October 25, 2019
I was that kid who loved playing in the dirt. One summer when I was about 10, we had some construction done on our water well, and there was a pile of dirt in our yard from the excavation. That was the coolest pile of dirt. I would make mountains and watch the dirt collapse under its own weight. I would haul the hose over and make flowing torrential rivers and debris flows gush down the mountainside and carve channels. I’d collect rocks from the driveway and place them in the path of the water and watch the stream divert and carve around them. I’d watch as the mud flowed out across the end of the dirt pile and built a delta in the grass. It was a fun pile of dirt.
Where I grew up in rural Pennsylvania, most kids in grade school don’t really get exposed to geology, and so few people know what geology actually is, or that it is useful, or that there are jobs in it. Fortunately, my parents let me play in piles of dirt, and collect rocks from the driveway, and fed my curiosity with science books from the local library. Fortunately, my parents were ok with me going to college to study geology, even though they weren’t really sure themselves if a geology degree could ever get me a job.
As a geology major in college, I liked the “hard rock” geology fields. I liked looking at highly deformed mylonites and gneisses. I liked thinking about the faults that extended the crust or that built giant mountains. I wanted to learn about the tectonic processes that constructed topography. I soon learned these topics fell under the field of structural geology. While I loved my surface processes classes, I thought sediments and soils were boring in comparison to hard rocks. “Qal” was just the stuff scattered on the surface that covered up the important bedrock that recorded crustal deformation. “Qal” was thin and ephemeral. “Qal” was dirt. Geologists looked at rocks.
As I started to do research that applied my structural geology skills to understand the earthquake histories of faults, I began to map my fist active faults. And, not surprisingly, I started by focusing on the bedrock. But soon I began to realize that that layer of “dirt” that covered the active faults was actually kind of really important. The dirt in the sediments and soils adjacent to active faults contained the geologic archive of past earthquakes. They were the strata that were offset during earthquakes. They were the strata that deformed during earthquakes. They were the strata that formed after and because of earthquakes. They were the strata that contained datable materials that recorded the age of past earthquakes. I started to realize how valuable that “dirt” was and how dynamic and fascinating those young sediments were.
Today, you could say I am a “dirt grade” structural geologist – one that combines structure and tectonics with geomorphology, surface processes, and sedimentology, to research active tectonics. This range of disciplines is necessary for research on active faults because you need to be able to recognize and differentiate between various tectonic (seismogenic and non seismogenic), geomorphic, sedimentological, and surficial processes that create and modify surficial deposits. You need to have knowledge of the physical, biological, and climatic processes that drive the evolution, aging, and modification of surficial deposits and soils with time. When studying “pre-historic” earthquakes, you need map and analyze “dirt” if you want to determine: where earthquakes have happened (tectono-geomorphic mapping), when earthquakes happened (dating offset deposits), and how big they were (measuring the magnitude of offset).
In Eastern California, we describe geomorphic surfaces offset by faults and dig soil pits to determine the relative ages of faulted alluvial fan deposits. We can use the slow aging and maturation of geomorphic surfaces and soils to develop a relative stratigraphy that can be used to bracket the timing of past earthquakes that have offset these deposits. When fan unit is first deposited, the surface has rough channel bars and swales and lacks a soil. Over time, physical weathering, bioturbation, cryoturbation, and dust accumulation causes these surfaces to become smoother, and accumulation of organic matter and substrate leaching builds soils that become thicker, darker, and more clay rich with time. If a fault ruptures through these deposits, you can measure the offset of linear features like streams, levees, and channel bars that were displaced to determine the magnitude of paleo earthquakes, and older deposits should have more offset than younger deposits. Techniques like these make up some of the first steps in determining a rupture history for an active fault.
On Vancouver Island, we have been working on the next steps, where we have been excavating trenches across faults to test if they have hosted earthquakes during the past ~15,000 yrs. This work has required working with the operator of a specialized spider backhoe that can maneuver through the forested hillslopes of the island, and excavate trenches across faults that are several tens of meters long. These trenches expose cross sections of the fault that allow us to make detailed maps of the sediments in the trench walls, to determine which stratigraphic units, if any, have been faulted. Faults in Quaternary deposits rarely have colorful gouge or mineralized slickensides, like we’d expect in a “hard rock” fault zone. Instead, they are often marked by subtle changes in sediment composition, texture, color, or soil development, or by alignments of clasts whose long axes are parallel to a fault surface, rather than to a depositional fabric. Detailed mapping of offset units in the trench wall is necessary to determine the relative timing of past slip events, to measure displacement during each event, and to locate datable material to determine the timing of events.
In order to determine ages of past earthquakes, we apply Quaternary geochronologic techniques to deposits exposed in pits or trenches. Yes, we date dirt. We search for detrital organic material that can be dated by 14C – a method that uses the radio-decay of carbon to determine deposit’s age – or for fine grained silt and sand that can be dated by Optically Stimulated Luminescence – a method that essentially dates the last time a deposit was exposed to light, or we collect samples in a depth profile for Cosmogenic Nuclide Dating – a method that can determine how long a surface has been exposed to the cosmic rays that bombard Earth’s surface. We then work with geochemists and geochronologists to help determine and understand the meaning of ages we obtain from faulted deposits.
So, while college-aged me thought dirt was boring, current me spends a lot of time working with “dirt.” And while I still love looking at a beautiful mylonite or highly deformed gneiss, I’m pretty happy that this structural geologist learned to love dirt, because it allowed me to research the earthquake histories of several faults, help contribute to local seismic hazard assessments, and better understand the processes that drive active deformation of the crust.
Christine Regalla, Assistant Professor, Northern Arizona University’s School of Earth & Sustainability