Earthquakes and landslides – Expert Q&A

The magnitude 7.8 earthquake that struck North Canterbury on Monday morning caused numerous landslides, blocking roads and damming rivers.

Caution earthquake damage sign

Early estimates suggest there could be tens of thousands of landslides through the area, causing hazards such as blocked rivers which could result in floodwaters if the dams break.

Dr Tom Wilson and Professor Tim Davies from the University of Canterbury geology department, along with Dr Tom Robinson from Durham University (previously at UC) prepared a Q&A about earthquakes and landslides.

Note: Download the Q&A with associated landslide hazard maps at this dropbox link.

Why are there so many landslides after this earthquake?

Short Answer: There are two main factors.  There has been very strong and long duration of earthquake shaking, and the earthquake is located in an area with very steep mountains.

Long Answer: The earthquake sequence has occurred in a mountainous area of New Zealand.  In fact, this area has some of the fastest uplift rates anywhere in New Zealand.  This rapid uplift means the mountains are very steep. The type of rock in this region is also very weak, and this weakness is increased by the wet climate.  So when there is strong and long duration shaking from a large earthquake, like the one we have experienced, it often leads to a large number of landslides. Some of the early assessments suggest there may well be many tens of thousands of landslides.

Is it normal to have this many landslides?

Short Answer: Yes for this size of earthquake, in this sort of landscape.

Long Answer: When earthquakes occur in steep mountainous landscapes it is common to experience lots of landslides.  Similar numbers of landslides have been observed after other large earthquakes in New Zealand and overseas, such as the recent earthquake in Nepal in 2015.  The earthquake in China in 2008 caused an estimated 80,000 landslides while the 2015 Nepal earthquake caused an estimated 22,000 landslide throughout the Himalaya’s. These landslides often cause substantial damage to communities and are usually the main cause of damage to roads and other infrastructure.

Could we have known this could happen?

Short Answer: Geological evidence indicated that large earthquakes and associated extensive landsliding has occurred throughout the South Island previously.  We can now use models to estimate where landslides might occur as a result of big earthquakes.  The problem is that we can’t reliably predict where or when big earthquakes might occur.

Long Answer: There is a lot of geological evidence we can observe that this part of New Zealand has had big earthquakes in the past. For example, a number of large fault lines have been identified over the past 50 years however, new fault lines are being found all the time. This earthquake seems to have occurred on a combination of known and unknown faults. By comparison, the Canterbury earthquake occurred on what were unknown faults at the time.

There is also a lot of evidence that large numbers of landslides and some very large individual landslides have occurred in the region, probably during previous large earthquakes.  Scientists, engineers and Civil Defence and Emergency Management officials had expected that mountainous areas in North Canterbury would experience extensive landsliding in a large earthquake.

Scientists can estimate where landslides may occur during big earthquakes to some extent by analysing controlling factors, such as the amount of shaking, the strength of the rocks making up the mountains, and the slope angle.  These models can be made rapidly after an earthquake – like this one from the North Canterbury earthquake.  They can also be used to estimate where landslides are most likely to occur from future earthquakes, such as the Alpine Fault.

What are the impacts of the landslides in mountainous landscapes?

Short Answer: Landslides can be highly destructive to people, structures and infrastructure networks.  In mountainous landscapes landslides caused by large earthquakes tend to extensively block roads in large numbers, which can take a long time and many resources to clear. Landsliding can also continue for long-periods after the earthquake due to aftershocks and rainfall, prolonging the hazard.

Long Answer: Landslides can damage any people or built structures they come into contact with.  Very few people live in the mountainous environment of North Canterbury and southern Marlborough, so roads tend to be the worst affected by landslides. Because roads in this region need to traverse steep terrain to link isolated townships, they are the most exposed infrastructure and it only takes one landslide to block a road.  In major earthquake with large numbers of landslides, the result is often large numbers of road blockages.

Landslides can also continue for many months and years after the earthquake, posing a risk to surviving infrastructure and anyone working to clear landslide debris. This is because the main earthquake causes the slopes to crack and break apart. Some slopes may not fail in the initial earthquake, but instead, fail during a later aftershock or rainstorm. Following the 1999 earthquake in Taiwan, major landsliding continued to occur for 4 years after the earthquake, and this year in Nepal during the monsoon thousands of landslides reoccurred in the area affected by the 2015 earthquake.

What happens when landslides occur in a river valley?

Short Answer: They can block the river causing water to build up behind.  When this ‘landslide dam’ fails it can lead to a large flood of sediment-laden water downstream.  These can be very destructive to people, structures and farmland downstream – so need to be carefully managed.

