Kaikoura quake: 100,000 landslides

The now-named Kaikoura Earthquake triggered up to 100,000 landslides in Northern Canterbury and southern Marlborough.

The largest of them, brought down on the south side of the Clarence River mouth in the wake of the overnight 7.5 quake, brought down an estimated one million cubic metres.

Tonight, GNS Science reported that like other quakes, this one, first registering near Culverden, appeared to have involved jumping from one fault rupture to another in a complex pattern.

This compound style of rupture was a feature of the magnitude 7.1 Darfield quake of 2010 where up to eight neighbouring faults ruptured almost simultaneously.

At daybreak today three GNS geologists took to the air in helicopters to make observations of quake damage in the North Canterbury-Marlborough region.

They described the huge Clarence River landslide as the biggest landslide in New Zealand since the Young River landslide near Wanaka in 2007.

The number and distribution of landslides is considered consistent with the severity of the ground shaking.

They also observed numerous surface fault ruptures, blocked rivers, and a section of the main north-south railway line shunted sideways by at least two metres.

Numerous landslides were blocking main highways and rail tunnels.

Over the coming days, GNS Science and GeoNet staff will deploy instruments in the quake region to gather intelligence on the sequence.

This includes portable seismometers to get more precise recordings of aftershocks and GPS instruments to measure deformation of the land surface.

With landslides blocking roads, they will probably need to rely on helicopter support.

Today, GNS Science seismic engineers spent time in downtown Wellington observing light surface damage at the Port and damage to some highrise buildings.

Aftershocks are likely to continue for many months.

Scenarios and probabilities

Most earthquake aftershock sequences decay over time, with spikes of activity and occasional larger earthquakes.

GeoNet has issued aftershock probabilities that describe the likely progression of the sequence over the coming months.

There are very different probabilities for each scenario.

Scientists have developed three scenarios based on what they know so far, but they add that their understanding is evolving as they do more analysis and receive more data.

Scenario One: Very likely (80% per cent and greater)
A normal aftershock sequence that is spread over many months. Felt aftershocks (magnitude 5 and greater) will occur from the epicentre near Culverden, right up along the Kaikoura coastline to Cape Campbell in Marlborough. This is the most likely scenario.

Scenario Two: Likely (60 per cent and greater)
In the next month, earthquakes of about magnitude 6 will occur in the North Canterbury and Marlborough regions, as well as potentially offshore in Southern Cook Strait and offshore Kaikoura.

Scenario Three: Unlikely (less than 40 per cent)
The least likely scenario is that in the next month, (it is unlikely but still possible) there will be rupture of longer known faults (with earthquakes of about magnitude 7), in the Marlborough and Cook Strait regions.

Within this sequence, aftershocks will most likely occur anywhere in the affected areas.

Under aftershock probabilities issued tonight by GNS Science, there was a 12 per cent chance of another quake of over magnitude 7 hitting within one day, along with a 71 per cent probability of a quake between 6.0 and 6.9 in the same period.

There was a 24 per cent chance of a plus 7.0 quake or two hitting in the next seven days, and a 32 per cent of one striking in the next 30 days.

This compared with a 93 per cent probability of up to six magnitude 6.0 to 6.9 quakes hitting in the next seven days, and a 98 per cent chance of up to eight quakes in that range striking within the next 30 days.

Over the next 30 days, there was a 99 per cent probability of between 28 and 53 quakes ranging between 5.0 and and 5.9, and between 18 and 39 of that size within the next seven days.