Modelling and sensing smart cities using microearthquakes and traffic

Posted on May 17, 2021

When an earthquake strikes, the Earth vibrates like the string of a guitar. Dr. Hom Nath Gharti knows that the planet is much more complex than a single strand of nylon or steel—a vast array of topographical variation and material complexity. He’s developed a tool that will help decode the underlying music of the Earth.

Dr. Gharti, an assistant professor in the Department of Geological Sciences and Geological Engineering, has been studying the dynamics of seismic wave propagation. His work is helping researchers across the world use earthquakes as tools to map the planet’s interior structures: a geological CT scan.

One of the biggest issues in the field was the inability of mathematical models to account for a certain element of seismic wave propagation: gravitational perturbation. “When an earthquake happens, it changes the gravitational field of the Earth,” said Dr. Gharti. “It is very small yet routinely detected, but to compute that signal numerically is very challenging.”

The inability to do so served as a continuous limitation to the accuracy of seismic modelling, in particular for long periods. As a postdoctoral researcher at Princeton University, Dr. Gharti solved the problem using a new approach called the spectral-infinite-element method. The resultant software allowed researchers to model gravitational perturbations with unprecedented accuracy and efficiency.

The software’s applications extend beyond the lab: for example, better modelling may enable satellites that can detect gravitational perturbation from space to detect earthquakes earlier. “It may only be by a few fractions of a second, but for infrastructure like nuclear power plants, those fractions could be critical,” he said.

The method also opens the door to a whole slew of possible modelling applications in the field of computational geomechanics. “After an earthquake, the Earth rings like a bell for days, and now we can decode those tones,” he said, “We plan to use the method to tackle other problems in planetary mechanics, like glacial rebound or sea-level rise.”

Dr. Gharti and his team decided to release the software as open-source, something they’d done previously with a program called SPECFEM3D, that modelled seismic wave propagation and quasistatic problems. He says the value of releasing something for the public is increased collaboration and that people can come up with creative applications that the developers may never have considered. “SPECFEM3D was primarily intended for seismic wave propagation and released a few years ago, and people are using it for things like draping of clothes, medical imaging, and detecting cracks in machines,” he said.  

Cracking the gravitational field problem represents just one of Dr. Gharti’s many contributions in computational geomechanics. He’s built an entire career out of identifying gaps in the field, and then creating software in order to bridge them.

Dr. Hom Nath Gharti

During his PhD research at the University of Oslo and NORSAR, he delved into the issue of microearthquake detection. Most people are aware of big quakes, like the one that devastated the Japanese coastline in 2011, but they may not know that smaller ones occur unnoticed with incredible frequency. The challenge in detecting these microearthquakes is that the signal they emit is usually drowned out by background noise.

Dr. Gharti was able to formulate an automated method of isolating the phenomenon. The breakthrough is significant for early detection because microearthquakes can serve as bellwethers for more consequential geological events. “If we have real-time monitoring systems, and we see that there are a large number of microearthquakes happening, we know that those may be precursors to landslides or even big earthquakes,” he said.

Improving our systems for understanding and detecting seismic events is of personal importance to Dr. Gharti, whose experience with earthquakes extends far beyond computer simulations.

While completing his undergraduate degree in geotechnical engineering at Tribhuvan University in Nepal, Dr. Gharti was queueing for lunch in the cafeteria when the entire building began to shake. The earthquake turned out to be relatively small, and no one was hurt, but the experience affected him profoundly. “It struck me instantly, I thought, ‘I should study earthquake engineering, so I can help my country and the whole world.”

Dr. Gharti experienced two more quakes, one quite severe, while completing his Master’s degree at the University of Tokyo. It was during this time in Japan in the early 2000s that he realized the impact that rising computational power could have on the field of seismology. “To study how a seismic wave propagates through the earth, you need to divide the earth into several pieces, in the order of millions or billions,” he said, “We need a supercomputer to solve a problem that big.”

Now at Queen’s, Dr. Gharti aims to apply his expertise in computational geomechanics to an emerging field: urban seismology. Normally, seismic data is collected by going into the field and detonating explosives, or by using sensors to record earthquakes, but we may be able to achieve the same result using the existing human infrastructure. “Trains, traffic, some built structures; we want to study if it’s feasible to use those to better understand the subsurface structure,” he said.

“We in the Department of Geological Sciences and Geological Engineering have been so pleased to welcome Dr. Gharti to Queen’s,” said department head Dr. Vicki Remenda. “He possesses expert knowledge in the mathematics behind complex geological problems and has developed software that supports research in a wide variety of applications in the earth sciences, including geodynamics and geotechnical engineering. Dr. Gharti is an excellent addition to our emerging strength in Geodynamics and Geophysics, and our existing strength in Geotechnique.”

Many motorists in Kingston may have noticed an audible humming as they drive over the Lasalle Causeway. That’s the exact type of phenomenon that Dr. Gharti aims to utilize. “By recording the ground vibrations, we could get a clearer picture of the subsurface, and know if there are any potentially unstable areas, like sinkholes,” he said. “We plan to develop some sort of near-real-time monitoring system that could detect these instabilities.”

Dr. Gharti has hired two undergraduate researchers to work on the project this summer, and he’s currently reviewing applications to fill two graduate positions in his lab. He says he’s been taught and mentored by so many amazing teachers since childhood, that he now feels duty bound to pay it forward to his own students.

“As a teacher and a professor, my main goal has always been to inspire the younger generation,” he said.