It’s important to map the underground structure. Why? Along with a few other reasons, scientists use it to map earthquake hazards. Using this information, builders can protect their work from damage from earthquake in areas with high risk of seismic activity.
The problem has been that old technology can make it difficult to map the structure quickly and accurately. New discoveries are paving the way for easier and faster mapping.
Previous Methods of Mapping Underground Structure
Before 2003, scientists would use seismic activity to map underground structure. When an earthquake happens, scientists would measure and monitor the P waves and S waves that move in the interior of the earth and the Love and Rayleigh waves that move on the earth’s surface.
The speed that these waves travel and the way they scatter tells us what types of soil and other geological structures they are moving through. The waves might move faster through one type of soil, or they might scatter more with a different soil structure.
The problem with this is that scientists have to wait for an earthquake. This can be slow and unpredictable. Scientists can create their own explosions to create the waves needed for measurement, but this comes with its own problems like cost and danger.
In 2003, scientists discovered that they could use normal ambient noise to map underground structure. Called the Ambient Noise Cross Correlation Technique, this freed researchers from the need for an earthquake. Using this, researchers can now use the sound of the ocean or human activity to measure the underground structure.
Improving Accuracy Using Leaky Mode Seismic Waves
A recent study published in Geophysical Research Letters, looked at using another form of seismic waves to measure underground structure.
Called leaky modes, these waves are ones that result from a normal mode seismic wave that is stuck between two layers of rock. As the wave bounces between the two layers, energy escapes and these weaker waves produced are leaky modes.
How does this new technique benefit us? Normal seismic waves are more sensitive to the S wave velocity structure underground. Leaky modes are more sensitive to P wave velocity structure, allowing the researchers to develop a more accurate and precise map.
This allows builders and planners to be more confident in the seismic hazard of a potential construction project.
Mapping Using Fiber Optic Cables
In a previous article, we wrote about scientists who are using fiber optic cables to measure seismic activity. Turns out, those same cables can also be used to map underground structure.
In a study published in Geophysical Research Letters in December 2021, researchers have used distributed acoustic sensing (DAS) to create a high-resolution image of near-surface structure. The resolution of this image is 2 orders of magnitude higher than previous ones.
Using an array of fiber optic cable, DAS measures how light pulses sent along the cables scatter and correlates this to seismic movement. Since its discovery, DAS has been used in many different applications.
In areas where the fiber optic cables moved more as the result of seismic activity, shear velocity of the soil was lower. Using this, researchers were able to create a very fine-scale map that can be used to “significantly improve urban seismic risk management.”