Shaking Things Up: A Graduate Student’s Take on the Relationship Between Earthquakes and Soil Conditions

About

Date: 

04/17/2017 (All day)

PI/Speaker: 

Kenneth Hudson

Original report written by Kenneth Hudson. Interview and editing by Bailee Abell.

Kenneth Hudson

“I feel inspired to be working on such an impactful project,” said Kenneth Hudson, a graduate student and Earth Research Institute fellowship recipient. Hudson is researching seismic activity and how earthquakes affect the Earth’s soil. Using data recorded from actual earthquakes, he is seeking clarity and meaning in the seismic movements of the Earth’s crust.

A master’s candidate in the UCSB Earth Science Department, Hudson began his involvement in this project over two years ago as an undergraduate student, when he started interning for his current advisor, Jamie Steidl. “I was drawn to [this project] because I have always been especially interested in the properties of wave propagation, whether it is electromagnetic radiation, acoustic, or seismic,” Hudson said. “I find it fascinating! It is also interesting to me because it is something that helps us better understand the way the ground shakes during large earthquakes, which helps us to design better buildings that won't fall down, ultimately saving lives and reducing economic damage.”

For almost three decades, UCSB has monitored densely instrumented geotechnical array field sites, studying the movements of seismic waves (relating to the vibrations of the earth’s crust). When seismic waves travel into soil and cause strong ground movement, the soil behaves nonlinearly; this means that the shear modulus of the earth’s material decreases from the maximum value observed during weak ground movement. This affects the site by changing the soil response to weaker movement after a large event. In other words, when large earthquakes occur and the ground movements become stronger than normal, future shaking is nearly unpredictable.

Results showing time lag between 4 pairs of vertical accelerometers from WLA against date of event. Both the average (all events and events < 30 Gals) show a statistically significant increase in time lag for a varying number of years after the 2012 swarm for all the pairings.“The scientific community has known about these ‘nonlinear effects’ for a while now, but only from small scale experiments,” Hudson explained. “This study actually uses the real thing—data recorded by instruments from actual earthquakes—to study these effects, producing results that we know match exactly how it actually happens in the real world, because it is in the real world.”

Hudson’s project aims to show what actually happens to wave propagation during large earthquakes that occur on the ground right under our feet, rather than in laboratory simulations. “We cannot accurately predict future shaking during large earthquakes, which is what this study is trying to help with,” Hudson said.

Granular Earth materials—combinations of discrete solid macroscopic particles, such as coal or sand—have been known to exhibit nonlinear responses due to large strain deformation in lab experiments since the 1970s. Recent studies have focused on in situ (or on-site) experiments of nonlinear effects during both induced ground motions and strong ground motion events. In situ studies of the long-term damage to soil observe that some recovery processes may last hours, while others appear to last years. Hudson’s study seeks to further constrain the nonequilibrium state following strong ground motions by observing unique in situ shear wave velocities during small to medium intensity shaking generated by earthquakes at several downhole arrays.

In Hudson’s research process, he wrote thousands of lines of MATLAB, a computer coding language, to analyze and interpret actual earthquake data. “I went about it by working hundreds of hours on my computer in the ERI office writing the code, talking with my advisor and other ERI scientists, and thinking hard about the math and physics of it all,” he said. “It was difficult fine tuning all the codes I wrote to avoid errors and to produce the finished project you see today. Also, trying to come up with interpretations about physical processes that have never really been contemplated before was a little mind boggling at times, but with the help of other ERI personnel, I was able to face all these challenges.”

Hudson found that significant nonlinear soil behavior occurs during real earthquakes; however, the truly meaningful results are found after large earthquakes take place. In contrast to previous assumptions, Hudson found that the soil takes a long time to “regain its strength” after an earthquake. Given this, researchers must take the soil recovery time into account when researching large earthquakes, subsequent aftershocks, and the design of earthquake-safe structures. These results lead Hudson to further develop models that include this observation of long-term recovery. “We need to change our models to better reflect the real world so we can make better predictions about future earthquakes and seismic wave propagation,” he said.

Hudson admits that the results surprised him; he was unaware that soil can remain “damaged” for a long time after a strong earthquake. Reflecting on the experience, “The most rewarding part of the project was the process itself, learning a lot about the background of this type of research, the physics and math behind all the analyses I did, and improving my code writing skills,” he said. “Also getting to write it up and present a couple of posters really helped me learn where my weaknesses in writing and presenting are, so I can improve on those in the future.”

The American Geophysical Union (AGU) selected Hudson as the recipient of the Fall 2016 Outstanding Student Paper Award, following his presentation at their annual meeting attended by geophysicists from all over the world. The award is given to the top 5% of participants in the Outstanding Student Paper Award Program.

“I was able to invest all my time this summer into writing computer code to analyze the data and produce results. I was able to get a lot more in depth and create better results because of all the time I was able to put into it.” Hudson said, attributing those opportunities to the support provided by the ERI fellowship. In addition to presenting at AGU, Hudson shared his work at the Southern California Earthquake Center. “I made meaningful connections with other scientists and was able to get my name out there as a young and rising scientist in this field,” he said.

 

Currently, Hudson is working on another project regarding seismic wave propagation. He is studying an underground hammer source and how the waves it generates from its motion are affected by changes in soil saturation.