When an earthquake occurs, two things need to be determined quite quickly: the epicentre and the magnitude.

The actual source of an earthquake, called its focus, is usually many kilometres deep in the Earth, where the rock as been strained and has finally reached its breaking point.  The epicentre is the place on the Earth's surface directly above the earthquake's focus.  The energy that is released travels outwards from the focus in waves - and people and buildings are shaken when those waves reach their area.
 

A seismograph makes a recording of how the ground moved, and from that recording we can tell exactly what time those waves reached the seismograph.  Two different types of wave travel outward from an earthquake at different speeds, rather like lightning and thunder in a thunderstorm.  The different times that the two waves arrive at any particular seismograph let scientists know how far away the focus of the earthquake was from the seismograph.  As there are several seismographs at different places, each one gives us a measure of distance to the focus, so scientists can combine all the information to calculate exactly where the focus was, its depth within the earth. and its nearest point to the Earth's surface - the epicentre. Finding the epicentre.  The maximum distance that the epicentre could be from each seismograph is calculated and circles are drawn with that distance as the radius.  The circles will intersect at the epicentre The magnitude number describes how big the earthquake is, not the sense of how badly you get shaken, but how big the disruption was within the Earth.  If you are near a large earthquake, you will be shaken very strongly.  If you are far enough away, you might not feel it at all.  To calculate magnitude seismologists use a seismograph to measure how much the ground moved, and then they take into account the distance from the focus.  This means that no matter what distance you are from the earthquake, you always get the same answer for the magnitude. The intensity of an earthquake, on the other hand, is its destructiveness due to the amount of ground movement at a particular place.  Intensity is measured accotrding to the Modified Meercalli scale which has ten steps for New Zealand earthquakes.  The intensity of an earthquake does not relate directly to the amount of energy release in an earthquake (which dictates magnitude) because intensity decreases with distance from the event, and because natural features such as rock type, soli type and the amount of water in the area, modify its destructiveness from place to place.

Finding the distance X.  The time interval between the arrival of the P. wave and the slower S. wave increases with increasing distance from the focus.

The magnitude scale was devised by Prfoessor Charles Richter in 1935 to compare local Californian earthquakes.  An earthquake of magnitude 4 is quite small, in fact you have to be quite close in order to feel it.  Magnitude 6 is big enough to do quite a lot of damage within a distance of a few kilometres.  The biggest that has occurred in New Zealand was of magnitude 8 in 1855.  It was felt widely over the whole country and caused a lot of damage in central New Zealand

It is important to understand that as you go up one step on the magnitude scale, you multiply the size by about 30.  So an earthquake of magnitude 5 is 30 times as big as a magnitude 4 earthquake, and a magnitude 6 is 30 times as big again, or 30 times 30 = 900 times.  This means that the magnitude 8 earthquake in 1855 was nearly a million times as big as an earthquake of magnitude 4.

Civil Defence needs to know the location of the earthquake and its magnitude quite quickly in order to judge whether the effects are likely to be very severe.  If the magnitude is small, or if the epicentre is offshore, it is likely no damage has been caused.  But if the magnitude is large, and especially if the epicentre is near a city of any size, many people could have been affected and Civil Defence may need to be involved.

The Institute of geological and Nuclear Sciences has developed procedures for providing this information, and expects that with further developments it will soon be able to perform this task more quickly and reliably.

North Canterbury Earthquake 31 August 1888

Although Christchurch has been little affected by earthquakes this century, its cathedral suffered damage on several occasions last century.  The 1888 magnitude 7.1 - 7.3 earthquake in northern Canterbury, some 100 km from Christchurch, caused partial collapse of the cathedral's spire.  This North Canterbury earthquake originated at a shallow depth and ruptured to the surface along the Hope Fault, west of Hamner Springs.  During the earthquake, the two sides of the fault slid past each other with little or no vertical displacement.  This type of faulting is called strike-slip faulting and recognition of it in 1888 was a world first.

Damage to houses during this earthquake was severe close to the fault rupture.  Sand fountains and cracks in the ground (other than the fault crack) were common on the nearby river flats and there were numerous landslides on the surrounding hills.

The fault that ruptured in this earthquake is one of several nearly parallel faults that crosses the North Canterbury and Marlborough areas.  These have formed due to the collision between the Pacific and Australian plates.