Location:

Period:

18 Jul 2006 05:06:36 - 20 Jul 2006 01:17:26 (1 day 20 hours 10 minutes)

Volcanoes in 100km radius:

None

Earthquakes:

M 7.0+:

M 7.7; 226 km SSW of Singaparna, Indonesia; (17 Jul 2006) (63km from the swarm center)

3 swarms found nearby.

2006

PS20060717.1(73.2km)

17 Jul

1 day 23 hours

64 earthquakes

PS20060717.2(67.3km)

17 Jul

4 hours

5 earthquakes

PS20060723.1(52.0km)

22 Jul

2 days 16 hours

17 earthquakes

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Seismic Swarm PS20060719.1: Post-Mainshock Activity in the Sunda Subduction Zone

Seismic swarm PS20060719.1 occurred in the waters south of Java, Indonesia, approximately 261 km south-southwest of Kawalu. The sequence began at 05:06 on 18 July 2006 and concluded at 01:17 on 20 July 2006, spanning 44 hours and 10 minutes. During this interval, ten earthquakes were recorded, with magnitudes ranging from 4.2 to 5.5 and focal depths between 7 km and 34 km.

The swarm commenced with a magnitude 5.3 event at 30 km depth, followed later the same day by two magnitude 5.0 shocks at 34 km and 7 km. Activity continued into 19 July with a magnitude 4.2 at 10 km, a magnitude 5.5 at 10 km, and additional events of magnitude 5.0 and 5.2, all clustered near 10 km depth. The sequence ended early on 20 July with a magnitude 5.1 shock at 10 km. Most events occurred at shallow to intermediate crustal depths consistent with the overriding plate and plate interface in this tectonic setting.

This swarm took place one day after a magnitude 7.7 mainshock located 226 km south-southwest of Singaparna, only 63 km from the swarm centroid. The mainshock ruptured the megathrust along the Sunda Trench, where the Indo-Australian plate subducts beneath the Eurasian plate at rates of approximately 6–7 cm per year. The subsequent swarm is interpreted as triggered seismicity, likely reflecting stress redistribution on nearby faults and afterslip processes within the subduction zone.

Indonesia’s southern Java margin lies within the Sunda subduction system, one of the most seismically active regions on Earth. Historical records document repeated great earthquakes and associated tsunamis generated by slip on the plate interface. The 2006 mainshock itself produced a modest tsunami along the southern Java coast, underscoring the persistent hazard. Since 2000, only two swarms have been identified in this sector, indicating that clustered moderate-magnitude sequences are relatively uncommon compared with isolated aftershocks or background seismicity.

Geological conditions in the region feature a well-developed accretionary prism, forearc basin sediments, and volcanic arc systems onshore. Crustal structure includes a relatively thin overriding plate that permits shallow seismicity, matching the depth distribution observed in the swarm. Continued monitoring of such sequences provides valuable data on post-seismic deformation and the potential for renewed large-magnitude rupture along adjacent locked patches of the megathrust.

References

USGS Earthquake Catalog (events 2006-07-17 to 2006-07-20)
SeismoSight internal swarm classification PS20060719.1