According to new research undertaken by researchers at The University of New Mexico and the Nanyang Technological University and published today in Nature Geoscience, two of nature’s most devastating phenomena, earthquakes and tsunamis, may pose a greater threat than previously thought.
The risk may have been systematically underestimated in some areas, the researchers discovered after developing a new method to assess earthquake and tsunami hazards represented by the farthest portion of offshore subduction zones. Given the new findings, it is recommended that tsunami risk assessments be updated.
In the event of future earthquakes and tsunamis, the findings have significant ramifications for mitigating risk in impacted regions around the world, particularly Southeast Asia and the Pacific Rim.
There are additional significant aspects to take into account besides earthquake magnitude, which is one component that influences the creation of tsunamis. It must be a shallow maritime earthquake that moves the seafloor. Although modest tsunamis have occasionally been produced by major (i.e., > M8) strike-slip earthquakes, thrust earthquakes (as opposed to strike slip) are significantly more likely to cause them.
When two tectonic plates collide and one of them slides beneath the other to form a subduction zone, an earthquake known as a megathrust is among the most powerful earthquakes ever recorded. The plates are always moving toward one another, but if the fault or interface between them is stopped, a slip deficit develops over time.
Like a debt, this slip deficit must finally be repaid, and earthquake day is pay day for tectonic plates. These earthquakes have the ability to move the seafloor higher and generate deadly tsunamis when they strike the shallowest portion of the fault closest to the ocean floor.
We applied this technique to the Cascadia and Japan subduction zones and found that wherever deeper locked patches are present, the shallow fault must also have a high slip deficit regardless of its own frictional properties. If these areas can slip seismically, global tsunami hazard could be higher than currently recognized. Our method identifies critical locations where seafloor observations could yield information about frictional properties of these faults in order to better understand their slip behavior.
Eric Lindsey
For geoscientists projecting seismic and tsunami inundation dangers, it is crucial to understand the probable rupture behavior of megathrusts, particularly in the shallow offshore region of the fault where the majority of damaging tsunamis are generated. Based on laboratory analyses of recovered fault zone material, it is frequently thought that the possibility of seismic behavior in the shallow region of the fault is rather low.
A model that connects how slip on the fault impacts the movement of these stations can be used to assess the rate of slip deficit buildup on the fault using geodetic measurements that monitor how the earth’s surface moves over time, such as using highly accurate GPS sensors deployed on land.
However, because it is far from land and below kilometers of water, where conventional GPS devices cannot operate, it is difficult for scientists to utilize this technology to “see” what is happening in the shallowest area of the fault.
Researchers from Nanyang Technological University (NTU) in Singapore and The University of Novel Mexico have now created a new geodetic method for estimating this value that takes the interplay between various fault components into consideration and produces a far more physically accurate answer.
The shallow part of the fault is also immobile if the deep portion of the fault is stuck between earthquakes because it is in a region known as a “stress shadow” and cannot slip due to a lack of energy buildup, according to Lindsey’s team. This fact has not been taken into account by previous models, they noted.
The team created a method using the same land-based data that greatly improves their ability to “see” the fault slip in the areas that are farthest from the shore. By taking this effect into account, this method allowed the team to reevaluate the threat posed by the offshore portions of subduction zones that are most likely to generate tsunamis.
“We applied this technique to the Cascadia and Japan subduction zones and found that wherever deeper locked patches are present, the shallow fault must also have a high slip deficit regardless of its own frictional properties,” said Eric Lindsey, an assistant professor in the UNM Department of Earth and Planetary Sciences who conducted the research while at the Earth Observatory of Singapore at NTU.
“If these areas can slip seismically, global tsunami hazard could be higher than currently recognized. Our method identifies critical locations where seafloor observations could yield information about frictional properties of these faults in order to better understand their slip behavior.”
This study is significant because it calls for a reevaluation of earlier tsunami hazard models on megathrusts around the world. The evaluation can be completed rather fast because it can be done using existing data. Hopefully, this will improve coastal communities’ readiness for upcoming disasters.
