Mapping Earthquake Geology
Before diving too deep into our earth, it’s important to first build a foundation of terminology. Two frequently merged concepts are earthquake geology, and seismology. They do come up in the same conversations, but are unique studies. Earthquake geology focuses on the impacts to the surface of our earth. This involves understanding the form and structure of the crust, and how faults and the surface change as a result of repeated earthquakes. Seismology is the study of seismic waves and taking a deeper look inside the earth. Seismologists use seismometers or geophones to measure movement under the surface. These instruments can even detect tornadoes and hurricanes above ground. There are two sources of seismology. Passive sources are natural movement, like from earthquakes, or artificial movement, like from construction or other surface activity. Active source seismology is a result of induced seismic waves. This can be from explosives, air bursts for underwater testing, or the classic combination of some metal plates and a sledgehammer. So why are so many people involved and invested in answering geological questions with GIS and geographic methods? Well, as the saying goes, “ What gets measured gets managed”. Creating a more complete map of the earth’s mineral resources provides a better foundation for planning for future extraction, conservation, and risk management. Knowing areas of higher risk allows communities to plan accordingly, and communicate potential hazards to their citizens. Considering that the very nature of earthquakes and seismology require understanding and documenting change, maps are essential for coming to significant conclusions. Scientists employ a number of geospatial tools, such as GNSS systems and remote sensing, to better document the earth’s makeup. These techniques will often be combined with more traditional geologic methods, like taking ground samples, in order to verify findings and better tune standards and signatures for digital sampling.The Role of GPS in Geology
GNSS technology, whether it takes the form of GPS (USA), GLONASS (Russia), BeiDou (China) or Galileo (EU), has far more uses and applications than we have generally given it credit for. There is of course huge value in knowing exactly where the device is on the earth, but what if we could capture information on the surrounding area as well? Following the standard use of GPS, there are a lot of things that geologists can learn about the planet’s changes. Primarily, they are interested in movement. By taking readings with a device at a fixed location over a long period of time, a technique known as campaign GPS, the scientist can get finely detailed measurements of vertical and horizontal movement. Changes in the device’s location are interpreted as movement. In order to translate movement to meaning, the trained eye of a geologist is needed. Observing two GPS units positioned on the opposite sides of a fault line, they can determine the expected rate of movement of that fault. If the observer notices one year that the movement is outside of the expected range, then that is an indicator there may be an earthquake on the way. Although more is better, if a scientist is working with only one GPS unit, they can still get useful scientific information. A unit placed on a volcano can help give information on if the surface is bulging or shifting, indicating a possible future eruption. Outside of the common application of simply measuring location, GPS units and their signals can be used more creatively to paint a better picture of the landscape. Multipath error is the result of a GPS unit giving or receiving conflicting or reflected signals due to the surrounding environment. Traditionally, multipath error is accounted for and thrown out as just that, error. If a researcher is willing to sift through the noise, however, it is possible to get an estimate of the changes in height of the surrounding environment by categorizing some of the recurring errors. These height differences and changes can be interpreted as snow depth, or growth in vegetation, depending on the study area’s geography.Geology and Remote Sensing
Considering that nowhere on earth can we escape geology, it makes sense to take science to the sky to better understand and measure it. By using satellites, drones, UAVs, and the many sensors and scanners that can be attached to them, geologists are able to make observations on a much larger scale more efficiently than if they were restricted to the ground. Lidar and synthetic aperture radar (SAR) are incredibly useful tools for geology. Lidar is especially useful as it allows for repeatable construction of a digital terrain model (DTM) that exposes the shape of the ground below tree canopies. This is great for seismologically active areas with heavy forest cover, such as the Pacific Northwest of the United States. A detailed 3D representation of the surface can show features associated with faults, allowing for a better geological inventory of the area. Of course, more traditional sensors play a great role in geology as well. The Advanced Spaceborne Thermal Emission and Reflection Radiometer (ASTER) is a fantastic resource for geologists as it hosts visible and near infrared (VNIR), short wave infrared (SWIR), and thermal infrared (TIR) sensors. These infrared bands are useful for studying the mineral makeup of the earth as different minerals will have different spectral signatures. By understanding the mineral content of the surface, it is possible to infer the types of rocks present, as rocks are simply collections of minerals. Once a geologist knows what rocks are present on the surface, this can allow them to make educated assessments of the bedrock of the area. The earth is of course not static, but rather a living, and even breathing force. The more we invest into understanding and mapping all the nuances and features of our planet, the more prepared we will be when it comes time to take action against some of the world’s most pressing challenges.In Conversation
From Acting to Earthquake Geology
Daniel: Wendy, welcome to the podcast. Can you introduce yourself?
