Author: N Raynes

HOW DOES AN EARTHQUAKE HAPPEN AND WHAT HAPPENS WHEN IT DOES?
What sequence of events occurs when an earthquake is
hazardous?
STAGE 1: PROCESSES WITHIN THE EARTH
Magma rises to the crust within the mantle. As it moves against the underside of the crust, friction is created. This friction causes the plates to move.
The plates move
against each other
Away from each other
Alongside each other in opposite directions
STAGE 2 : PROCESSES ON THE EARTH
As Pressure builds up between plates foreshocks are often felt. These are a cluster of small earthquakes that can be felt in the days or weeks before a large earthquake
A sudden rupture of the plates along a fault line is experienced as a severe shaking for 15-40 seconds.
The centre point of the rupture below the earth’s surface is called the focus. Seismic waves radiate in all directions from the focus. The centre point on the earth’s surface above the focus is called the epicentre.
There are THREE types of seismic waves radiating from the focus -
P Wave (primary wave) - a sound wave that travels through solids, liquids and gases. It is the first movement felt by people when an earthquake occurs and feels like a sonic boom. It does little damage other than rattling windows.
S Wave (secondary wave) - the second wave arrives shortly after the P Wave as it travels through solids only. It makes the ground shake vertically and horizontally, like a jelly, and can be very destructive.
L Wave (long wave) - arriving after the P and S waves it moves through the earth in waves making the earth’s surface ripple like water on a lake. It moves the ground sideways and can be very destructive.

STAGE 4: AFTERSHOCKS
Numerous aftershocks can be felt for hours and days afterwards. Some of these can be large and cause further
destruction to already weakened structures.
STAGE 5: RECOVERY
A state of emergency is declared after the main earthquake and is coordinated by Civil Defence. The immediate
concerns after tending to the injured are food and shelter for survivors. Unsafe areas are evacuated and schools, halls and marae used as shelter centres. The state of emergency may not be lifted for many weeks.
STAGE 6; REHABILITATION
The rebuilding of roads, buildings, services and economic facilities may take months and is very costly.

CASE STUDY : THE NAPIER EARTHQUAKE 1931
IMPORTANT EVENTS IN THE HAWKES BAY EARTHQUAKE IN 1931

Offers of assistance came from all over New Zealand. The naval vessels HMS Dunedin and Diomede rushed to Napier with relief and workers from Auckland. Eventually Napier’s CBD was rebuilt in the Art Deco style.

At 10.47am on Tuesday, February 3rd 1931, these processes within the earth showed themselves on the earth’s surface when two violent earthquakes struck Hawkes Bay within 30 seconds of each other.
Centred 17 km beneath Patoka (30 km northwest of Napier) the earthquake sent seismic waves in all directions travelling at 6 km per second. The first quake was 7 on the Richter scale. Miss Nancy Hobson described the start of the earthquake as being ‘like on the deck of a ship in rough seas.’

First on the scene were sailors from HMS Veronica berthed in the port at the time. They joined firemen, policemen and locals in the search for survivors. Fire fighting was difficult because water pipes had been broken. The Veronica sent the message for help. Residents left their homes and camped on the beach front, lawns and open spaces. The race course became the field hospital and Nelson Park the refugee centre. Many children were sent to relatives or friends in Palmerston North.

674 aftershocks occurred after
February 3rd.

The Pacific Plate is being subducted 23km beneath Napier and continues to a depth of 80 km beneath Mt Ruapehu. Riding over it is the deformed edge of the Indo-Australian plate forming the ranges of the Hawkes Bay. The plates are colliding at a rate of 55 mm per year.

A total of $300 million of damage was caused as buildings collapsed. Within minutes in Napier, fire spread through the inner city destroying 2.5 hectares of the C.B.D. People trapped in the rubble died. The inner city of Hastings collapsed killing 90 people. All roads (except the highway from Hastings to Wellington), railway lines, and telephone links were out and much of the power supply.