Location:

Period:

16 Aug 2013 02:31:05 - 16 Aug 2013 06:55:58 (4 hours 24 minutes)

Volcanoes in 100km radius:

None

Earthquakes:

3 swarms found nearby.

2013

S20130721.1(4.8km)

20 Jul

2 days 6 hours

46 earthquakes

2016

PS20161113.1(66.6km)

13 Nov

1 day 19 hours

33 earthquakes

S20161113.1(8.3km)

13 Nov

2 days 16 hours

59 earthquakes

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Seismic Swarm PS20130816.1: Earthquake Sequence Southeast of Blenheim

On 16 August 2013, a seismic swarm designated PS20130816.1 was recorded 29 km southeast of Blenheim in New Zealand’s South Island. The sequence began at 02:31 and concluded at 06:55, encompassing 10 earthquakes over 4 hours and 24 minutes. This event represents the sole swarm documented in the region since 1 January 2000.

The swarm initiated with a magnitude 6.5 earthquake at a depth of 8 km. Subsequent events included a magnitude 5.4 at 9 km depth, followed by a magnitude 5.5 at 5 km. Additional shocks registered magnitudes of 4.5, 5.9, 5.1, 5.1, 5.9, 5.0, and 5.0, with depths ranging from 5 km to 19 km. The sequence displayed a pattern of moderate-to-strong aftershocks clustered within the first few hours, with later events occurring at progressively greater depths.

The location lies within the Marlborough Fault System, a network of strike-slip faults that accommodate oblique convergence between the Pacific and Australian plates. This tectonic setting has produced a long history of seismic activity, including the 1848 Marlborough earthquake and the 2016 Kaikōura event that ruptured multiple faults across the region. Blenheim sits near the Wairau Fault, part of the broader Alpine Fault system that extends southwestward along the South Island.

Swarm activity in this area remains infrequent based on records since 2000, with PS20130816.1 standing as the only classified swarm. Depths between 5 km and 19 km align with typical shallow crustal seismicity along the plate boundary, where brittle failure occurs in the upper lithosphere.

16 Aug 2013 02:31:05, magnitude 6.5, 8 km depth
16 Aug 2013 02:37:27, magnitude 5.4, 9 km depth
16 Aug 2013 02:45:27, magnitude 5.5, 5 km depth
16 Aug 2013 03:06:13, magnitude 4.5, 5 km depth
16 Aug 2013 03:09:08, magnitude 5.9, 8 km depth
16 Aug 2013 03:21:31, magnitude 5.1, 17 km depth
16 Aug 2013 03:51:35, magnitude 5.1, 19 km depth
16 Aug 2013 05:31:16, magnitude 5.9, 14 km depth
16 Aug 2013 05:58:28, magnitude 5.0, 17 km depth
16 Aug 2013 06:55:58, magnitude 5.0, 19 km depth

Ongoing monitoring by GNS Science continues to track regional strain accumulation along these faults, providing context for future seismic hazard assessments in the Marlborough district.

References

GNS Science geological reports on the Marlborough Fault System.
GeoNet earthquake catalogue for South Island events.