Seismic Swarm Analysis: October 31, 2017 Event West of Ambon, Indonesia

A seismic swarm occurred 41 km west of Ambon, Indonesia, on October 31, 2017. The sequence began at 11:31 and concluded at 16:47 local time, encompassing 10 earthquakes over 5 hours and 16 minutes. This event, classified internally as Swarm PS20171031.2, featured magnitudes between 4.9 and 6.1, with most hypocenters at shallow depths of 4–15 km and one outlier at 60 km.

The swarm initiated with a magnitude 5.8 event at 4 km depth, followed rapidly by 5.6 and 5.7 shocks. Peak activity included a magnitude 6.1 earthquake at 6 km depth. Subsequent events registered 4.9, 5.7, 5.1, 5.1, 5.1, and a final 5.0 at 10 km depth. Such clustered activity without a dominant mainshock distinguishes swarms from typical aftershock sequences and often signals fluid migration or stress transfer along fault networks.

Ambon lies within the tectonically complex Banda Arc region of eastern Indonesia. This area forms part of the Pacific Ring of Fire, where the Indo-Australian Plate converges with the Eurasian and Philippine Sea plates. Subduction along the Banda Sea produces frequent seismicity, with the arc characterized by intermediate-depth earthquakes and volcanic chains. The Maluku Islands experience elevated hazard due to oblique convergence and back-arc spreading.

Historical records since 2000 indicate only two prior swarms in the vicinity: one in 2000 and another in 2015. These infrequent clusters underscore the episodic nature of swarm activity amid the region's steady background seismicity driven by plate boundary forces.

Seismic swarms like this one provide insights into local stress regimes. The predominance of shallow events suggests activation of upper-crustal faults, potentially influenced by the island's proximity to active volcanic systems. Monitoring such sequences aids in refining hazard assessments for Ambon and surrounding communities.

References

• SeismoSight internal swarm classification records
• Regional tectonic summaries from Indonesian geological surveys
• Banda Arc seismicity overviews from global monitoring networks