A University of Canterbury seismic network has captured valuable data on the magnitude 7.1 Darfield earthquake of September 4th.
Monday 1st November 2010
A network of scientific instruments designed to lie in wait for the anticipated Alpine Fault earthquake has captured detailed information about the Darfield earthquake and its aftershocks.
The network, CanNet, was prompted by studies of the South Island’s Alpine Fault by Mark Yetton in the mid-to-late 1990s. John Berrill, Professor of Engineering at the University of Canterbury at the time, envisaged a network of 80 instruments deployed throughout Canterbury to study movement of the fault when it eventually ruptures.
At the time, a network of that size was beyond available funds so a decision was taken to design and manufacture a suitable low-cost recorder.
Early this decade, the Earthquake Commission proposed supporting the network by buying the instruments and running them as part of the GeoNet project. The university retained control of the network’s scientific direction and design.
It was a stroke of luck that nearly 30 strong motion CanNet accelerographs, waiting for the Alpine Fault quake, completely surrounded the epicentre of the Darfield quake when it struck on the morning of September 4th.
Having such a dense network of instruments ready to record a large nearby quake is normally not economically viable. Foreknowledge of the location of an earthquake’s epicentre is beyond the science at the present time, so networks tend to be less dense to cover as much area as possible. After a large earthquake has occurred scientists then increase the density of instruments in the nearby area to study the aftershocks and continued movement of the fault.
CanNet is unique in that the group designed and built their own low-cost instruments, allowing them to be deployed in advance of a large earthquake. This meant that a dense network was already in place to study the main earthquake and early aftershocks. Luck contributed by placing the epicentre of the unexpected earthquake in amongst the instruments.
The nearest sensor to the earthquake centre was located at Greendale, west-south-west of the epicentre at Darfield. Not surprisingly this instrument recorded the strongest shaking of 1.25g – i.e. a force 1.25 times as strong as gravity.
In addition to providing very accurate data on ground motion during the earthquakes, the CanNet instruments have provided detailed records of the P and S waves generated by each event. This allows the location and depth of each earthquake to be determined with great accuracy.
The unique data, which includes the mainshock and early aftershocks, is seen as tremendously important by New Zealand and overseas scientists and engineers.
[source: GeoNet News, Special Edition, “Darfield Earthquake, 4 September 2010 – The GeoNet Response,” published October 2010]