Q&A: How ‘slow slip’ earthquakes may be driven by deep hydraulic fracturing
The Cascadia Subduction Zone is a massive geologic fault that last ruptured in January 1700.
But while this fault has stayed quiet for centuries, it regularly generates small tremors that accompany gradual, nondisruptive movement along the fault.
The tiny tremor events and slow slippage are known collectively as “episodic tremor and slip.” Seismic waves associated with these tremor events are recorded and tracked by the UW’s Pacific Northwest Seismic Network. Other groups track the associated slow motion of the plates using GPS measurements. These paired types of events occur regularly and seem to fluctuate with tidal cycles, but they originate deep underground and their cause has been mysterious.
A pair of papers published Jan. 29 provides new confirmation of speculations about a cause of these events. Taken together, the papers show that fluids deep underground create fractures in the rock, and that this creates rumblings that match what we observe at the surface.
Marine Denolle, an assistant professor of Earth and space sciences at the UW, and Joan Gomberg, an affiliate associate professor at the UW who is based with the U.S. Geological Survey, are co-authors on the AGU Advances paper about the experimental findings.
“The life and death of cracks” – Harvard University press release, Jan. 29, 2024
Denolle began advising lead author Congcong Yuan as a faculty member at Harvard before joining the UW faculty in 2022. Denolle and Gomberg sat down with UW News to answer some questions about the study, and what it means for the Cascadia region.
UW News: What are slow slip earthquakes? And how are slow slip events related to larger, more damaging earthquakes?
Joan Gomberg: When cracks form really quickly, that’s an earthquake. When the rock beneath the surface breaks and moves really fast, and that sends out these big loud vibrations that travel as waves and can knock down buildings. So you care about that a lot.
But sometimes the same thing happens really slowly. So slowly that it doesn’t send out big waves. And it makes these little, tiny little rumblings and shaking, but nothing gets knocked down.
Marine Denolle: The slow earthquakes in Cascadia are a bit more predictable and tractable than large earthquakes. Slow events are accompanied by the “pops” that we detect as tremor at the surface every 12 to 15 months, so they are semi-periodic. These tremor signals, or the “pops” are tracked by the Pacific Northwest Seismic Network — not the larger-scale, slow, absolute fault displacement.
JG: These pops, even though they don’t hurt anything, are really just telling you that something is happening. They’re telltale evidence that yes, something is moving, something is going on. In seismology we call it a passive marker. It’s just a little something chattering, saying: ‘Hi, I’m here, I’m moving!’
Can you describe this experiment that forms the basis of the new AGU Advances paper?
MD: At Harvard we had the apparatus to 3-D print materials, inject high pressure fluid in it, and have a high-speed camera to observe how hydrofracturing cracks the material. What we wanted to do is listen to the sound of the fracture and find the source of the acoustic emissions — sound waves — or vibrations, to map the geometrical expansion of the fracture. We can’t see through rocks, so we wanted to make this experiment with a transparent sample, where we have the ground-truth between acoustic emissions and visualization of the fracture growth.
Because we could see through the sample this slow-growing fracture that has all these pops, we realized that this looks like what we’d see in nature for slow-slip earthquakes, except that we had to invoke the use of fluids to drive the fracture.
Our results show a potential model for slow earthquakes. They are related to the faster earthquakes, in the sense that they relieve stress and they may load stress nearby for future earthquakes. Understanding the behavior at all scales and at all speeds is part of understanding earthquakes that eventually will matter for damaging ground motion.
Your research found that hydraulically driven fractures are causing the seismic signals we observe at the surface. Deep in the Earth, where is the fluid coming from?
JG: Most rocks are in solid form but they have H2O bound up inside of them. It’s not fluid, but it’s got hydrogen and oxygen, and under certain conditions deep in the Earth, when the temperatures and pressures get large enough, that actually does get released. It isn’t melting. It’s just with sufficient pressure and temperature the water is released from of the minerals.
Two papers are being published at the same time. Can you explain how they relate to one another, and to the seismology we observe in nature?
MD: The Nature Physics paper by our colleagues is about the mechanics of these fractures. And our paper in AGU Advances is about how can we provide an analogy, or a model, to the natural world. How can this mechanistic model provide an explanation for the observations that we see in the Earth?
One similarity we observed between the experimental and natural systems was how much seismic energy was released for events of different size. And the other one was the intermittency of the fracture’s growth. When there is a little bit of viscosity in the fluid, the fracture sticks for a while and then pops a little bit before it progresses. Sticks for a while, then pops. And these irregular pops are what has been observed in the natural system.
The evolution of the rupture, the slow-moving, fracturing in the lab was as intermittent and as irregular as what we would see in nature. So it looks like the overall evolution style was similar.
JG: This result shows that it’s all about the role of water — fracturing rock and squishing water into it. If you look at these rocks, it’s very clear that they’re full of veins. Many times there’s a black rock but it has all these white squiggly lines through it that very likely formed as fluids squirted into opening fractures.
This study connects those fluid-filled veins to the observed seismicity. They always say that invoking fluids is a geophysicist’s last resort: If you don’t know how to explain something, say, ‘Oh, it must have been the fluids.’ And so I was skeptical. But this makes me a bigger believer in fluids.
Why use a 3-D printed model for a process that occurs in the Earth?
JG: In the Earth you can measure the seismic waves at the surface, even though they’re being generated way down. But you can never see the crack. These processes occur many, many miles beneath the surface, and you’ll just never see them. The only time you actually see this environment is that sometimes these rocks that are buried miles down just naturally rise up to the surface over millions of years. This is what the geologists call “exhumed.” And you can see they have all these little fractures in them that have been filled with water and minerals and other stuff. But it’s long after the fact — you didn’t see them form, you only see the aftermath.
The experiment in the lab was a way to try to simulate this, so that you could see everything as it was happening. This allowed us to see the cracks form and connect that to the seismic signals we detect at the surface.
What do you think is most exciting about this result?
MD: It’s exciting to see a new demonstration of how the tectonic tremors we observe at the surface could be deep hydraulic fractures in which cracks form and open due to pressurized fluids. As geophysicists, we just assume that the tectonic movements are shear, or sliding motion between two solid objects. But we show experimentally that hydraulic fracture is consistent with the geological record.