Natural hazards

Earthquakes

Although the Auckland region lies in one of the lowest earthquake activity regions of New Zealand, earthquakes of varying magnitude are likely to occur at some stage in the future.

Earthquakes that have shaking intensities greater than VII on the Modified Mercalli scale are likely to cause widespread damage. The extent of this damage will depend on the ground conditions of a particular site (e.g. liquefaction potential, ground shaking amplification, instability), building and infrastructure condition, response and recovery plans, and the community awareness of what to do.

The potential affects of a damaging earthquake:

Ground shaking that is severe enough to damage buildings built to earthquake code standards (~0.26 g) is expected to occur in Auckland, on average, once every two thousand years. The expected impacts of an earthquake of this magnitude include:

• 30-90% of liquefiable soils liquefy, causing very high probability of damage to structures founded in or on these soils. Also significant risk of slope failure
• loss of key engineering lifelines: Communications out for days; Energy supplies at Ports of Auckland sites are vulnerable to liquefaction; Water supply, wastewater, and stormwater possibly out for several months
• damage to bridges
• high economic losses
• high social disruption - real or perceived need for relocation of communities
• high risk of loss of life and to human health.

Leaning Apartment Houses in Niigata, Japan Earthquake of June 16, 1964, Niigata, Japan. The magnitude 7.4 earthquake killed 26 and destroyed 3,018 houses and moderately or severely damaged 9,750 in Niigata prefecture. This aerial view of leaning apartment houses in Niigata illustrates how soil liquefaction and poor foundations can result in damage. Most of the damage was caused by cracking and unequal settlement of the ground such as is shown here. About 1/3 of the city subsided by as much as 2 meters as a result of sand compaction. Photograph Credit: National Geophysical Data Center. Source: NOAA/National Geophysical Data Center.

Modified Mercalli Scale (MM)

The Modified Mercalli Scale measures the intensity of an earthquake. The intensity of an earthquake is its destructiveness due to the amount of ground movement, at a particular place.

MM magnitude	Effects
I	Not felt in general
II	Felt by a few on top of buildings.
III	Hanging objects may swing slightly.
IV	Felt indoors by many, dishes rattle, walls creak.
V	People run outside, crockery dislodged from shelves, hanging pictures, move.
VI	Felt by everyone, heavy furniture moved, plaster cracks.
VII	Frightens everyone, damage to weak buildings, difficult to stand up.
VIII	General fright and some panic, unreinforced chimneys fall, but only superficial damage to ordinary buildings.
IX	Panic is general, some damage to strong buildings, ground cracks, some houses shifted off their foundations.
X	General panic, wooden buildings seriously damaged, landslides, rivers slop over banks.
XI	General panic, broad ground cracks, soil slumps, great damage to underground pipes, few buildings remain standing.
XII	General panic, total destruction, objects thrown up in air.