Graphic shows two main Christchurch fault ruptures - 06/09/2011

Scientists have produced a semi-transparent graphic giving a simplified view of the two faults that ruptured causing the earthquakes on 22 February and 13 June in Christchurch.

The viewpoint is above Gebbies Pass on Banks Peninsula looking north over Lyttelton Harbour and the Port Hills towards Christchurch.

The ground surface is semi-transparent so the underground faults can be seen through it. The dashed lines show where the faults would be visible on the ground if they had broken all the way to the Earth’s surface.

This is not the only possible interpretation of the faulting that occurred in the February and June earthquakes, but it is the simplest model that gives a reasonable fit to observed ground displacement data (GPS and satellite radar).

Scientists are continuing to work on improved models of the earthquakes using seismic data and ground displacement data.

Graphic showing two fault lines

The colours on the faults show the amount the two sides of the fault slipped past each other during the earthquake. Red shows slip of more than 1.8 metres, yellow is more than 1.2 metres, and green is more than 0.6 metre. The arrows on the faults show the direction of slip, with the length of the arrow proportional to the size of the slip.

The squares on the fault surfaces measure 1km by 1km. The faults do not break through to the ground surface. Slip occurs down to about 7-8km depth and up to within about 1km of the surface.

Both earthquakes caused parts of the Port Hills to move south, or southwest, and up. Parts of the Port Hills have risen by more than 40 centimetres.

The magnitude 6.3 February earthquake was the larger of the two, with slip of more than 2.5 metres between the two sides of the fault at 4km depth. The south side of the fault moved generally up and west while the north side moved the opposite way.

The largest slip was centred under the Avon-Heathcote Estuary. The fault trends in a WSW-ENE direction from about Cashmere to just offshore of the estuary, and it slopes towards the southeast.

The magnitude 6.0 June earthquake had a maximum slip of about 1.5 metres at 4-5 km depth. The west side of the fault moved generally south and up, while the east side moved the opposite way.

The largest slip was centred under Brighton Spit and Sumner. The June fault has a different trend to February’s. It is oriented in a NNW-SSE direction between Brighton Spit and near the mouth of Lyttelton Harbour, and it slopes down towards the west.

It is not uncommon to have two nearby faults breaking in different orientations like this. Both the faults are responding to stresses in the ground that are oriented in a WNW-ESE direction, midway between the orientations of the two faults.