Interesting Facts Regarding The Sept. 4th Earthquake Near Christchurch

A selection, from a new fact sheet:

• The fault that caused the earthquake on 4 September 2010 has been named the Greendale Fault. It is a previously unknown fault under the Canterbury Plains.
• The rupture on the fault broke through to the ground surface from many kilometres below and created a 29 km long east-west running scarp in the land between the Hororata River and Railway Road near Rolleston.
• Up to 4.6 metres horizontal and 1.5 metres of vertical of permanent offset has been recorded across the fault at the ground surface. This is thought to be the first time that this fault has moved in several thousand years. It is highly unlikely that it will move again, producing another large earthquake, within the next few thousand years.
• Movement on the Greendale Fault that caused the earthquake on 4 September also placed stress on the rocks around the fault, and particularly at the ends of the fault. Aftershocks are happening as the rock readjusts to this movement. This readjustment takes some time, so aftershocks generally continue for a few months after a big earthquake. However, they do tend to become less frequent and tend to generally get a little smaller as well.
• It is unclear at this stage how this earthquake has affected other faults in the region. The chance of another major earthquake in the central South Island is not likely to have changed. Earthquake activity in Canterbury and elsewhere in New Zealand following the earthquake appears to be normal.
• Cracks seen in the ground in areas of Waimakariri district, Christchurch city and lower Selwyn district are not faults. They are where the ground has cracked, often because of liquefaction, during the earthquakes. These cracks only go down to a depth of up to several metres. The Greendale Fault itself continues down kilometres into the earth’s crust.
• Parts of the Waimakariri River stopbank system downstream of State Highway 1 were damaged during the earthquake. At present there is approximately 1 in 20 year protection for this area. A programme of repairs is now being undertaken by Environment Canterbury staff, and 1 in 30 year protection is expected within two months. Flood protection upstream of State Highway 1 has not been compromised and remains at 1 in 500 years.
• Liquefaction and lateral spreading within the Halswell catchment has affected the capacity of the Halswell river and drainage system, and water levels are raised. Environment Canterbury staff are currently dredging the Halswell River channel and digging out side drains, and will move on to stabilising river banks. This work will take several months.
• The level of flood protection on the Avon River has not been significantly affected by the earthquake.Christchurch City Council investigations show that the risk of flooding from the Avon and Heathcote rivers is no greater than before the earthquake. However, heavy rainfall could cause localised surface flooding in areas affected by liquefaction because of damage to stormwater systems.
• Banks Peninsula is an extinct volcano and the earthquake activity is not related to it. Volcanic earthquakes are very different in style to the 4 September earthquake and subsequent aftershocks. Measurement of warm springs in the Lyttelton Harbour basin show an increase in flow, but no significant increase in temperature after the earthquake.