The North Canterbury region has had a complex geological history including periods of marine transgression, tectonic uplift and erosion (Rattenbury et al. 2006). Most of the rocks occurring below the Quaternary sediments are ‘basement’ Torlesse Group and erosion-resistant igneous intrusions. There are smaller outcrops of weak marine
sedimentary rocks. Glaciation in the Quaternary led to the formation of many lakes (e.g., Rotoiti, Rotorua, Sumner). Large rivers run from these lakes cutting gorges and valleys. During the Quaternary, the Southern Alps and mountain ranges throughout Marlbourgh, Nelson and northern Canterbury uplifted and the region was subject to widespread
18
erosion. Alternating glacial and interglacial climatic fluctuations then gave rise to the sediment deposits (soils) that occur across the region.
1.7.1.2 Tectonic History
North Canterbury has a long tectonic history record extending back to the early Ordovician (Rattenbury et al. 2006). It is still tectonically active today. Located on the Australian-Pacific plate boundary the region is crossed by a number of major active faults, several of which have generated damaging earthquakes in historical time.
1.7.1.3 Geological Units
Basement Torlesse greywacke, formed from the Cambrian to early Cretaceous, is generally more resistant to erosion than the younger overlying sedimentary rocks. In places where the rock has been heavily fractured by faults or through folding and shearing processes (e.g., schist and mélange) the rock is weaker and more prone to slope failure (Rattenbury et al. 2006).
The overlying Waima Formation and Greta Formation (Motunau Group) are Neogene siltstones (GNS 2013). They are weak and friable. Undifferentiated Cookson Volcanic Group is predominantly basalt. It is fractured, but more erosion resistant than the sedimentary rocks into which it was intruded. Paleogene limestone and calcareous mudstone is erosion resistant, often forming prominent ridges. The sandstone, siltstone, mudstone and conglomerate formed in the late Cretaceous to Pliocene (Rattenbury et al. 2006). The sandstones, while weakly cemented, are relatively hard like the
conglomerates. The siltstones and mudstones are softer and more prone to erosion. The clay-rich material they form through erosion are highly plastic, water saturated and are prone to slope failure. Most of the limestone and volcanic units, with the exception of the uncemented and weathered units, are strong.
The late Pleistocene river gravels are weathered and eroded (GNS 2013). Quaternary sediments are generally weak, weathered and water-saturated (Rattenbury et al. 2006). Deposits with high clay content are less prone to slope failure. Loess, common in
Northeast Canterbury, is usually several metres thick and has relatively high dry strength. However, loess displays tunnel gully erosion on slopes and develops debris flow with increasing moisture content.
19 1.7.1.4 Regional Climate
Canterbury has a temperate climate (Macara 2016). NIWA released a report in 2016 detailing Canterbury’s climate. One of the rainfall data collection sites was Culverden, centrally located in the Hurunui District (Figure 1). Winter is the wettest season, with a high average rainfall (9 days/month) and highest number of days of rainfall (27 days) (Figure 2). This is despite the November having the highest monthly rainfall at an average of 63mm out of an annual average of 576mm (Figure 3).
Hanmer Springs, compared to four other locations in other parts of Canterbury, showed the lowest moisture deficit and highest runoff (Macara 2016). This can be extrapolated to mean that the Hurunui District has a higher soil moisture content than much of the rest of the region. As expected, and within the regional temperate trend, winter is the wettest season with most runoff and summer is the driest.
The temperature data for the Hurunui District does not have a proxy within Macara (2016)’s report. The town of Kaikōura’s geography and coastal location is different from the majority of the Hurunui District. A temperature map within the report
indicates that Kaikōura temperatures are typically lower on average than the Hurunui District’s. The area’s hilly topography creates a large degree of variability.
Figure 2 Mean # of rainfall days per month for Culverden adapted from NIWA report using data from 1981-2010 (Macara 2016).
Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec
.1mm a day 8 7 7 8 9 9 10 8 8 9 8 8 1mm a day 7 6 6 6 7 7 8 7 7 8 7 6 0 2 4 6 8 10 12 Me an # o f Da ys Months
20
Figure 3 Mean monthly rainfall for Culverden adapted from NIWA report using data from 1981-2010 (Macara 2016) . The Canterbury plains has a history of drought. The Hurunui District in particular was affected by the 1997-1998, 2000-2001 and 2015-2017 droughts (MPI 2013,2015). The drought was on-going at the time of the earthquake.
1.7.1.5 Geohazards
The primary geohazards in North Canterbury include: landslides, earthquakes, co- seismic hazards (e.g., liquefaction, strong ground shaking), coastal erosion and tsunami. Coastal erosion and tsunami are not a concern for the farm case studies in this thesis as they are all located sufficiently inland. Earthquakes or heavy rainfall typically triggers landslides. They can also occur in weak shallow marine deposits originating in the Cretaceous and Cenozoic (Rattenbury et al. 2006). Coastal erosion and rivers
undercutting slopes also trigger landslides. Hundreds of faults were mapped in North Canterbury pre-November 2016. Unconsolidated water-saturated Quaternary
sediments can amplify earthquake ground motions leading to stronger shaking than experienced on a bedrock site. Such amplification has the potential to result in greater shaking damage.
Since the beginning of written human record in the Kaikōura area (about 1840), there have been many strong earthquakes (Table 1). The first of these were the M 7.0+ 1848 Marlborough and 1888 North Canterbury earthquakes. The North Canterbury
earthquake ruptured part of the Hope Fault, which with a slip rate of 20-40mmyr-1 is the most active fault in the area.
0 10 20 30 40 50 60 70
Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec
Me an Ra in fall (m m ) Months
21
Table 1 Earthquakes that have impacted the study area. Local refers to earthquakes that occurred within the 2006 Kaikōura Geological Map area. Notably the 1855 M 8.1 Wairarapa earthquake occurred on the North Island. Modified from
(Rattenbury et al. 2006).
Rural New Zealand Earthquakes Current Study Area
Year Location/ Name Magnitude (M)
1848 Marlborough 7.5 1888 North Canterbury 7.0-7.3 1901 Cheviot 6.9 1922 Motunau 6.4 1948 Waiau 6.4 2016 Culverden/ Kaikōura 7.8
Outside the Study Area
1855 Wairarapa 8.1
1929 Arthur's Pass 7.0
1929 Buller 7.7
22
1.7.2 Hurunui District
Figure 4 The Hurunui District wards. Adapted from (Hurunui District Council, 2012).
The Hurunui District is 864,640 ha (Figure 4)(Hurunui District Council 2012). In 2013, the population was 11,529 (Statistics New Zealand 2013b). Agriculture and forestry are the major industries. The District also has growing viticulture and tourism industries based around the areas vineyards, coastline, hot springs and lakes. The climate across the district includes coastal micro-climates and alpine climates (Hurunui District
Council 2012). North Canterbury contains river valleys, mountains and ocean coastlines.