Long Answer: Landslides can block rivers which lead to a ‘quake-lake’ being formed where the water backs up behind the ‘landslide dam’.  Eventually, the lake becomes so full that the water spills over the top of the dam, usually causing the dam to breach.  The breach may be quite slow or it can be very rapid, but usually occurs within 1 week of the dam forming. However, in some cases, landslide dams have survived for thousands of years.

If rapid failure of the dam occurs, then large, fast moving flood waters full of sediment and debris can travel large distances down river. Like any flood, this may break through river banks and can be very destructive.  This is why it very important to stay away from rivers where landslide dams may have formed.  A small breakout flood has already occurred in the Clarence River, and several other rivers appear to have landslide dams in them.  Identifying where landslide dams have formed is currently a big focus of the science agencies’ so that Civil Defence Emergency Management can put risk management plans in place.

Landslide dams often fail without any warning so it is important to be aware of any signs that might suggest a dam has formed or broken. If the river flow is unusually low or muddy, this may be a sign a dam has formed upstream. If river levels start to rise suddenly and rapidly without any obvious reason, this may be a sign that a landslide dam upstream has breached.

In the longer term, the large volumes of sediment which landslides have deposited in the catchments of rivers will be transported downstream.  This can lead to rivers aggrading (depositing sediment and building up the height of the river bed), which can lead them to break out of their channels causing flooding.  Given the large volumes of sediment the rivers need to move, this process will continue to occur for many years.  This happened following the 1999 landslide from Mt Adams in Westland, which deposited 100,000’s cubic metres of sediment into the Poerua River.  Over the next decade large volumes of sediment were transported by the river and deposited over the previously productive farm land downstream.

How long it might take to get blocked road and rail networks open?

This is hard to estimate.  This is going to depend on how many landslides have affected the road or rail line, the size of these landslides, their accessibility, and the on-going risk of aftershocks and rainfall causing further landsliding.  With continuing aftershocks and rainfall, those landslides may not be safe to go near for several months.

A good example is what happened, and continues to happen, in Nepal after the 2015 earthquake.  After that earthquake there was extensive landsliding throughout the affected mountainous regions, with up to 22,000 landslides identified.  Nepal experiences yearly monsoons, with heavy rainfall occurring every day from mid-July to mid-September. This rainfall, along with large aftershocks, has continued to reactivate landslides formed in the initial earthquake. During the 2016 monsoon, these landslides caused further extensive damage to the roads in the region, leading to the only road to China being closed for 3 months after only being reopened since the earthquake a few months earlier. Even if workers can get in and clear landslides quickly, it will be a continuous job lasting months to years, and could cost billions of dollars to repair.

This is a really difficult issue, especially given the importance of reopening these essential links to the local communities and economy.

So how do we avoid this happening again to our infrastructure?

Short Answer:  We need to identify where natural hazards exist and plan and design our infrastructure to be more resilient to earthquakes by either designing them to resist damage and minimize disruption or locating them where the hazard is lowest. We also need to ensure there are viable alternative routes in the network so there is redundancy. This way if one route is affected, other routes can still provide access.

Long Answer: Due to New Zealand’s long, thin geography and small population, we often have few alternative routes or networks.  So when a major part of a network is disrupted, such as State Highway 1 around Kaikoura, our networks can be badly disrupted which isolates communities and impacts supply chains, such as all the freight moved by roads. Ensuring we learn from this event to increase resilience to these networks must now be a priority for New Zealand.

Generally speaking, there are three main ways to make our infrastructure more resilient to natural hazards like earthquakes and landslides.  Firstly, site our infrastructure so it is as safe a location as possible, using natural hazards scientists and engineers to robustly identify hazards and mitigate the risks.  Secondly, design and build infrastructure to resist or avoid being damaged by natural hazards, such as engineered bridges, using rock shelters and tunnels, and engineered foundations wherever possible.  Thirdly, adding redundancy to our networks where there are high functioning alternative routes which perform if another route or line is taken out.  All these options cost money, sometimes of lot of it, so as a society we need to judge what sort of safety and performance we want from our vital infrastructure systems during disasters.  Making changes is sometimes easiest after a disaster.  But a critical issues is that many of our other existing infrastructure networks are highly vulnerable to natural hazards and this need to be addressed now.  While this is fresh in our minds, this is the time to discuss this and take action.

Clearly there will be some challenging decisions to make whether to realign the badly impacted stretches of State Highway 1, but in many other parts of New Zealand we have further long stretches of infrastructure vulnerable to natural hazards.  Ensuring our infrastructure can sustain impacts like earthquakes and landslides and remain functional, or that there are functional alternatives, is crucial.  Sometimes a big disaster like this can be the catalyst to trigger bold decisions which will enhance our resilience to future disasters.

It’s very promising to hear the government is directly considering these issues already.