Dating

In order to make sense of the sediment cores that can be retrieved from lakes near to the Alpine Fault such as Lake Christabel, it is worth having a think about what happens to the environment when the fault ruptures in a large earthquake. Under normal conditions, alpine lakes fill up very slowly with sediment that is fed into them by rivers. The particles settle onto the lake bed gradually, to create a sequence of finely layered mud. When an earthquake occurs, a number of consequences affect the landscape. The soft surface sediment on the bed of the lake gets deformed and folded, and the shallower slopes at the side of the lake collapse to create flowing avalanches (turbidites) that sweep down and across the lake floor. In the nearby mountains, large landslides occur that choke the river valleys with a chaotic mix of large and small rock fragments. In the months and years following the earthquake, the landslide debris is gradually washed into the lake, to form a recognisable layers on top of the turbidite deposit. Eventually, conditions return to normal, with the finely layered sediments gradually covering over all of the evidence of the earthquake and its aftermath. It may be hundreds of years before another earthquake sttikes that is near enough and strong enough to leave its mark in new layers of the lake sediment. Now lets have a look at the real thing – an example of a sediment core that has been retrieved from a New Zealand’s alpine lake. Back in the lab at the University of Otago in Dunedin, Jamie Howarth opens a core tube to reveal the layers of sand and mud from Lake Christabel. Here is a section of the core that shows the finely laminated lake sediments formed in normal conditions (on the right). In the centre you can see that the layers are slightly folded – this is the indication of an earthquake that has deformed these layers. They would have been at or just below the surface of the lake floor at the time. Here Jamie is indicating the remains of a leaf next to the blade of the knife. This is not far below the earthquake layer, and can be used to get a radiocarbon age which will help to date the earthquake event. This dark coarse layer is the next layer that was added to the sequence on top of the folded sediment. It is the base of an earthquake generated turbidite deposit. The material gets gradually finer to the left (‘upwards’) as the cloud of particles slowly settled onto the lake floor. The section shown here is the landscape recovery phase. Dating of the base and top of this layer in several cores has shown that it can take 50 years for the landscape to recover from an Alpine Fault earthquake. During that time, hillsides are destabilised, debris flows cover flat areas near to the mountains, and rivers are prone to changing course due to being overloaded with sediment. Finally we see the thinly layered sediment  indicating that normal conditions have returned to the lake environment. This map shows what can be done when this research is carried out at a number of lakes along the Alpine Fault. The coloured lines (purple, orange, green etc) show earthquake records that have been identified so far in some of the lakes along the length of the fault. You can see that the last earthquake rupture (in 1717 AD) was over 300 km long. The one prior to that around 1600 AD ruptured the northern end of the fault. Information about previous earthquakes is still incomplete, but the picture is starting to become clearer. With more research, Jamie and his colleagues will be able to show a more detailed history of the last 10 Alpine Fault earthquakes including the dates, lengths of rupture and magnitudes of the events.

Lake Christabel   J.Thomson@GNS Science This is Lake Christabel in New Zealand’s South Island. It is one of the many beautiful alpine lakes  to be found close to the Alpine Fault. Lake Christabel was formed when a huge landslide blocked the valley, thus damming the river that then backed up to form the lake.The present day outlet runs over the old landslide deposit of large chaotic boulders. Hidden beneath the waters of Lake Christabel are very distinctive sediment layers that tell the story of huge earthquakes that rocked the nearby mountains during ruptures of the Alpine Fault. Jamie Howarth from GNS Science, and Sean Fitzsimons from Otago University, have been investigating several such lakes to read the earthquake histories. I joined them on a recent expedition along with Delia Strong and Rob Langridge from GNS Science. The aim was to retrieve sediment cores from the lake to investigate the earthquake records. First of all a seismic survey was undertaken to find the best spots to sample on the lake bed. Sean is in the lead boat, towing a second dinghy that carries the equipment. The survey uses an acoustic source that sends pulses down into the water. The boat is towed along so that noise interference produced by a nearby motor is avoided. As the sound pulses are reflected back from the lake bed and its layers of underlying sediment, they are translated into a two dimensional vertical section image of the lake floor. A number of survey lines are made across the lake to give some idea of the 3 dimensional structure of the lake sediments. Once the best locations for sampling have been chosen from the survey results, the corer is prepared with a fresh 6 metre pipe that will be pushed into the lake floor to retrieve a sediment core. The corer is transported to the chosen point on the lake surface, and then dropped off the side of the boat once it is in position. After being connected with several airlines which are required to control the pressure coring process, the corer is lowered the 90 metres to the lake floor. The large barrel sits at the bottom, and is sucked into the mud to create a stable platform for coring. High pressure air is then applied to the piston which pushes the 6 metre coring pipe into the mud, releasing clouds of bubbles up to the surface. These bubbles allow Sean and Jamie to monitor what is going on with the corer at depth. Lake Christabel Corer Retrieval J.Thomson@GNS Science When the coring is complete, an airbag is attached to the line and filled up with air so that it  pulls the whole assembly out of the mud. The airbag bursts up to the surface from below in a spectacular fashion. Lake Christabel Corer Retrieval J.Thomson@GNS Science About a minute later, the corer assembly also emerges from the depths. It is not a good idea to be too close to this as it could easily sink a boat that was in the wrong place. The corer is then plugged and loaded into the boat to be brought back to shore, with its precious cargo of sediment. The PVC tube containing the core is then cut into 1.5 metre lengths for ease of transport. Each tube is carefully labelled to avoid any confusion  about where it was taken from and its relationship to the other samples. Lake Christabel Flight  J.Thomson@GNS Science Once all the sampling has been completed, the expedition is over. It takes several helicopter loads to transport the two boats, safety gear, corers, generators, samples and all our personal equipment back to the road end. The samples are then taken to Otago University for analysis. My next post will describe how alpine lakes like Lake Christabel have shown themselves to be very useful natural seismometers through this research approach.