Slow-slip events are very similar to earthquakes, as they involve more rapid-than-normal movement between two pieces of the Earth’s crust along a fault.
However, unlike earthquakes where the movement occurs in seconds, movement in these slow-slip events, or ‘‘silent earthquakes’’, can take weeks to months to occur.
The Hikurangi subduction zone is where the Pacific Plate dives down or ‘‘subducts’’ beneath the eastern North Island. It is the main plate boundary fault under the North Island and is at one point just 40km from Gisborne.
Dr Wallace said the findings had implications for Gisborne.
“The triggering of slow- slip immediately following Kaikoura gives us new insight into the sensitivity of the plate boundary offshore Gisborne to stress changes from distant earthquakes.
“We had not observed this type of triggering of slow-slip events by distant earthquakes offshore Gisborne before.
“The increase in the number of earthquakes observed during the slow-slip offshore Hawke’s Bay also gives us important information about the ability of slow-slip events to trigger small earthquakes.
“It is possible that in some rare cases slow-slip events could trigger even larger earthquakes, and this research helps scientists to build up a picture of the likelihood of something like that happening.”
Instruments detect slow slipThe research paper added that the national network of Global Positioning System (GPS) instruments operated by GeoNet detected the slow-slip events, which occurred on the Hikurangi subduction zone plate boundary in the weeks and months following the Kaikoura earthquake.
The slow-slip occurred less than 15km deep below the surface (or seabed) and spanned an area of more than 15,000sq km off the Hawke’s Bay and Gisborne coasts. This is comparable to the land area of New Caledonia.
A slow-slip event was also triggered on the subduction zone at 25-40km beneath the Kapiti coast region, and up to half a metre of post-earthquake slip (called “afterslip”) under Marlborough. The Kapiti slow-slip event and Marlborough afterslip are still continuing today, although they have slowed substantially over the last several months.
“This is probably the clearest example worldwide of large-scale slow-slip being triggered over long distances by seismic waves from a large earthquake.
“One of the most intriguing factors is that the quake triggered slow-slip off the Gisborne coast, up to 600km away from its epicentre in North Canterbury.”
Seismologist Dr Yoshihiro Kaneko, of GNS Science, said the slow-slip was triggered by stress changes in the Earth’s crust caused by passing seismic waves from the Kaikoura quake.
The triggering effect was likely accentuated by an offshore ‘‘sedimentary wedge’’ —a mass of sedimentary rock piled up at the edge of the subduction zone boundary under the sea floor off the North Island’s east coast.
This layer of more compliant rock is particularly susceptible to trapping seismic energy which, in turn, promotes fault slip at the base of the sedimentary wedge where the slow-slips occur.
Dr Kaneko said the study also suggested that the northward travelling rupture during the Kaikoura quake directed strong pulses of seismic energy towards the North Island, and this also influenced the long-distance triggering of the slow-slips beneath the North Island.
The east coast slow-slip event was also responsible for sparking a series of small to moderate earthquakes off the east coast in November last year. The largest earthquake in this sequence was a magnitude 6.0 quake off the southern Hawke’s Bay coast at Porangahau on 22 November.
Although scientists are still in the early stages of trying to understand the relationships between slow-slip events and earthquakes, Dr Wallace says that these exciting observations highlight additional linkages between slow-slip events and other earthquake processes.
The scientists are planning to produce separate research papers on the post-earthquake slip on the subduction zone beneath the Kapiti and Marlborough regions once those events finish.
