New Zealand quake study reveals ruptures can be much bigger than we thought possibleStephen Hicks, University of Southampton
No one could have expected what was to hit New Zealand in 2016. The country is certainly no stranger to being shaken up by moving tectonic plates. Yet on November 14 2016, it was struck by what may be the most complex rupture ever recorded, overshadowing even the highly destructive sequence of earthquakes that hit Christchurch in 2010 and 2011. New research into the event shows we may have to rethink our understanding of how far earthquake ruptures can travel.
Around midnight, without warning, a magnitude 7.8 earthquake ripped through the country’s South Island, with the main rupture lying close to the coastal town of Kaikoura. As the rupture advanced to the north-east, it left a trail of devastation.
Submerged rocky plateaus beneath the coast rose up from the ocean and became new reefs, suddenly releasing thousands of tonnes of gushing seawater. This deafening cascade lasted minutes. Houses were sheared from their foundations and deposited into adjacent fields. Railway lines were dragged from their beds and re-routed.
Tens of thousands of landslides roared down slopes as mountains, hills, and cliffs could not stand up to the shaking. Huge volumes of mud, sand, and gravel rushed onto the abyssal plains of the Pacific Ocean. Within minutes, a three-metre-high tsunami inundated local coastlines. Two people were killed, although far more deaths could have been expected for an event of this size. This earthquake had everything.
New research, published in the journal Science, used evidence from satellites, ground sensors and field maps to show that the 2016 quake ruptured at least 12 major fault-lines. Like dominoes tumbling and crashing against each other, each fault unzipped and shifted blocks of the crust by more than 20 metres, the height of a four-storey building. Many quake records also tumbled. Never before has such a cascading rupture across so many faults been observed in such detail.
Dr Ian Hamling, the lead author on the study, is an earthquake scientist based at GNS Science in New Zealand. Based in nearby Wellington, Hamling experienced the shaking. “I’ve lived in New Zealand for four years and have felt a few earthquakes,” he told me. “I didn’t expect this one to be as complex as it was.” Within hours, the data coming into GNS told a unique story, leaving Hamling “stunned”.
For earthquake scientists around the globe, the events of New Zealand raise key questions. How often does this type of quake occur and could it happen elsewhere?
The geology of this part of New Zealand is a labyrinth. It is the pivot point between two plate boundaries. To the north, one tectonic plate dives beneath the other. Further south, two plates slide alongside each other, forming the country’s Southern Alps. As a result, the crust in the Kaikoura region is highly broken up and fractured. Similar mazes of fractured rock can be found in many of the world’s earthquake-prone regions.
For areas around the world that host large earthquakes, scientists use a model they call “segmentation”. Segments are discrete areas of faults, tens to hundreds of kilometres long, that typically rupture on their own during large quakes. This concept comes from recent recordings of seismic shocks and descriptions of ancient quakes in historical records.
When excavating evidence left by past earthquakes, scientists have often assumed geological scars running across multiple segments were caused by separate events. The new research into the 2016 quake shows that a single rupture can even jump across large gaps between segments, which do not necessarily need a clear physical connection.
With advances in satellite imaging, scientists are now able to dissect complex quakes. The New Zealand quake occurred partly on land, where it could be easily monitored. Satellites were poised to detect tiny changes in ground movement, networks of monitoring instruments were in place, and dedicated teams were rapidly deployed to map out the plethora of faults. In this case, the complexity of the rupture was clear.
But what if an earthquake were to strike in the remotest parts of the planet, such as in the deserts of central Asia, or below the deepest oceans? In isolated areas, complex events may remain undetected and could occur more often than previously assumed.
Maps showing the estimated hazard posed by quakes in different regions are generally based on the assumption of single segment ruptures. In earthquake scenarios where fault segments link up, there is a bigger area available to rupture, ramping up the quake’s energy. Magnitude seven quakes become magnitude eight; eights become nines. Hamling said: “This event will definitely start to feed into our hazard models.” New Zealand is now showing the world that calculations of earthquake hazard need a rethink.
These lessons will also affect early-warning systems for earthquakes. These systems rapidly assess the first few seconds of an incoming seismic signal to estimate the degree of shaking when potentially damaging waves arrive. Initial seismic waves during the Kaikoura earthquake probably gave no indication that the quake could develop into a magnitude 7.8 rupture due to the domino effect. So understanding the cascading process during the New Zealand rupture could improve our abilities to warn whether a quake is destined for “greatness”.
The Kaikoura quake will likely remain unparalleled for some time. Yet as our models better simulate the true complexity of Earth and new observations illuminate hidden parts of the planet, we may find that the 2016 events and the wisdom gained could be overshadowed in the not so distant future.