What do these slow-slip events mean for future large earthquakes?

Updated 3pm, 9/12/2016

This is an important, but difficult question that our experts here at GNS Science, along with Victoria University have been working on answering. The slow-slip events (or silent earthquakes) cover a large area of the plate boundary underneath the North Island and have made calculating the likelihood of future large aftershocks trickier.

Although still very unlikely, we now estimate that the probability of a magnitude 7.8 or larger earthquake in the coming year has increased to approximately 5% – due to ongoing slow-slip events. This is a small increase in the likelihood which is generated by the Kaikoura Earthquake alone. This is approximately 6 times greater than it was prior to the Kaikoura Earthquake.

Due to the large extent of slow slip, the adjusted forecast covers a larger region than the standard aftershock area to now include the lower half of the North Island and the upper South Island. There are several faults in these areas capable of large quakes, including the subduction zone and crustal faults like those that ruptured during the Kaikoura earthquake.

 
Cross section of the North Island of New Zealand showing how the
Australian and
Pacific Plates meet. The slow-slip events (orange-yellow
patches) are superimposed
onto the cross-section. Bottom right Insert
shows a map view of the slow-slip events.

What does this mean?

Our forecasts tell us what is likely (or unlikely) to happen in the future, but they can never definitively say if a large earthquake will occur or not. We’re aware that these messages could be unsettling, and that’s a very normal reaction. What we do want you to take away from this (and this applies to all New Zealanders, at all times—not only now) is to follow Civil Defence’s advice and make sure that you’re prepared for earthquakes and tsunamis. We know that being prepared makes a real difference in helping you get through an event and recovering afterward. Many of you have already got you and your family prepared, so well done you guys!

Tell me about slow-slip events again

The current slow-slip events that have followed the M7.8 Kaikoura earthquake are occurring along the fault between the Australian and Pacific Plates, known as the Hikurangi subduction zone.  The movements along the fault are equivalent to a magnitude 7.0 earthquake in the Hawke’s Bay-Gisborne region, and magnitude 6.9 earthquake in the Manawatu-Kapiti region.

We have observed many similar slow-slip events in these areas of this size, but this is the first time we’ve observed slow-slip occurring simultaneously in multiple areas around the North Island in the 15 years we've been detecting them. This is also the first time we’ve been able to observe slow slip in New Zealand after a magnitude 7.8 earthquake, so it’s possible this is a normal pattern after such a large quake.

The Hawke’s Bay-Gisborne region slow-slip event only lasted about a week, and it has mostly finished. Slow-slip events in these regions happen less than 15 kilometres deep. There were a number of earthquakes offshore Porangahau in the southern Hawke's Bay in the 2-3 weeks following the Kaikoura earthquake that were likely triggered by the east coast slow slip event.

The slow slip event beneath the Kapiti and Manawatu regions appears to be ongoing at a relatively steady rate since the Kaikoura M7.8 earthquake. Slow-slip events in these regions tend to occur between 25-45 kilometres deep.

If we can’t feel slow-slip events, why are you focusing on them?

Slow-slip events occur beneath the North Island, where the Australian and Pacific Plates meet. In the lower North Island, the slow-slip events happen slightly deeper than the part of the subduction zone where the plates are currently stuck together.  These “stuck zones” are thought to periodically rupture in large earthquakes. When slow-slip events occur, stresses are applied to this stuck-plate zone. This increased stress happens during all slow-slip events. The number of earthquakes during some slow-slip events can increase, but this doesn’t always happen.

There have been hundreds of slow slip events observed at subduction zones around the world that have not triggered larger, damaging quakes. So, if slow-slip events do trigger large damaging quakes, it is very rare indeed. In New Zealand we typically have at least 2-3 slow-slip events each year. Scientists have only discovered in the last 15 years that slow slip events occur, so trying to understand their relationship to larger, damaging quakes is still in its very early stages.

How did we go about incorporating slow-slip events into aftershock forecasts?

In New Zealand, scientists at GNS Science have been developing aftershock forecasts for the several large earthquakes since the Canterbury earthquake sequence. We’ve refined these over the last 6 years, but have never had to incorporate slow slip events into the mix.  

Experts from GNS Science and Victoria University evaluated many strands of evidence to determine the likelihood of an earthquake equal to or larger than the Kaikoura Earthquake. We have also consulted with several international experts who study slow-slip phenomena in their respective countries to provide additional perspective. 

This is our first run at including slow slip into the forecasts, and it is probably the first attempt worldwide to implement this, so it is definitely a work in progress and our estimates have large uncertainties. As scientists’ understanding of this phenomena improves we hope to develop better ways to incorporate the mechanics of slow-slip events and their relationship with earthquakes into our forecasts.

Being prepared

Civil Defence have great resources on how to prepare your home, how to make an emergency plan for your family, and what to do during and after an earthquake. Check with your local and regional council for your region's tsunami evacuation zones - remember, these zones apply regardless of where the tsunami is coming from. Knowing where these zones are can help you plan your evacuation route before a tsunami occurs.  You will not have time to do this if an earthquake occurs.  Remember if you are near the coast and you feel a long OR strong earthquake, then you should evacuate inland or to higher ground immediately.

As always, we'll keep you updated with any new information as it comes to hand. We'll also be updating our aftershock forecasts periodically. 

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