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EXECUTIVE SUMMARY <\/SPAN><\/P>

<\/P>

GNS Science (GNS) has been commissioned by Rotorua District Council (RDC) to describe <\/SPAN><\/P>

landslide hazards within the Rotorua district. The project has assessed landslides throughout <\/SPAN><\/P>

the district in order to differentiate areas where the landslide hazard is similar. The report <\/SPAN><\/P>

and accompanying map explain and show the variation in relative landslide susceptibility <\/SPAN><\/P>

throughout the district. This information will help RDC develop policies and rules for dealing <\/SPAN><\/P>

with landslide hazards in the new district plan. <\/SPAN><\/P>

<\/P>

The landslide susceptibility map prepared using the methodology outlined in this report is in <\/SPAN><\/P>

good agreement with the known landslide information in the Rotorua district. The agreement <\/SPAN><\/P>

between the susceptibility model and historical landslide data provides confidence that the <\/SPAN><\/P>

relative landslide susceptibility displayed on the map effectively discriminates relative <\/SPAN><\/P>

landslide hazard. <\/SPAN><\/P>

<\/P>

Two rainfall events generated multiple landslides over a 14 year period between 1996 and <\/SPAN><\/P>

2010. Analysis of rainfall data indicates that 200 mm of rainfall in 24 hours is the threshold for <\/SPAN><\/P>

causing multiple landslides. Given that the rainfall events were limited in their extent in the <\/SPAN><\/P>

district and a threshold of 200 mm of rain in 24 hours (approximately a 1-in-20 year return <\/SPAN><\/P>

period event based on HIRDS data), to have two multiple landslide generating rainstorms <\/SPAN><\/P>

(greater than 20-year return period storms) in 14 years matches rainfall data (the rainfall and <\/SPAN><\/P>

landslide datasets were collected independently). <\/SPAN><\/P>

<\/P>

The landslide damage caused by the 2004 Rotoehu earthquake is equivalent to MM8 in the <\/SPAN><\/P>

epicentral area around Lakes Rotoehu and Rotoma. The 1987 Edgecumbe earthquake <\/SPAN><\/P>

caused landslide damage equivalent to MM6 in the same area. In Rotorua City MM8 <\/SPAN><\/P>

intensity has a return period of about 2500 years but for areas east of the city MM8 has a <\/SPAN><\/P>

return period of 500 years. Thus the landslide distribution in the epicentral area of the <\/SPAN><\/P>

Rotoehu earthquake is an analog for the type of landslide damage that might be expected <\/SPAN><\/P>

during MM8 shaking in Rotorua city, which is expected to occur an average once every 2500 <\/SPAN><\/P>

years. <\/SPAN><\/P>

<\/P>

Large pre-existing landslides have been searched for by systematically examining vertical <\/SPAN><\/P>

aerial photographs. No pre-existing large landslides have been identified in the Rotorua <\/SPAN><\/P>

district. Based on the lack of evidence for large landslides in these terrains nationally, there <\/SPAN><\/P>

is no large, pre-existing landslide hazard in the Rotorua district. <\/SPAN><\/P>

<\/P>

First-time landslides not triggered by high-intensity rainfall or earthquake-shaking occur <\/SPAN><\/P>

occasionally. Some of these occur in association with rainfall but there are often other <\/SPAN><\/P>

contributing factors such as recent slope modification. <\/SPAN><\/P>

<\/P>

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