Kaikōura an example of how large quakes trigger slow-slip events at a distance

Tuesday, 12 September 2017

The November quake ripped up the ground throughout the northeast of the South Island. This image shows the damage to a road in Kaikōura.

The slow-slip events that followed the Kaikōura quake are thought to be some of the best examples in the world of slow slipping being triggered by a large distant earthquake.

The 7.8-magnitude earthquake last November triggered a large slow-slip event between 250 kilometres and 600km away, on the shallow part of the Hikurangi subduction zone beneath the east coast of the North Island, off the Hawke's Bay and Gisborne coasts.

An uplifted section of shoreline around the Kaikōura peninsula. Water used to reach the rock wall and beach before the earthquake.

The slow-slip happened less than 15km below the surface or seabed and spanned an area of more than 15,000sq km.

The massive landslide brought down onto Ohau Point, north of Kaikōura by the quake. Road crews working from the north and south came together at the point in August for the first time since the earthquake.

It was thought to be the first time scientists had recorded a large-scale slow-slip event triggered by passing seismic waves from a distant large earthquake, GNS Science geophysicist Dr Laura Wallace said. She is lead author of a study into the slow-slip events published on Tuesday in Nature Geoscience.

* November earthquake kick-started slow-slip event in upper South Island

* Slow-slip earthquakes detected in Kapiti and Manawatu, joining Gisborne and Hawke's Bay**

There were many known episodes of something called tectonic tremor related to slow-slip triggered by passing seismic waves from a distant earthquake, but tectonic tremor on its own was often quite a small movement on the plate boundary - maybe less than 1cm, Wallace said.

During tectonic tremor, a small amount of slow-slip was probably going on. In contrast, the Kaikōura quake triggered a very large amount of slow-slip.

Large-scale slow-slip triggered by a distant earthquake on shallow parts of a subduction zone may be quite common, Wallace said.

The problem was that such areas were offshore and mostly did not have instruments close enough to the plate boundary for the slow-slip to be recorded.

Authors of the study also speculated that passing seismic waves from distant earthquakes could also trigger earthquakes on shallow subduction interfaces.

Wallace emphasised that did not mean an increase in the risk of earthquakes.

'If triggering were to happen, it would just be the straw that broke the camel's back . . . You would probably be having an earthquake in the near future anyway,' she said. No clear examples of that happening had been seen yet and if it did happen it would probably be very rare.

The largest slow-slip following the Kaikōura quake was greater than 10cm offshore in southern Hawke's Bay. A series of small to moderate earthquakes were also sparked in the area at the time, the largest being a 6-magnitude quake off the southern Hawke's Bay coast from Porangahau on November 22.

Some GeoNet GPS sites on the North Island east coast above the subduction zone detected eastward motion of up to 30mm in a one-two week period after the Kaikōura quake, the paper said. Some sites in the northern end of the zone moved up to 10mm eastward in the five hours after the earthquake.

The area was also moved 1-4mm northward by the quake. It was the first observed 'near-simultaneous rupture' of the central and north Hikurangi subduction zone slow-slip region – with a slow-slip event rupture length of about 300km – since the GeoNet GPS network began operating in 2002.

The changes in dynamic-stress in the slow-slip source area – caused by the passing of seismic waves from the earthquake – were about 1000 times greater than the static-stress changes caused by the quake itself.

'The seismic waves passing through had a much bigger impact than the Kaikōura Earthquake itself,' Wallace said.

The triggering effect of the passing seismic waves on the slow-slip is thought to have been boosted by what is known as a sedimentary wedge. The mass of sedimentary rock involved in this case is piled up at the edge of the subduction zone boundary under the seafloor off the North Island's east coast.

That layer of rock was particularly susceptible to trapping seismic energy,' GNS Science seismologist Dr Yoshiro Kaneko, another of the researchers involved in the study, said. The trapping of seismic energy in turn promoted fault slip at the base of the sedimentary wedge where the slow-slip events happened.

The northward travelling rupture during the Kaikōura quake- which ruptured a complex sequence of at least a dozen faults over about 150km of plate boundary in the northeastern South Island - also directed strong pulses of seismic energy towards the North Island, Kaneko said.

The study suggested that also influenced the long-distance triggering of the slow-slip events beneath the North Island.

Slow-slip events are similar to earthquakes, involving more rapid than normal movement between two pieces of the Earth's crust along a fault. But unlike earthquakes, where the movement happens in seconds, movement in slow-slip events – also known as silent earthquakes – can take weeks or months..

The Kaikōura quake also triggered a slow-slip event on the subduction zone at 25-40km depth beneath the Kapiti coast region, and up to half a metre of post-earthquake slip (called afterslip) under Marlborough. The Kapiti event and Marlborough afterslip are still happening, although they have slowed substantially in the past several months. It is expected research into those events will be produced, once they finish.