Tens of thousands of landslides were generated over 10,000 km2
of North Canterbury and Marlborough as a
consequence of the 14 November 2016, MW7.8 Kaikōura Earthquake. The most intense landslide damage
was concentrated in 3500 km2
around the areas of fault rupture. Given the sparsely populated area affected
by landslides, only a few homes were impacted and there were no recorded deaths due to landslides.
Landslides caused major disruption with all road and rail links with Kaikōura being severed. The landslides
affecting State Highway 1 (the main road link in the South Island of New Zealand) and the South Island
main trunk railway extended from Ward in Marlborough all the way to the south of Oaro in North
Canterbury.
The majority of landslides occurred in two geological and geotechnically distinct materials reflective of the
dominant rock types in the affected area. In the Neogene sedimentary rocks (sandstones, limestones and
siltstones) of the Hurunui District, North Canterbury and around Cape Campbell in Marlborough, first-time
and reactivated rock-slides and rock-block slides were the dominant landslide type. These rocks also tend to
have rock material strength values in the range of 5-20 MPa. In the Torlesse ‘basement’ rocks (greywacke
sandstones and argillite) of the Kaikōura Ranges, first-time rock and debris avalanches were the dominant
landslide type. These rocks tend to have material strength values in the range of 20-50 MPa.
A feature of this earthquake is the large number (more than 200) of valley blocking landslides it generated.
This was partly due to the steep and confined slopes in the area and the widely distributed strong ground
shaking. The largest landslide dam has an approximate volume of 12(±2) M m3
and the debris from this
travelled about 2.7 km2
downslope where it formed a dam blocking the Hapuku River. The long-term
stability of cracked slopes and landslide dams from future strong earthquakes and large rainstorms are an
ongoing concern to central and local government agencies responsible for rebuilding homes and
infrastructure. A particular concern is the potential for debris floods to affect downstream assets and
infrastructure should some of the landslide dams breach catastrophically.
At least twenty-one faults ruptured to the ground surface or sea floor, with these surface ruptures extending
from the Emu Plain in North Canterbury to offshore of Cape Campbell in Marlborough. The mapped
landslide distribution reflects the complexity of the earthquake rupture. Landslides are distributed across a
broad area of intense ground shaking reflective of the elongate area affected by fault rupture, and are not
clustered around the earthquake epicentre. The largest landslides triggered by the earthquake are located
either on or adjacent to faults that ruptured to the ground surface. Surface faults may provide a plane of
weakness or hydrological discontinuity and adversely oriented surface faults may be indicative of the
location of future large landslides. Their location appears to have a strong structural geological control.
Initial results from our landslide investigations suggest predictive models relying only on ground-shaking
estimates underestimate the number and size of the largest landslides that occurred.