Chapter 17 - Natural Hazards Awatere, Clarence and Hope faults while the New Zealand Alpine fault which originates in Milford Sound meets the Wairau fault at Blenheim. Damage from a major earthquake (i.e. greater than 6.5 magnitude Richter Scale) will include rock falls and landslides in the mountains. Large landslide deposits occur in a number of places throughout the mountains on both sides of the Wairau river, including one which dammed the Goulter River to form Lake Chalice, and are probably the result of major prehistoric earthquakes. An earthquake could cause lowering or raising of the lower Wairau Valley floor, with implications for drainage and flood control. Lowering could occur due to compaction following liquefaction of water saturated, unconsolidated sediments as well as, in the case of the Alpine Fault, vertical fault movement. The effects are likely to be greatest from Grovetown to the coast. Damage to buildings, rail, roads and other man made structures would be widespread. In addition, the intensity of an earthquake can vary considerably depending on subsurface materials. In general, damage is higher for structures built on unconsolidated, water-logged sediments, such as on the valley floors, than on basement rocks. Other forms of earthquake damage could include collapse of aquifers, or their offsetting by fault movement, leading to failure of groundwater supplies. The DSIR (1989) calculated the mean return periods for earthquakes in the Lower Wairau area as follows: Intensity Likely effects Approx. Return (modified magnitude Period (yrs) Mercalli) (Richter Scale) MM VI Felt by everyone - heavy objects move and 4.9 to 5.4 5 - 10 plaster cracks. MM VII Very strong - cracking of buildings, plaster 5.5 to 6.1 20 falls from ceilings, but well designed buildings little affected. MM VIII Destructive - damage considerable in all 6.2 to 6.8 100 but well constructed buildings. MM IX Ruinous - some damage to well designed 6.9 500 buildings and dwellings may be thrown out of plumb, ground fissures conspicuous. MM X Destructive - many buildings destroyed. 7.0 to 7.3 MM XI Very destructive - few buildings standing. 7.4 to 8.1 MM XII Catastrophic - damage total. 8.1 to 8.9 17.6.2 Objectives and Policies Objective 1 To avoid, remedy or mitigate the adverse effects of seismic hazard. Policy 1.1 Recognise that Marlborough is in the highest zone of earthquake risk in New Zealand. Policy 1.2 Establish an emergency recovery project for Marlborough. Policy 1.3 Maintain emergency procedures in association with Emergency Response. 17 - 13