WHAT IS COMPUTER MODELING?

What is computer modeling?

Of the many problems scientists face, problems of scale are a major hurdle. Here, a “scale” is not something used to measure weight but describes proportions of quantities like size and time. Geologists often deal with immense size and time scales, like entire mountain ranges that formed over millions of years. However, mountains today offer only a snapshot of their history. Like a detective, geologists use clues from rock chemistry, physics, and other sciences to understand how such geologic features formed and how they can affect humanity today.

One tool geologists in the 21^st century use to predict natural processes is computer modeling. These geologist-created models are powerful enough to simulate geologic processes, such as mountain uplift, over entire continents for millions of years.

You may have seen geologic computer models in movies, like the 2015 film San Andreas where seismologists made earthquake prediction models for Los Angeles, CA. For geologists off the big screen, our models rely on data from the real world, like the location of earthquake-causing faults, to set up a virtual scenario. Then, mathematical formulas are applied to the setup to make geologic predictions, like what part of Los Angeles could experience intense shaking in an earthquake. Critics sometimes claim that computer modeling does not characterize the real-world as well as it should. Therefore, computer models are compared with real-world geologic observations to provide scientists a more complete picture of how Earth’s processes might work.

How do computer models help the BCZN?

The BCZN uses computer models to learn more about how bedrock weathers, or breaks down, into soil. One model predicts how forces within the Earth’s shallow surface (0 – 50 m, or 0-164 feet-deep) are affected by topography (the form of a landscape including ridges and valleys). This is called topographic stress. This model is powerful because it can simulate topographic stresses over large areas and represent forces acting over thousands to hundreds of thousands of years. Such stresses cause two types of weathering: physical weathering by fracturing rocks, and chemical weathering, which occurs when water flows through open fractures and chemically reacts with rocks to form weaker material. In 2017, UCLA critical zone scientist Dr. Seulgi Moon and colleagues used a three-dimensional computer model to simulate topographic stresses under synthetic ridges (Fig. 1A) and real critical zone research sites in Colorado and South Carolina (Moon et al., 2017). One of their goals was to see how predictions of critical zone structure can change under different topography and regional forces, like mountain uplift that can put pressure on nearby landscapes. Ultimately, Dr. Moon and her coauthors show how topography can play an essential role in rock weathering. Using these models, the researchers claim we can learn more about landslides, groundwater flow, and other important critical zone processes relevant to everyday life.

The BCZN will use topographic stress models to predict where bedrock fractures in the critical zone may exist. But it is vital to use other methods of probing the critical zone (like boreholes and seismic surveys) to compare results between modeling and the real world. Where the model agrees with geologic data, we can be more confident that simulations are working. However, where the model disagrees with our data, we open new questions that science can answer.

Justin Higa

graduate student, UCLA

Image

Weathered bedrock at a critical zone site with fractures that may have formed due to topographic stress effects

Works Cited

Moon, S., Perron, J. T., Martel, S. J., Holbrook, W. S., & St. Clair, J. (2017). A model of three-dimensional topographic stresses with implications for bedrock fractures, surface processes, and landscape evolution: Three-dimensional topographic stress. Journal of Geophysical Research: Earth Surface, 122, 823–846. https://doi.org/10.1002/2016JF004155