Earthquake-induced landsliding in New Zealand and implications for MM intensity and seismic hazard assessment

Technical Abstract

Landsliding and ground damage caused by 22 historical earthquakes in New Zealand have been studied to determine: (a) relationships between landslide distribution and earthquake magnitude, epicentre, faulting, geology and topography, and (b) improved environmental response criteria and ground classes for assigning MM intensities and seismic hazard assessments in New Zealand. It has been shown that in New Zealand, very small landslides occur at about M5, but significant landsliding occurs only at M6 or greater. Most widespread landsliding (mainly disrupted slides or falls of rock and soil) has been caused by shallow earthquakes (< 45 km) of M 6.2- 8.2. The minimum MM intensity threshold for landsliding is MM6, while the most common intensities for significant landsliding are MM7 and 8. Very large landslides occur mainly at MM9 and 10, and are used in this study to redefine zones of MM10 shaking for the 1855 Wairarapa and 1929 Murchison earthquakes.
The maximum area affected by landslides (or in which they occur) ranges from about 100 km2 at M 5 to 20,000 km2 at M 8.2. The expression: Log10 A (area km2) = 0.96 M (magnitude) - 3.7 can be used to estimate the average area likely to be affected by landsliding during earthquakes in NZ. Overseas earthquakes generally affect larger areas probably because of topographic and climatic differences The intensity threshold for liquefaction in New Zealand was found to be MM7 for sand boils, and MM8 for lateral spreading, but both may occur at one intensity level lower in highly susceptible materials. The minimum magnitude for liquefaction is M 6, but is more common at M 7 and greater. Liquefaction ground damage is most common at MM8-10, at epicentral distances of 10-100 km. Landslide size is strongly dependent on magnitude, intensity, and distance. In N Z, smaller slides are formed at maximum epicentral distances of almost 300 km (for M 8.2, at MM6). Large and very large  failures occur at distances of up to about 100 km (for M 6 or more, at MM8-10). Landslides during overseas earthquakes often occur at greater maximum distances due to poorly understood combinations and interactions of topographic, geologic, climatic, and seismic factors. If other factors are about equal, landsliding in NZ is likely to be slightly more severe and widespread during winter than in summer.

Next to earthquake magnitude and intensity, landslide occurrence is most strongly controlled by topography, rock and soil types, with failures mostly on moderate to very steep slopes (20°-50°). The most common landslides during earthquakes are rock and soil falls on very steep cliffs, escarpments, gorges, gravel banks, and high unsupported man-made cuts. Such features are highly hazardous and more susceptible to rapid failure because of rock defects, low strength, and topographic amplification of shaking. Large dip slope failures of Tertiary sandstone and mudstone often occur on gentle to steep slopes (10°-40°). Very large rock avalanches are caused by earthquakes of M 6.5 or greater, on slopes steeper than 25°-30° and more than 100-200 m high, especially on strongly shaken high narrow ridges.

The good correlation between landsliding and the fault rupture zone indicated by aftershocks suggests that overall landslide distribution can be used to indicate the approximate location of the epicentre and fault rupture zone for some earthquakes, but allowance must be made for topographic effects. Historical seismicity shows that shallow M 5 and 6 or greater earthquakes that trigger damaging landslides are more likely in northwest Nelson, the central Southern Alps, Fiordland, Marlborough, Wellington, Wairarapa, Hawke's Bay, and East Cape areas. The central North Island, Auckland, Central
Otago and Southland are regarded as lower hazard areas.

More detailed and expanded environmental response criteria (landslides, subsidence, sand boils, lateral spreads) in the MM intensity scale are proposed, along with provisional ground type classes of varying landslide susceptibility (similar to those for buildings). It is hoped that these can be used for assigning more reliable and consistent earthquake intensities in areas where there were few buildings.

Relationships developed in this study can be used to assess earthquake-induced landslide hazard and risk in New Zealand. Further studies are recommended to incorporate results from this project into a GIS-based National Landslide Hazard Model, which could be used to predict landslide hazard in different parts of New Zealand for triggering events such as moderate to large earthquakes and rainstorms. Other research that is recommended includes detailed studies of some earthquakes (e.g. 1929 Murchison, 1855 Wairarapa) to refine the ground type classes, palaeoseismic studies in known "seismic gaps" on major
active faults, and continued earthquake reconnaissance studies in New Zealand and overseas.