Location:

Period:

17 Jul 2006 08:19:26 - 19 Jul 2006 07:25:06 (1 day 23 hours 5 minutes)

Volcanoes in 100km radius:

None

Earthquakes:

M 7.0+:

M 7.7; 226 km SSW of Singaparna, Indonesia; (17 Jul 2006) (15km from the swarm center)

3 swarms found nearby.

2006

PS20060717.2(138.3km)

17 Jul

4 hours

5 earthquakes

PS20060719.1(73.2km)

18 Jul

1 day 20 hours

10 earthquakes

PS20060723.1(122.3km)

22 Jul

2 days 16 hours

17 earthquakes

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Seismic Swarm PS20060717.1: Aftershock Sequence Following the 2006 Java Earthquake

The seismic swarm designated PS20060717.1 occurred along the Sunda subduction zone south of Java, Indonesia, from 08:19 on 17 July 2006 to 07:25 on 19 July 2006. Over 47 hours and 5 minutes, 64 earthquakes were recorded, with the sequence centered 239 km SSW of Kawalu. This activity represents the immediate aftershock phase of a magnitude 7.7 mainshock that struck at a depth of 20 km.

The mainshock epicenter lay approximately 15 km from the swarm centroid, consistent with rupture along the megathrust interface where the Indo-Australian Plate subducts beneath the Eurasian Plate. The Java segment of the Sunda Trench accommodates convergence at roughly 6–7 cm per year, generating frequent large earthquakes and occasional tsunamis. Depths of events in the swarm ranged from 0 km to 35 km, with the majority clustered between 10 km and 25 km, aligning with the expected seismogenic zone of the subduction interface.

The sequence exhibited classic aftershock behavior, beginning with the M 7.7 event followed by numerous moderate aftershocks, including multiple events of M 5.5–6.1 within the first 24 hours. Notable larger aftershocks included an M 6.1 at 15:45 on 17 July and several M 5.9 events distributed throughout the period. The rapid decay in frequency after the initial 12 hours matches empirical aftershock patterns observed in other subduction-zone mainshock sequences.

Geologically, southern Java lies within the Ring of Fire, where the oceanic slab descends at a moderate angle beneath the forearc. Historical records document repeated great earthquakes along this margin, including events in 1843, 1867, and 1921, all producing significant shaking and local tsunamis. The 2006 mainshock itself generated a modest tsunami that affected the southern Java coast, underscoring the persistent tsunami hazard in the region.

Post-2006 monitoring by regional networks has confirmed continued background seismicity south of Java, with several M 6+ events occurring in subsequent years along the same segment. The 2006 sequence remains one of the best-recorded aftershock series in the modern instrumental catalog for the Java megathrust, providing valuable data on stress transfer and fault behavior at the plate interface.

References

USGS Earthquake Catalog
Global CMT Catalog
Indonesian Meteorological, Climatological, and Geophysical Agency (BMKG) reports