows: two magnitude-5.3 shocks at 15:56 and 15:57 on 22 September, followed by a magnitude-5.0 event at 16:00, all at depths of 10–19 km. Activity resumed early on 23 September with a magnitude-6.2 earthquake at 00:14 (depth 10 km), succeeded by a magnitude-4.9 shock at 00:21. Subsequent events included magnitude-5.2 and 5.3 earthquakes at 05:34 and 06:13, respectively, both at 10 km depth, and a final magnitude-5.0 event at 10:28.

This swarm took place within the tectonically active offshore region east of the Boso Peninsula, where the Pacific Plate subducts beneath the Philippine Sea Plate along the Izu-Bonin Trench system. The area experiences frequent seismicity driven by plate convergence rates of approximately 5–7 cm per year. Historical records indicate that the broader region has hosted multiple earthquake swarms since 2000, with a total of 13 documented episodes. These occurred in 2000 (two swarms), 2005 (four swarms), 2008 (one swarm), and 2011 (six swarms), highlighting a recurring pattern of clustered seismic activity without a single dominant mainshock.

Swarm sequences such as PS20160923.1 typically reflect fluid migration or stress transfer along fault networks rather than classic foreshock-mainshock-aftershock patterns. The concentration of events at shallow depths (mostly 10 km) aligns with the brittle-ductile transition zone in this subduction setting, where increased pore pressure can trigger multiple moderate-magnitude ruptures over short periods. No significant surface deformation or tsunami was associated with the 2016 swarm, consistent with its moderate energy release and offshore location.

Geological studies of the Boso Peninsula and adjacent seafloor confirm the presence of active thrust faults and strike-slip structures accommodating oblique convergence. Long-term monitoring by Japanese agencies shows elevated background seismicity rates in this sector, underscoring the importance of continued surveillance for potential larger events linked to the Nankai Trough and Sagami Trough systems farther west.

Japan Meteorological Agency seismic catalog (2016–2023 updates)
USGS Earthquake Hazards Program regional tectonic summaries
Geological Survey of Japan, AIST: Subduction zone structure reports