Abstract
A study of landsliding caused by 22 historical earthquakes in New Zealand was completed at the end of 1997. The main aims of that study were to: (a) study the nature and extent of landsliding and other ground damage (sand boils, subsidence and lateral spreading due to soil liquefaction) caused by historical earthquakes; (b) determine relationships between landslide distribution and earthquake magnitude, epicentre, isoseismals, faulting, geology and topography; and (c) establish improved environmental response criteria and ground classes for assigning MM intensities and seismic hazard assessments in New Zealand.
 Relationships developed from the study indicate that the minimum magnitude for earthquake-induced landsliding (EIL) in N.Z. is about M 5, with significant landsliding occurring at M 6 or greater. The minimum MM intensity for landsliding is MM6, while the most common intensities for significant landsliding are MM7-8. The intensity threshold for soil liquefaction in New Zealand was found to be MM7 for sand boils, and MMS for lateral spreading, although such effects may also occur at one intensity level lower in highly susceptible materials. The minimum magnitude for liquefaction phenomena in N.Z. is about M 6, compared to M 5 overseas where highly susceptible soils are probably more widespread.
 Revised environmental response criteria (landsliding, subsidence, liquefaction-induced sand boils and lateral spreading) have also been established for the New Zealand MM Intensity Scale, and provisional landslide susceptibility Ground Classes developed for assigning MM intensities in areas where there are few buildings. Other new data presented include recent earthquake studies (e.g., Murchison 1929), a preliminary landslide size/frequency distribution for earthquakes over the last 150 years, and a preliminary EIL Opportunity and hazard model for New Zealand. Implications for earthquake-induced landsliding for seismic hazard assessments in New Zealand are briefly discussed. Suggestions are also made for future EIL research, including further studies of historical earthquakes, and large prehistoric landslides in the central Southern Alps, northwest Nelson, and Fiordland, to help determine past and future earthquake activity and hazard from active faults in those regions.
Highlights
Landslides have occurred during many historical earthquakes in New Zealand
The Keefer (1984 a) study: (a) identified the main types of landslides caused by earthquakes; (b) ·determined relationships between earthquake magnitude and the area affected by landslides, and the maximum distance of earthquake-induced landslides, spreads and flows from the epicentre and fault rupture zone; and (c) established threshold magnitudes and minimum shaking intensities that trigger landslides in susceptible materials
Since the revision was primarily aimed at making the Modified Mercalli (MM) scale appropriate for modem construction, most of the changes related to building damage, some were intended to better define earthquake shaking effects on the environment such as landslides and ground damage resulting from liquefaction phenomena
Summary
Since 1840, at least 22 earthquakes have resulted in substantial or widespread and damaging landsliding (Figure 1 and Table 1). The substantial damage caused by earthquake-induced landsliding (EIL) has been second only to building damage caused by strong shaking. Some buildings have been destroyed, and many roads have been damaged and closed by landslides and rock falls. Sixteen deaths resulted from landslides triggered by the 1929 Murchison earthquake alone. This paper presents a summary of that report (referred to here as the 1997 study), and other related research since it was completed
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