Location:

Period:

13 Jun 2008 23:43:45 - 14 Jun 2008 03:27:38 (3 hours 43 minutes)

Volcanoes in 100km radius:

Kurikomayama(15km), Naruko(41km), Chokaisan(65km), Akita-Komagatake(73km), Hijiori(79km), Iwatesan(85km), Hachimantai(95km), Akita-Yakeyama(95km)

Earthquakes:

M 7.0+:

M 7.0; 27 km SSW of ?funato, Japan; (26 May 2003) (71km from the swarm center)

9 swarms found nearby.

2008

S20080614.1(10.0km)

13 Jun

3 days 16 hours

67 earthquakes

2011

PS20110309.1(132.4km)

9 Mar

7 days 12 hours

159 earthquakes

PS20110311.3(164.2km)

11 Mar

1 day 17 hours

44 earthquakes

PS20110312.2(189.6km)

11 Mar

20 hours

6 earthquakes

PS20110311.8(144.4km)

11 Mar

9 hours

5 earthquakes

PS20110319.2(116.4km)

18 Mar

1 day 17 hours

8 earthquakes

PS20110323.1(183.2km)

22 Mar

23 hours

6 earthquakes

PS20110413.1(198.6km)

13 Apr

8 hours

6 earthquakes

2022

PS20220316.1(170.8km)

16 Mar

1 hours

7 earthquakes

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Seismic Swarm PS20080614.1 Near Furukawa, Japan

The seismic swarm designated PS20080614.1 occurred in northeastern Japan, centered approximately 29 km northwest of Furukawa in Miyagi Prefecture. This sequence unfolded over 3 hours and 43 minutes between 23:43 on 13 June 2008 and 03:27 on 14 June 2008, registering six earthquakes. The events took place within the tectonically active Ou Backbone Range, where the Pacific Plate subducts beneath the Okhotsk Plate, generating both interplate and intraplate seismicity.

The sequence began with a magnitude 6.9 event at 23:43:45 on 13 June at a depth of 7 km. Subsequent shocks included a magnitude 5.1 at 23:49:05 (11 km depth), a magnitude 4.4 at 00:00:04 on 14 June (5 km depth), a magnitude 5.5 at 00:20:11 (6 km depth), a magnitude 5.4 at 00:20:47 (10 km depth), and a final magnitude 5.1 at 03:27:38 (40 km depth). These shallow focal depths indicate rupture within the upper crust, consistent with regional thrust-faulting mechanisms driven by compressional stress from plate convergence.

This swarm reflects typical behavior in the Tohoku region, where clusters of moderate earthquakes often accompany larger intraplate events along active reverse faults. The 2008 activity aligns with the broader tectonic framework of the area, which has experienced repeated strong shaking due to both subduction-zone megathrusts and inland crustal faults. A notable prior event was the magnitude 7.0 earthquake on 26 May 2003, located 27 km south-southwest of Ofunato and approximately 71 km from the swarm centroid; that shock also occurred on a reverse fault within the same stress regime.

Seismic swarms in this setting frequently arise from fluid migration or stress transfer along fault networks, though the precise trigger for PS20080614.1 remains tied to the immediate tectonic loading. The concentration of events at depths between 5 km and 11 km, with one deeper outlier at 40 km, highlights the vertical extent of brittle failure in the regional crust. Such patterns underscore the importance of dense seismic monitoring for distinguishing swarm sequences from classic mainshock-aftershock series.

The geological history of the Furukawa vicinity includes Quaternary volcanism and uplift along the Ou Mountains, superimposed on older accretionary complexes. Ongoing convergence continues to load regional faults, making the area prone to both isolated large events and clustered moderate activity like the 2008 swarm.

References
SeismoSight internal swarm classification PS20080614.1
USGS Earthquake Catalog (events since 2000)
Geological Survey of Japan, active fault database