Earthquakes in New Zealand

Global Distribution of Earthquakes

What is an Earthquake?

An earthquake is a sudden slipping or displacement of part of the earth’s crust. 

When it happens, energy is released and travels outwards in waves.

Why do they happen in New Zealand?New Zealand is slowly being deformed by the interaction of two large pieces of crust or tectonic plates. This causes stresses in the rock that makes up the crust, and from time to time it fractures, releasing energy which we may feel as earthquakes.

Some of these fractures are many kilometres long; we often refer to the visible ones as fault lines. The Alpine fault, the Wellington fault and the Wairarapa fault are famous features of our landscape. But most earthquakes occur on unnamed, unknown faults beneath the earth's surface.

New Zealand sits on the boundary between two tectonic plates.

Indo-Australian

Plate

Pacific Plate

The surface of the earth is made up of a crust which varies in depth from 5 to 70 km. The word shallow is used to describe earthquakes that occur in the earth's crust.

Canterbury’s Unstable World

Earthquakes which have caused damage in New Zealand

CANTERBURY (2010)

(07/09/10)

Focus = the point of energy release.Epicentre = the point on the earth’s surface directly above the focus.

A seismograph or seismometer is an instrument that measures motions in the ground, including those of seismic waves generated by earthquakes.

This image represents one day's recording of the seismometer located in McQueens Valley on Banks

Peninsula, Canterbury.

Earthquakes generate several kinds of seismic waves including P, for "Primary" and S, for "Secondary"

waves.

P wave

S wave

The P waves move in a compressional motion similar to the motion of a slinky, while the S waves move in

a shear motion perpendicular to the direction the wave is travelling.

Richter ScaleEach whole number increase corresponds to an increase of approximately 30 times the amount of energy released.Each increase of 0.2 corresponds to a doubling of the energy released.

How the magnitude of an earthquake is calculated.

This data also enables Seismologists to determine the location of the epicentre and the depth of the focus of the earthquake

Canterbury Earthquake 04/09/10 4.35am

• Epicentre: 10 kms S.E. of Darfield; 40 kms W. of Christchurch.• Magnitude: 7.1 • Focal Depth: 10 kms

This scale is more useful than the Richter Scale when describing the impact of an

earthquake.

Canterbury Earthquake 04/09/104.35am - Modified Mercalli Scale Felt Intensities

AftershocksAn aftershock is a smaller earthquake that occurs after a previous large earthquake in the same area (the main shock). If an aftershock is larger than the main shock, the aftershock is re-designated as the main shock and the original main shock is re-designated as a foreshock.

Aftershocks are smaller earthquakes formed as the crust around the displaced fault plane adjusts to the effects of the main shock.

Most aftershocks are located over the full area of fault rupture and either occur along the fault plane itself or along other faults within the volume affected by the strain associated with the main shock.

Canterbury Earthquake Animation

Thousands of aftershocks have struck the Canterbury region since the huge 7.1 magnitude earthquake at 4.35 am on Saturday, 4 September, 2010.

Click here to view Paul Nicholls’ animation.

X

Canterbury Earthquake 04/09/10 Aftershocks05/09/10 9.44 pm

Modified Mercalli Scale

Felt Intensities

• Epicentre: 20 kms W. of Christchurch • Magnitude: 3.5• Focal Depth: 2 kms

Canterbury Earthquake 04/09/10 Aftershocks06/09/10 2.48 pm

Modified Mercalli Scale Felt Intensities

• Epicentre: 20 kms S.W. of Christchurch • Magnitude: 4.1• Focal Depth: 6 kms

Canterbury Earthquake 04/09/10 Aftershocks06/09/10 3.24 pm

Modified Mercalli Scale Felt Intensities

• Epicentre: 30 kms S.W. of Christchurch • Magnitude: 5.4• Focal Depth: 15 kms

Canterbury Earthquake 04/09/10 Aftershocks08/09/10 7.49 am

Modified Mercalli Scale Felt Intensities

• Epicentre: 10 kms S.E. of Christchurch • Magnitude: 5.1• Focal Depth: 5 kms

The Destructive Effects of Earthquakes1.Direct shaking 3. Landslides2. Liquefaction 4. Tsunami

• The level of damage done to a structure depends on the amplitude of the seismic waves and the duration of shaking.• The geology of an area can affect the level and duration of shaking, but more important are local site conditions. • The amplitudes are largest close to large earthquakes and the duration generally increases with the size of the earthquake. • Generally shaking in soft sediments is larger and longer than when compared with the shaking experienced at a "hard rock" site.

• Most earthquake-related deaths are caused by the collapse of structures and the construction practices play a tremendous role in the death toll of an earthquake.

Southern Italy, 1909 – death toll >100,000; survival rate 36%. More than half the victims died as a result of building collapse. San Francisco, 1906 – death toll about 700; survival rate 98%.

• Building practices can make all the difference in earthquakes, even a small earthquake beneath a city with structures unprepared for shaking can produce tens of thousands of casualties. - Engineers have a saying: ‘Earthquakes don’t kill people, buildings do.’

• After the 1931 Napier Earthquake attention in New Zealand was focused on weaknesses in building construction, especially poor building standards and the lack of any provision for earthquake-resistant design. • This led to a draft by-law in 1931, which was incorporated into a building code in 1935. The code recommended standards of design and construction so that buildings could resist the horizontal motions created by ground shaking.

• Building codes in 1965, 1976, 1984 and 1992 have added requirements to accommodate changes in building materials and design. • For a major earthquake the goal is to protect life by ensuring a building will not collapse and people can escape from it, even if the building itself is badly damaged.

Christchurch Liquefaction Susceptibility

Earthquake-induced liquefaction

A sand volcano or sand boil is a cone of sand formed by the ejection of sand on to the surface

from a central point. The sand builds up as a cone and a crater is commonly seen at the summit. The cone can range in size from

millimetres to metres in diameter.