Location:

Period:

4 Mar 2021 14:00:55 - 10 Mar 2021 14:47:05 (6 days 46 minutes)

Volcanoes in 100km radius:

None

Earthquakes:

188

6 swarms found nearby.

2001

PS20010822.1(74.8km)

21 Aug

1 day 13 hours

8 earthquakes

PS20010824.1(74.6km)

24 Aug

10 hours

5 earthquakes

2011

PS20111118.1(47.9km)

18 Nov

5 hours

6 earthquakes

2016

PS20160901.1(83.1km)

31 Aug

1 day 4 hours

21 earthquakes

2021

PS20210304.1(40.9km)

4 Mar

3 days 9 hours

34 earthquakes

PS20210311.1(13.3km)

11 Mar

1 day 6 hours

6 earthquakes

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Seismic Swarm S20210304.1: Offshore East Coast of New Zealand's North Island

An earthquake swarm designated S20210304.1 occurred offshore the east coast of New Zealand's North Island from 14:00 on 4 March 2021 to 14:47 on 10 March 2021. Over 144 hours and 46 minutes, 188 events were recorded. The sequence took place within the Hikurangi subduction zone, where the Pacific Plate subducts beneath the Australian Plate at rates of approximately 40–50 mm per year. This tectonic setting generates frequent seismic activity, including both interplate thrust events and intraslab earthquakes at varying depths.

The first 100 events exhibited magnitudes ranging from 3.5 to 4.9, with the majority between 3.6 and 4.2. Depths clustered between 10 km and 82 km, indicating a mix of shallow crustal and deeper intraslab sources. Notable larger events included multiple magnitude 4.9 shocks at shallow depths around 10 km on 4 and 5 March, alongside deeper events exceeding 70 km. Activity showed a typical swarm pattern with no single dominant mainshock, instead featuring episodic bursts of similar-sized events distributed across the first two days before gradually declining.

The east coast offshore region has a well-documented history of seismic swarms driven by subduction-related stress accumulation and fluid migration along the plate interface. Since 2000, five swarms have been identified in the area: two in 2001, and one each in 2011, 2016, and 2021. These episodes reflect the persistent tectonic loading along the Hikurangi margin, which also hosts New Zealand's highest seismic hazard for large megathrust earthquakes.

Analysis of the initial 100 events confirms predominantly normal-faulting characteristics consistent with slab bending and extension within the subducting Pacific Plate. Depths up to 105 km align with the known geometry of the subducted slab beneath the North Island. No surface rupture or significant tsunami risk was associated with this swarm, typical for events of this scale in the region.

References

GeoNet New Zealand Earthquake Catalogue
USGS Earthquake Hazards Program
GNS Science Hikurangi Subduction Zone Reports