EXPERT REACTION: Strong earthquake in NZ triggers tsunami

The M=7.8 New Zealand earthquake occurred in an area of high seismic hazard in the northern part of the South Island.

The rupture started about 90 km north of Christchurch and caused slip on a fault (or faults) extending about 150 km to the northeast. According to the USGS, the largest slip was near the town of Seddon, which also experienced shaking during the Cook Strait earthquake sequence of 2013. L

arge aftershocks have occurred in the region and are likely to continue for days or weeks. They may cause further damage to already weakened structures and/or additional landslides.

This was a very large earthquake that occurred near the interface between the Pacific Plate and the Australian Plate.

Although as far as we know it did not occur on that plate interface, which is named the Hikurangi Subduction Zone, the direction of slip on the faults involved in this earthquake is similar to what is expected from earthquakes on the Hikurangi Subduction Zone, which underlies Wellington.

It is important to understand the implications of this earthquake for the potential triggering of earthquakes on the Hihurangi Subduction zone and other faults in the region around Wellington.

Preliminary data suggest that this is a structurally complicated earthquake, involving a mixture of reverse faults that thrust the northern South Island up over the Pacific Plate, and strike-slip faults, which slid northern crustal blocks sideways and towards the northeast relative to their southern counterparts.

The majority of the energy release appears to be sourced from rupture of a large fault or fault system that could be up to 200 km long and that strikes northeast and dips to the northwest, beneath the northern part of the South Island.

Due to ongoing convergence between the Pacific and Australian tectonic plates, faults in this orientation accommodate a mixture of reverse faulting and strike-slip. Because of the large size of the fault, and the initial interpretation that the rupture started at the southwest end of the fault and propagated to the northeast, the seismic energy was released over a period of up to 2 minutes.

In Christchurch, the shaking felt quite different to the fatal 2011 Christchurch earthquake, which had a tremendous amount of high frequency energy and very strong ground accelerations; this most recent earthquake would have been dominated by lower frequency shaking manifested as a ‘rolling-type’ ground motion felt for up to 1-2 minutes.

This earthquake has triggered liquefaction in coastal areas and in susceptible sediments, and landsliding along steep susceptible cliffs in the northern South Island. For an earthquake of M 7.5-7.8, several large aftershocks (magnitude >6) are expected, and for each magnitude 6 aftershock we expect 10 more magnitude 5 aftershocks over the coming days and weeks.

This region has been one of the most seismically active in New Zealand over the last few years, including magnitude 6.5 and 6.5 earthquakes that occurred as part of the Cook Strait earthquake sequence in 2013; it is likely that these sequences are related given their close spatial and temporal association.

A magnitude 7.8 earthquake at a shallow depth of 23km below the ground surface and 53km toward north of Amberley occurred in North Canterbury in New Zealand on November 13, 2016 at 11:02:56 UTC.

This earthquake was the results of very complex tectonic activities between Australian and Pacific Plates colliding under New Zealand (approximately 30-60 mm/year of relative movement between tectonic plates). The complexity of fault lines and tectonic activities is due to the fact that tectonic plates in some locations such as east of the North Island move toward each (creating compressive stresses), and in some other regions such as South Island slide past each other (generating shear stresses), both creating shallow earthquakes.

Let’s remember series of devastating earthquakes in the last 6 years in the same region including Christchurch earthquakes in September 2010 and February 2011 directly beneath City of Christchurch. It should be noted that due to the complexity of plate boundaries in this area, particularly different orientation of fault lines, it is very likely that several fault lines are activated in one earthquake.

It is foreseen that due to the main earthquake and follow up aftershocks, as a result of liquefaction of soil, significant damage to transport infrastructure mainly roads, and lifelines including water and sewage, oil and gas distribution pipelines, may happen. Indeed, although liquefaction of loose materials may happen well below the ground surface, it can be manifested as the ground surface subsidence. Furthermore, considering the terrain in this region in New Zealand, serious rock falls and landslides may happen within the radius of 100km away from the epicentre of this earthquake (even coastal cliffs) and authorities should make assessment of slopes before letting people back in the affected areas. Indeed, aftershocks can further degrade the weakened slopes and cause landslides deteriorating the condition.

Furthermore, Seiche (standing wave in an enclosed or partially enclosed body of water; different to Tsunami) may happend at great distance from the earthquake source in lakes such as  lake Sumner, lake Brunner and lake Poerua or even much further away from the epicentre, such as Lake Wanaka in South Island, due to this earthquake and its aftershocks. Tourists and travellers should be warned about this and let’s not forget The 1964 Good Friday earthquake in Alaska that produced damaging waves up to 2m high in lakes.