Wendy: I’m Wendy, and I actually started off as an actress — my undergraduate degree was in theatre. I took some geology classes because I thought they were interesting, but I moved from the East Coast to Los Angeles to be an actor, not because I wanted to but mostly because I didn’t want a real job. I’d been a professional actress for a few years when the Hector Mine earthquake hit — a magnitude 7.1 out in the desert, widely felt in southern California including my fourth-floor Hollywood apartment. Car alarms went off, the building was creaking, my pet rabbit was screaming. I was fascinated. I went to the USGS the next day and asked if I could volunteer. They said no — turns out after a large earthquake in an urban area they’re a bit busy. I went back later, volunteered, and eventually they hired me into outreach and education for the USGS Earthquake Hazards Program in Pasadena. Eventually I went back to grad school and got my master’s and PhD in earthquake geology.
Earthquake Geology vs Seismology
Daniel: What’s the difference between earthquake geology and seismology?
Wendy: Earthquake geology studies how earthquakes impact the surface of the Earth — how faults change the surface, what repeated earthquakes do, how we can map and understand it. Seismology uses seismic waves coming from earthquakes to look inside the Earth, like a telescope peering inward. They’re looking much deeper than I would. I’m looking right at the crust along the surface. Together we get the whole enchilada.
Daniel: Are seismologists only interested in waves from earthquakes, or can they generate them?
Wendy: Both. Passive source seismology uses waves we don’t create — earthquakes, construction, volcanic eruptions, anything that shakes the surface. Active source seismology is where we put energy into the ground on purpose — sometimes a sledgehammer hitting a plate along a line of geophones, sometimes explosions, sometimes air bursts underwater. Seismometers are amazing instruments — they can detect tornadoes and hurricanes too.
GPS as a Geological Sensor
Daniel: How do you map what’s happening on the surface?
Wendy: At the macro level, GPS instruments. We use them to see how the surface moves through time, vertically and horizontally. The Plate Boundary Observatory is hundreds of GPS instruments across the western US — if one instrument west of a fault and one east of it are moving apart, there’s a fault in the middle accommodating that motion, and eventually there’ll be an earthquake. Same with volcanoes — if a volcano starts to inflate because magma is rising, GPS instruments on it move apart. Think of putting two dots on a partially inflated balloon and watching them spread as it blows up.
Daniel: How dense does the network need to be?
Wendy: Even one instrument gives a lot of information. There’s been subsidence in the San Joaquin Valley from groundwater extraction — one instrument there would still measure subsidence, even the recharge after a big rain filling the aquifer back up. It’s like the Earth breathing. GPS stations also measure snow depth, soil moisture, vegetation growth — using multipath signals that we originally tried to get rid of. Very clever people said “wait, maybe we can learn something from that data we were throwing away.” As the angle of reflection changes, you learn what’s happening in the near-surface environment.
Wendy: I worked on a study in South America installing campaign GPS stations across parts of the Andes — looking at how glacier melt was letting the mountains rebound. Isostatic rebound: the ice is heavy, as it melts the mountains rise. We were trying to disentangle that from the tectonic uplift that created the Andes from the subduction zone.
Daniel: What’s a meaningful amount of movement?
Wendy: Depends on environment. On a volcano, centimetres of change and everybody sits up. The San Andreas is moving about 55 millimetres a year — about 5 centimetres — so we expect that much differential motion between stations on either side.
Mapping Fault Lines from Space with ASTER
Daniel: What about synthetic aperture radar?
Wendy: Earthquakes change the surface, and SAR satellites can map exactly where that deformation happened — vertical, horizontal, however the fault moved. You quickly figure out the geometry of the fault. Those maps help direct geologists in for the on-the-ground mapping, especially where there’s significant cracking or liquefaction. We have the on-the-ground geology and the in-the-sky geology now, which is amazing.
Daniel: Tell me about using ASTER to map faults from spectral signatures.
Wendy: For my PhD I was working on the Karakoram fault zone in northwest India, on the southwestern edge of the Tibetan Plateau. Same length and motion as the San Andreas, but in really remote areas. We were working in Ladakh at over 15,000 feet, K2 was in my field area, geopolitically contested between India, Pakistan, and China. Geologists hadn’t been able to access large parts of the region and the maps were sparse. I was lamenting this over beers with grad school friends and one of them said “wow, we can map the mineralogy of the surface of Mars — why don’t you use ASTER to map the Himalayas?” I went to the Mars Space Flight Facility, used and adapted some of their codes, and mapped the area using ASTER on the Terra satellite.
