Extremely thick oceanic and continental tectonic plates can slide against each other without producing megathrust earthquakes, which are triggered at subduction zones – where one tectonic plate is pulled beneath another.
But in findings published Monday in the journal Nature Geoscience, researchers show that the raggedness of the sea floor may be to blame for these massive earthquakes. The most recent megathrust quake – a magnitude 9.0 – occurred in Tohoku, Japan, in 2011, and set off a 131-foot-high tsunami, killed 15,000 people and caused an estimated $235 billion in physical damage.
“We’ve shown that the fault zone along plate boundaries may be thicker than we originally thought, which can accommodate the stress caused by the creeping plates,” said lead author Ake Fagereng. “However, when you have an irregularity on the sea floor, such as large bumps or mounds, this can cause the plate boundaries to slip tens of meters and trigger a giant earthquake.”
For generations, scientists have examined subduction zones and the megathrust earthquakes that originate there, along with the presence of underwater volcanoes. However, they failed to account for the lack of megathrust earthquakes in areas expected to produce such quakes – including the “Ring of Fire” in the Pacific Ocean, the most seismically active region on Earth.
Studying rocks exposed to a 180-million-year-old extinct fault line in New Zealand, the team was able to determine that rocks in a fault zone can be tens to hundreds of feet thick and act as a sponge that absorbs the pressure that builds as two tectonic plates slip past each other. The rocks were naturally carried to the Earth’s surface from depths of about 9 to 16 miles.
“By exhuming rocks from this depth, we’ve been able to gain an unprecedented insight into what a fault zone actually looks like,” said Fagereng.
The team believes their findings, along with detailed subsurface maps of the ocean floor, could help to develop accurate models for projecting where large earthquakes are likely to occur, helping potentially affected areas prepare for disasters.