Location:

Period:

14 May 2023 08:11:57 - 14 May 2023 16:22:52 (8 hours 10 minutes)

Volcanoes in 100km radius:

Kurose Hole(36km), Hachijojima(53km), Mikurajima(60km), Miyakejima(80km), Kozushima(92km)

Earthquakes:

11 swarms found nearby.

2000

PS20000629.1(101.2km)

28 Jun

1 day 0 hours

10 earthquakes

PS20000703.1(80.9km)

2 Jul

7 hours

5 earthquakes

S20000706.1(79.8km)

5 Jul

1 day 6 hours

27 earthquakes

PS20000712.1(75.8km)

11 Jul

2 days 5 hours

10 earthquakes

PS20000720.1(73.7km)

19 Jul

16 hours

6 earthquakes

PS20000723.1(82.6km)

23 Jul

1 day 5 hours

6 earthquakes

PS20000730.1(73.1km)

30 Jul

12 hours

5 earthquakes

PS20000803.1(84.9km)

2 Aug

21 hours

7 earthquakes

S20000803.1(74.3km)

3 Aug

1 day 20 hours

45 earthquakes

PS20000815.1(82.2km)

15 Aug

10 hours

7 earthquakes

2001

PS20010704.1(117.8km)

4 Jul

20 hours

12 earthquakes

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Seismic Swarm PS20230514.1 South of Shimoda, Japan

A seismic swarm designated PS20230514.1 occurred on 14 May 2023, approximately 162 km south of Shimoda on Japan’s Izu Peninsula. The sequence began at 08:11 and concluded at 16:22, registering eight earthquakes within an eight-hour-and-ten-minute window. All events were recorded at shallow to moderate depths between 9 km and 18 km, with magnitudes ranging from 4.8 to 5.8.

The sequence opened with a magnitude 5.4 event at 08:11:57 (18 km depth), followed rapidly by a magnitude 5.8 shock at 08:21:41 (12 km depth). Subsequent events included magnitudes 5.0 (9 km), 5.6 (13 km), 4.8 (14 km), 5.0 (10 km), 5.2 (10 km), and a final magnitude 5.1 (10 km) at 16:22:52. The clustering of these events within a narrow temporal and spatial window is characteristic of swarm behavior rather than a typical mainshock-aftershock sequence.

The region lies within the tectonically active Philippine Sea Plate margin, where subduction and back-arc processes generate frequent seismic and volcanic activity. Shallow crustal earthquakes in this offshore area commonly reflect stress release along minor faults associated with the broader Izu-Bonin subduction system. Historical records since 1 January 2000 document eleven swarms in the same locale, with notable concentrations in 2000 (ten events) and 2001 (one event). These earlier episodes demonstrate recurring swarm activity over more than two decades, underscoring the persistent seismic character of the source zone.

Analysis of the 2023 swarm indicates a rapid onset and decay, with the largest event occurring early in the sequence. Depths remained consistently above 18 km, suggesting failure within the brittle upper crust. Such patterns align with fluid migration or localized stress perturbations often observed in subduction-related swarms, although detailed source mechanisms require further instrumental study.

Continued monitoring of this offshore segment remains important for understanding long-term seismic hazards along Japan’s southern Izu margin. The May 2023 activity adds to the documented history of episodic swarms and contributes to refined assessments of regional strain accumulation.

References
SeismoSight internal swarm classification PS20230514.1
Japan Meteorological Agency regional seismic bulletins (2000–2023)