Wendy: ASTER is the Advanced Spaceborne Thermal Emission and Reflection Radiometer — a multispectral imager. I used the thermal infrared bands which let you map mineralogy. Minerals are the building blocks of rocks, so once you know the mineral combination you can identify the rock. To validate it I collected hand samples — GPS-located precisely — and scanned them in the lab with an instrument that collected the same kind of data. The granite signature I was seeing from space was the granite signature I was holding in my hand. Then I could apply that signature with confidence to areas I couldn’t physically access.
Piercing Points and Strike-Slip Faults
Daniel: How does mineralogy map back to fault lines?
Wendy: The Karakoram is a strike-slip fault — sliding horizontally — so anything crossing it gets displaced. Imagine a road crossing the San Andreas; after an earthquake one side of the road appears to have moved to the right relative to the other. Those are piercing points. They can be roads, fences, basaltic dikes, contacts between rock formations, rivers. By matching offset features across the fault you measure horizontal motion. Then if you can date when those features formed — the dike, the diversion of the river — you get rates of motion over time. Dates equal rates.
LiDAR, Digital Deforestation, and the B4 Project
Daniel: What about LiDAR?
Wendy: Perfect tool for the surface. Tectonics builds the ground up, erosion wears it back down — LiDAR gives you the input you need to understand the landscape’s evolution. It’s 3D — unlike a Google Earth image where you might see a fault next to a cliff but not know the cliff’s height or whether there’s a scarp at the fault, LiDAR gives you exact 3D shape and quantitative measurements.
Wendy: We did the B4 project — a LiDAR scan of the San Andreas before the next big earthquake — from the Salton Sea all the way up through Parkfield. After the next earthquake we’ll be able to go back and see exactly how the ground changed.
Daniel: Tell me about digital deforestation.
Wendy: A LiDAR scanner from a plane (or a drone) sends thousands of laser pulses to the ground. Each pulse returns multiple times — the first return is typically the top of the tree, the second a branch, the third a leaf, and so on. The final return is the ground surface. If you keep only the last return you remove the canopy and see the bare ground. In heavily forested places like Oregon and Washington with active fault lines, this virtual deforestation lets you see the surface and locate faults much more easily.
LiDAR as a Teaching Tool
Daniel: Have you used LiDAR for storytelling and teaching?
Wendy: Yes. People are used to looking at Google Earth, but they get distracted by roads, houses, land use. They’re not thinking about the shape of the ground underneath. A grayscale LiDAR image makes the ground the most obvious thing, so novice learners focus on the landscape and make critical observations: “look at these linear hills,” “look — on one side of this line there are hills, on the other it’s flat.” That’s a fault. We did experiments where we gave people no background and they still figured it out quickly. The exception is craters — depending on the sun angle some people see a crater as a hill, and you have to turn the image to match what their brain expects.
The Job of Science Communication
Daniel: Why did you decide to focus on science communication?
Wendy: Early in my career I saw firsthand how important it was to communicate with people who need to use the science — lawmakers, religious leaders, a mum at home with her kids. Especially in hazards work, I feel there’s a societal obligation for scientists to make sure the science actually gets used. It’s one thing to do the science — until you communicate it, it’s not actually finished. The organisation I work for is primarily funded by the National Science Foundation, so it’s taxpayer dollars paying my salary. It’s my job to tell people why the work matters and how it keeps them safer. Families move to the West Coast from places where earthquakes are unheard of and they’re terrified. Five minutes spent on an email can alleviate real anxiety. That to me is the best part of my job.
Daniel: Should all scientists also be communicators?
Wendy: Yes, but not all in the same way. Maybe you don’t give public talks — maybe you write a blog. Different skills, different comfort levels. Not everyone needs to be on TV.
Daniel: What do you do when the science is inconvenient?
Wendy: Be honest. After a large earthquake you have to tell a community there’ll be more — that’s terrible to be the messenger, but it’s our job. You can deliver it with compassion and empathy. Don’t say “panic, there’ll be more.” Say “this is normal, that was terrifying, more is coming, here’s what to do.” Give people some actionable item so they have agency. The data is data — you have to tell people.
Daniel: Thoughts on climate change communication?
Wendy: Amazing work has been done, but it’s politicised. The path forward needs hope. For so long we’ve been trying to get people to see climate change is happening — most do now. So we can focus on solutions. We’re the species with cars on Mars and a helicopter on Mars. We’ve done impossible things before. Instead of “the end is nigh,” let’s focus on solutions. Effective communication is also about values and belief systems — there’s a lot more social science in communication than people realise. Katharine Hayhoe and others speaking to minds and hearts are the people I’m in awe of.
Daniel: Where can people connect with you?
Wendy: My website is drwendybohon.com, and the organisation I work for is at iris.edu — that’s the Incorporated Research Institutions for Seismology, not the IRS.
