A study published April 19 in the journal Nature Geoscience estimates that the chances of a major earthquake occurring in the Alpine Fault on New Zealand’s South Island within the next 50 years are as high as 75 percent, and likely to be a powerful earthquake of magnitude 8 or greater.
This result is much higher than the previous study that came up with only a 30 percent chance. The new study estimates that the Alpine Fault has an 82 percent chance of a magnitude 8 or greater earthquake.
The Alpine Fracture Zone, which runs for about 600 kilometers between Milford Sound on the west coast of the South Island and Marlborough in the northeast, is one of the most likely threats of natural disaster in New Zealand.
Seismic Gate Mechanism
This study uses an earthquake gate mechanism that exists in nature to produce results that differ significantly from previous studies. Scientists do not yet know exactly how it works, except that some subtle changes in the topography along the fault zone can play a decisive role in the outbreak of earthquakes, such as blocking earthquakes of a few magnitudes or less and allowing earthquakes of a few magnitudes or more to erupt, so it is called an earthquake gate.
Observations of the Alpine Fault Zone show that this mechanism plays a key role in whether earthquakes will develop to magnitude 8 or higher. Some magnitude 7 earthquakes are stopped by this gate, and those that break through the gate and reach magnitude 8 or stronger release more than 32 times the energy of a magnitude 7 earthquake.
One of the researchers, geologist Nicolas Barth of the University of California, Riverside, said, “Seismic gates are like traffic wardens on a one-way street at a construction site. Sometimes you encounter a green ‘let go’ sign, and other times you encounter a red ‘no passing’ sign and have to wait until the situation permits before you can pass.”
This study found that there seems to be some pattern in the sequence of earthquakes that are stopped and rushed through this gate. Study leader Jamie Howarth of the University of Victoria in Australia said understanding the mechanism of the earthquake gate is important for predicting future magnitudes of earthquakes in the Alpine Fault Zone.
The data show that the last three earthquakes in this region have rushed through the gate, reaching magnitudes of 8 or more. “Our model shows that if three consecutive earthquakes have rushed through this gate, the next one is likely to do the same. So we expect the next quake to reach magnitude 8.1 similar to 1717.”
Seismic gates are an important new area of research focused on by seismologists recently, with The Southern California Earthquake Center, an international research group of more than 100 institutions, topping the list.
In addition to data from New Zealand, the group is also looking at the Cajon Pass, Calif. It is likely that the interaction of the San Andreas and San Jacinto faults in this area will also create a mechanism for earthquake gates that will play a decisive role in the next magnitude of the earthquake.
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