Earthquake Swarms

What is an earthquake swarm?

Unlike the familiar mainshock-aftershock earthquake sequences, where numerous smaller earthquakes tend to follow a larger earthquake, earthquake swarm sequences do not have a dominant mainshock, and may have several or many earthquakes of similar magnitude that happen throughout the swarm duration.

Swarms can contain any magnitude range of earthquakes and the largest magnitude earthquake does not always happen at the beginning of swarm sequences. Although like mainshock-aftershock sequence, swarms are still clustered close in time and space.

Because there is not necessarily a large earthquake at the start of a swarm, we know that the driving mechanism of swarm sequences, or the energy driving the swarm, must be different to a mainshock-aftershock sequence. It is often thought to be local physical changes like the movement of fluids, which in volcanic environments can include gas or magma. Because we typically only see the symptoms of that energy release (in the form of the swarm of earthquakes) it can be difficult to know what is driving the swarm and whether the energy input is ongoing or has stopped.

Although we tend to talk about earthquake clusters being either mainshock-aftershocks or swarms it is important to remember that these terms represent endmembers of a spectrum of observed earthquake clustering behaviour.

Where do earthquake swarms occur?

Earthquake swarms are relatively common and happen in all regions of New Zealand and throughout the world. One of the more frequent areas of swarm activity is within the Taupō Volcanic Zone, the area from Ruapehu up through Whakaari/White Island.

Stories on recent earthquake swarms in New Zealand:

2021 Okataina

2020 Rotorua

2019 Kawerau/Taupo

2019 Northland

2018 Turangi

What causes swarm sequences?

Swarms can be generated when fluids (e.g., water) migrate and interact with pre-existing faults. In volcanic environments, this can be fluid released from deeper magma or circulating within active geothermal areas (in volcanic areas such as the Taupō Volcanic Zone). The earthquakes triggered by fluids occur as fault slip on the cracks and faults through which the water is moving. The fluid reduces the clamping force on the faults and lubricates the fault enough to allow it to slip.

Magma movement can also act as the ‘driving mechanism’ for swarms, creating the earthquakes as magma-filled cracks push their way through the Earth’s crust. In such a case the earthquakes commonly occur near the crack tip (ahead of the magma where the crack is starting to open), or off to the side of the crack. Swarms of volcanic earthquakes like this occurred in the weeks before the 2012 August eruption at Tongariro. In addition to being caused directly by the magmatic activity, magma accumulation at depth can also change stress in the surrounding areas.

In these cases, we might see swarms of normal tectonic earthquakes as far as 20 km from a volcano.

A slow-slip event is essentially an earthquake in slow-motion, and typically involves centimetres to tens of centimetres of movement along a fault, over weeks to years. We commonly see slow slip events at the Hikurangi subduction zone, usually at least one or two per year. Sometimes swarms of small to moderate-sized earthquakes occur during slow slip events at the Hikurangi subduction zone. These swarms likely occur due to stress changes in the Earth’s crust that occur during the slow slip events. Our scientists are working hard to try and better understand the relationship between earthquake swarms, slow slip events, and large earthquakes at the Hikurangi subduction zone so we can improve our ability to understand earthquake occurrence.

Can swarms be forecasted like aftershock sequences?

No. Unfortunately, the duration, size, and number of earthquakes that will happen in each swarm sequence cannot be forecasted. There are several factors that limit our ability to forecast swarm sequences including:

In a mainshock-aftershock sequence, the main driver of continued earthquake activity is the largest earthquake or “mainshock.” Although aftershock sequences can last for months to years, we have tools we can use to forecast the number of earthquakes that may happen over a set period because the driver – or energy input has already ended and behaves in a predictable way.

How we produce earthquake forecasts

What can I do about swarms?

Earthquakes can occur anywhere in New Zealand at any time, In the event of a large earthquake: Drop, Cover and Hold.

All New Zealanders should be prepared in case of a natural disaster-induced emergency. Follow the Ministry of Civil Defence’s guidance on how to make sure that you and your family will get through an emergency, and EQC’s advice on how to secure your household and belongings.