Further information:

The 7.8 earthquake which hit South Island of New Zealand on 13 November 2016 (11:02:56.970 UTC) was due to a reverse fault and the highest intensity of earthquake (to be VIII Intensity known as “Severe Intensity”) was felt at Amberley 53km away from the epicentre.

Moderate to heavy damage is expected near Amberley and it appears that the fault activities and movement are extended toward north with Peak Ground Acceleration estimated to be up to 0.35g (or peak velocity estimated to be 35 cm/s). Awatere River banks particularly near Seddon station should be inspected for rock falls and landslides. Regions near Blenheim and Renwick, particularly Woodbourne Airport and then towards Cook Strait could also experience large ground shaking particularly during aftershocks.

Analysing the initial data indicates that possibly a series of fault lines along almost a straight line between Amberley and Wellington have been activated in this earthquake. It is expected that parts of transport infrastructure namely South Island Main Trunk Railway in the north-east part of Canterbury suffer from ground movements and slope failures due to steep landscape.

In addition, due to aftershocks and rupture of tail of the major faults particularly towards the north of Cook Strait and Wellington Harbour (near Petone and Lower Hutt), some damage to near shore or offshore structures, such as excessive deflection of retaining walls and structures, may occur.

Yet another major NZ earthquake not on the Alpine Fault system or any known major mapped faults in the region marking the Pacific and Australian Plate margin.

The consequent stress loading on the Alpine fault may be bringing it closer to failure and a magnitude 8 earthquake on the Wellington Fault must be in the minds of NZ emergency management, the worst case scenario.

On the positive side, not many casualties and seemingly only one building collapse in this recent earthquake, that an old unreinforced concrete building. Landsliding onto the coastal railway and highway to be expected given the terrain.

The reported tsunami is a puzzle given the earthquake was under the land, it would be useful to see the tide gauge record to confirm it was a tsunami and not just a drawdown of the sea level.

The seismicity of NZ is going through an active phase, and in Australia too where there have been 3 times the expected number of magnitude 5+ earthquakes in the last 2 years, including one M6.2 earthquake along the NT/SA border, almost a one in ten year earthquake.

The 13 November 2016 Magnitude 7.8 earthquake occurred in northern South Island, New Zealand.

This earthquake shows evidence of complex rupture history. Its rupture area straddles a major plate boundary transition between a region (to the north) where the Pacific Plate is subducting beneath the Australia Plate, at the Hikurangi Subduction zone, and a region to the south where the Pacific and Australia plates move past each other on or near the Earth’s surface along the Alpine Fault and Marlborough Faults plate boundary system.

This earthquake initiated within the Pacific plate south of this transition and ruptured northward crossing over to the subduction zone. Analyses of seismic moment release (a measure of the energy of the earthquake) show that the major moment release occurred along the northern segment where the rupture potentially occurred on the plate boundary interface. 

The transition in plate boundary style, where this earthquake occurred, contains numerous faults with a wide range of orientations. The main rupture and aftershocks may exploit this complicated tectonic zone resulting in a range of earthquake mechanisms for the aftershocks and in comparison to the main earthquake.

The Magnitude 7.5 earthquake that struck some 70 km north of Christchurch today has had a devastating impact on the NE part of New Zealand’s South Island.

The tourist town of Kaikoura is effectively cut off as numerous landslides have blocked road and train access, but it also appears that the small tsunami generated by this event was largest in that area as well.
 
New Zealand sits astride the boundary between the Pacific and Australian tectonic plates and as such experiences numerous earthquakes. Recent events in 2010 and 2011 around Christchurch have now been followed by this most recent one.

The earthquake struck just to the south of New Zealand’s most active fault – the Hope fault – and seismologists are still unsure as to whether it was one or two faults that moved, but there have been many large aftershocks as well.

Yet again, New Zealand’s earthquakes are proving to be anything but easy to understand.

In addition, it is always difficult to know exactly what type of tsunami warning to issue as a result of such an event - a relatively “small” earthquake for tsunami generation and near but not at the coast.

As far as we know at the moment the resultant tsunami was generally small and may have only had a local impact around Kaikoura. The tsunami warning was regional though and populations, highly sensitised to earthquakes, responded well to evacuation advice. However, the worry is now that since there was little or no visible tsunami for much of the region – will this be seen as a false alarm and how will people respond next time?

Large aftershocks continue, with the latest - a Magnitude 6.8 - generating yet another tsunami information statement from the warning centres.

While the earthquake itself may not generate a tsunami, it is quite possible that large local tsunamis can be generated by earthquake groundshaking causing submarine landslides offshore. The Kaikoura Canyon, close to Kaikoura’s coast is just such a location and it has the potential to produce a devastating tsunami for an area already badly affected by the earlier earthquake.