Location:

Period:

21 Aug 2001 06:52:06 - 22 Aug 2001 20:22:37 (1 day 13 hours 30 minutes)

Volcanoes in 100km radius:

None

Earthquakes:

M 7.0+:

M 7.1; 295 km NE of Gisborne, New Zealand; (21 Aug 2001) (19km from the swarm center)

7 swarms found nearby.

2001

PS20010824.1(1.5km)

24 Aug

10 hours

5 earthquakes

2011

PS20111118.1(117.8km)

18 Nov

5 hours

6 earthquakes

2016

PS20160901.1(119.4km)

31 Aug

1 day 4 hours

21 earthquakes

2020

PS20200126.2(134.3km)

25 Jan

14 hours

5 earthquakes

2021

PS20210304.1(115.7km)

4 Mar

3 days 9 hours

34 earthquakes

S20210304.1(74.8km)

4 Mar

6 days 0 hours

188 earthquakes

PS20210311.1(87.3km)

11 Mar

1 day 6 hours

6 earthquakes

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Seismic Swarm PS20010822.1: Analysis of Activity Offshore Gisborne

A seismic swarm designated PS20010822.1 occurred 288 km northeast of Gisborne, New Zealand, from 06:52 on 21 August 2001 to 20:22 on 22 August 2001. The sequence lasted 37 hours and 30 minutes and included eight earthquakes, all recorded at a focal depth of 33 km. The events began with a magnitude 7.1 mainshock, followed by seven aftershocks ranging from magnitude 4.6 to 5.3.

The full sequence is as follows:

21 August 2001, 06:52:06 – magnitude 7.1
21 August 2001, 12:22:22 – magnitude 5.0
21 August 2001, 13:44:41 – magnitude 5.0
22 August 2001, 03:02:55 – magnitude 4.6
22 August 2001, 03:16:12 – magnitude 5.1
22 August 2001, 04:39:29 – magnitude 5.3
22 August 2001, 05:28:04 – magnitude 5.1
22 August 2001, 20:22:37 – magnitude 5.0

The magnitude 7.1 event was located 295 km northeast of Gisborne, approximately 19 km from the swarm centroid. This mainshock aligns with the regional pattern of moderate-to-large earthquakes along the Hikurangi subduction margin.

The Gisborne offshore region lies within the Hikurangi Margin, where the Pacific Plate subducts obliquely beneath the Australian Plate. Subduction occurs at rates of 40–50 mm per year, producing both interface and intraslab seismicity. Earthquakes at depths near 33 km typically occur within the subducting slab or near the plate interface. The margin has a well-documented history of seismic activity, including the 1947 Gisborne earthquakes (magnitudes 6.2 and 5.8) and more recent events such as the 2016 Te Araroa earthquake (magnitude 7.1). Ongoing GPS and seismic monitoring by GeoNet confirms persistent strain accumulation and episodic slow-slip events in the northern Hikurangi region.

Swarm sequences in this tectonic setting often reflect fluid migration or stress triggering along pre-existing faults. The rapid succession of events following the initial magnitude 7.1 shock, all clustered at identical depth, is consistent with aftershock redistribution rather than independent mainshocks. No surface rupture was reported, as expected for an offshore event at 33 km depth.

Post-2001 monitoring has improved resolution of similar swarms through expanded seismic networks. The 2001 sequence remains a reference example of intraslab activity in the northern Hikurangi Margin and contributes to refined hazard models for the east coast of the North Island. Continued research integrates these historical data with contemporary geodetic observations to better understand subduction dynamics and associated seismic risk.