On shaky ground
Trying to protect people from the worst effects of earthquakes
Recent events in both Christchurch and Japan are a grim reminder of what dire human consequences earthquakes can have when they occur in densely populated areas. New Zealand is prone to earthquakes because the Alpine fault, a region where the Pacific and Indo-Australian tectonic plates meet, runs down the middle of the south island. Many countries in the region such as Indonesia and Japan, also lie near fault lines and consequently are prone to earthquakes too.
Dr Phil Cummins, a scientist at the ANU Research School of Earth Sciences, is currently leading a project involving Geoscience Australia, technical agencies of the Government of Indonesia, and Bandung Institute of Technology, Indonesia. The project aims to greatly increase our knowledge of the potential for damaging earthquakes to occur in Indonesia.
Determining where an earthquake may occur is quite a complicated business. “Although earthquakes are often caused by slippage along fault lines where tectonic plates join, that doesn’t mean that they only occur right at the plate boundary.” Dr Cummins says, “ The process of plate movement creates residual stress in the plate that can extend far away from the plate boundary. Sometimes this intraplate stress can cause earthquakes that rupture shallow crustal faults some distance from the actual plate boundary. These smaller faults can be very dangerous, especially if they are close to population centres and we either don’t know where they are or don’t know what their earthquake activity might be. This is unfortunately exactly what happened in Christchurch.”
One of the recently completed phases of Dr Cummins project was a terrain mapping of two active faults in Java using a technique known as LIDAR (Light Detection And Ranging). “There have been quite a few studies using Radar,” Dr Cummins explains, “But the problem with that in a heavily vegetated country like Indonesia is that the microwaves reflect off the leaves so what you’re actually looking at is the profile of the tree tops which is not the same as that of the underlying ground.”
Using the LIDAR system, the team have been able to map the topology of the ground to accuracies of better than a metre, enabling them to see fault lines and stress related features in unprecedented detail. This coupled with vast amounts of seismic data generated by a monitoring network installed by the Indonesians after the 2004 tsunami, will enable the scientists to construct an elaborate model of the region.
“None of the work we’re doing can prevent earthquakes or even necessarily predict exactly when they will occur,” Dr Cummins says, “But what I think we will be able to do is highlight the worst potential danger spots and make some recommendations about what would be appropriate in terms of building codes for future development.”
The movement of plates along a fault does not necessarily create damage to buildings. There are cases where the fault can slip rapidly but smoothly, shifting buildings sideways by several metres but not actually destroying them. The most damaging effects are caused by violent ground shaking associated with a “jerky” slippage, the waves from which can radiate out many kilometres in every direction depending of the composition of the Earth’s crust in the region.
The distance of the earthquake from population centres has a tremendous influence on the outcome. Large offshore earthquakes occurring at subduction zones – where one plate slides beneath another – are known to geologists as megathrust earthquakes and are by far the most powerful. They almost always involve a sudden shift to the ocean floor that can generate massive tsunamis such as the one that recently hit Japan. Smaller earthquakes that occur onshore, however, are the ones that often generate high frequency shaking that can have devastating effects on buildings in nearby population centres such as Christchurch.
Buildings must be designed to resist each specific hazard that in turn, can depend on the location and local terrain. If the underlying soil is subject to liquefaction – a process in which vibrations of specific frequencies cause the soil to flow like a liquid – piles need to be driven through the soil layer into solid rock below. In other cases, building must be designed to flex and dissipate energy without collapsing.
In the vast majority of cases, people hurt in earthquakes are injured by the buildings they’re in or near, not by the ground movement itself, so the best opportunity to improve the situation lies in improving buildings. “In order to do that in an effective and efficient manner, we first must better understand the underlying science” Dr Cummins says.
The media give the impression that natural catastrophes are becoming increasingly common and increasingly deadly, though in the case of earthquakes, there’s no reason to believe that the frequency is increasing at all. What is happening though is that human population is growing, people are moving to densely populated areas and also being pushed into regions that weren’t previously inhabited. What this does is greatly increase the impact earthquakes have on humans making better understanding them and the measures we can take to reduce casualties all the more important.