The Christchurch earthquake
Today’s speaker was Dr. Richard Hamblin, a retired geologist who spent much of his career map-making with the BGS. However, whilst on a geology holiday in New Zealand in February 2011, Richard and his wife were ‘fortunate’ to experience the second major earthquake to hit Christchurch in five and a half months.
Richard’s lecture encompassed the reasons why earthquakes happen, the earthquakes in New Zealand and more specifically in Christchurch, how the earthquakes impacted on the fabric of Christchurch and how this affected Richard and his wife.
The geology behind the earthquakes. The Earth has three major internal structural components i.e. the core, mantle and crust. The mantle is composed of ductile material that moves in convection cells at approximately 1cm. per year. Overlying the mantle are the plates of crust that are either continental or oceanic. The plates move with the underlying convection cells. Upwelling from the mantle causes pressure reduction and the melting of rocks to form a basaltic magma which rises up to form mid-ocean ridges (MOR). The rotation of the convection cells means that the oceanic crust is moved on the currents to areas where it is taken down (subducted) below the continental crust. This process takes place under great stress which causes earthquakes and volcanoes. The volcanoes formed above subduction or Benioff zones are an acid (now known as felsic) and basic (now mafic) magma mix which together form an intermediate suite of andesitic cones. This is the result of the oceanic crust basalts moving up and mixing with continental crust on their journey to the surface.
The ring of volcanic and earthquake activity encircling the largest oceanic (Pacific) plate is also known as the ‘Ring of Fire’. From the Pacific MOR the Pacific plate is moving west and the Nazca plate moving east away from the MOR. These plates are subducted on their plate margins forming typical andesitic cones e.g. The Andes, the Cascades in North America (remember Mount St. Helens), the volcanoes of Japan, New Zealand and the western Pacific islands.
Richard went on to explain that New Zealand is divided into three regions in terms of earthquake activity. North Island is affected by the Pacific plate moving under the Australian plate at an angle of 45 degrees. The resulting row of andesitic cones forms an island arc on the land surface in a NE/SW direction. The arc ends in the sea at White Island where lava is still flowing. For the archetypal volcano shape think ‘Mount Doom’ from Lord of the Rings – a classic andesitic, compound, stratovolcano. Further west on North Island Auckland sits on a second island arc system, caused by stretching in the Australian plate. Here there is more continental (acid/felsic) crust absorbed into the magma making it more viscous and explosive. The resulting cones are smaller but there are more of them. Volcanism has not stopped in this area.
Between North and South Island the plate margin changes to one of conservative movement i.e. the plates slide laterally past each other. This area has had the two worst recorded earthquakes with magnitudes of 8.2 and 7.8 on the Richter Scale.
In South Island the Alpine fault system marks the boundary between the two converging plates. The margin forms a 400km wrench fault with attendant lateral movement of 2cm per annum, calculated from the amount of dislocation in the Dun mountain ophiolite. Convergence has created upward movement with the vertical uplift being greater on the Pacific plate, hence the Southern Alps are moving up faster. Raised beaches e.g. Dolomite Point, are evidence of this uplift. Mount Cook is rising rapidly and simultaneously being eroded. The sands and gravels being the products of erosion have been transported down rivers flowing southeast. Christchurch is sited just off a volcanic centre on these sediments. There had been no recorded earthquakes on the eastern half of South Island but there is a row of volcanic centres on the east coast. The likely explanation for these is that convergence is so great a partial melt was created in the mantle, with spinel minerals as evidence in surface rocks.
The two most recent Christchurch earthquakes were close in time but unrelated. The first, magnitude 7.1, took place on 4th September 2010 and was tied to movement on a reverse fault. The second quake, magnitude 6.3 and at a depth of 5km, on 22nd February 2011 was due to movement on a wrench fault with enough energy to move a hedge about 3 metres.
Richard’s eyewitness account included the following facts: the earthquake was extremely violent with entirely horizontal movement and no warning. The road surface rose up about 9 inches. Most single storey, wooden buildings were undamaged but the foundations failed and were removed by liquefaction. In the centre of Christchurch, which was built on Tertiary and Quarternary silts, sands and muds, whole buildings collapsed particularly stone or brick buildings such as churches. Mud and water from liquefaction covered roads and made trees topple. There was no electricity. Richard and his wife were denied access to their hotel, and spent overnight in a motel where they experienced frightening aftershocks that rolled in like thunder, with magnitudes of up to 5. Richard added that a state of emergency was declared for the following five days. However the Hamblins were stranded and were given refugee status, conferred upon them by the Red Cross. They were eventually airlifted out followed by their luggage five weeks later.
In light of Richard’s experience his two really helpful tips for travellers were to always carry your passport with you and to have important telephone numbers written down on paper in case mobiles do not work.
To conclude Richard summarised his experience by suggesting that Christchurch had been built in the worst possible place. Built on recent sediments the vibrations caused trouble to unstable, weak footings. The death toll of 200 could have been far worse had the population not moved onto the streets in the aftermath of the initial quake, A scary, but incredible experience!