By Noel Bartlow, Ph.D., Assistant Researcher, Berkeley Seismology Laboratory
The Cascadia Subduction Zone has occasional large earthquakes and frequent slow-slip events. A new study quantifies how these slow-slip events accommodate tectonic plate motion.
Citation: Bartlow, Noel (2020), Faults slip slowly in Cascadia. Temblor, http://doi.org/10.32858/temblor.077
Subduction zones such as the one beneath the U.S. Pacific Northwest and British Columbia are capable of generating very large and destructive earthquakes. But not all of the tectonic motion accommodated in these areas causes earthquakes that can be felt. Episodic tremor and slip, a type of aseismic fault slip or slow slip, accounts for a large amount of fault motion on the deeper extent of the Cascadia Subduction Zone. A new study reveals what this means for future large earthquakes in the region.
A quiet Cascadia comes to life
Just off the Pacific Northwest coast, the Juan de Fuca Plate collides with the North American Plate. Here, the Juan de Fuca Plate slides beneath North America, forming the Cascadia Subduction Zone. The contact between these two plates, called the plate interface, is stuck due to friction. Slip on the plate interface is necessary to accommodate the collision of the two plates. The Cascadia Subduction Zone plate interface slips every few hundred years in very large earthquakes with magnitudes approaching or even above 9.0. These earthquakes generate dangerous tsunamis similar to the 2011 magnitude-9.0 Tohoku-Oki earthquake and accompanying tsunami in Japan. The last such event in the Cascadia region occurred more than 320 years ago on January 26, 1700.
In between large earthquakes, the plate interface is not silent. Instead it chatters to life every few months with episodic tremor and slip events. In these events, the plate interface slips as it would in an earthquake but takes much longer to do so, releasing the same energy as an earthquake with a magnitude of up to 6.8 over a period of a few days to weeks. These events do not produce dangerous shaking, but they do contain information about how the subduction zone is behaving. An episodic tremor and slip event differs from other slow-slip events in that episodic tremor and slip events recur frequently and are also accompanied by numerous tiny earthquakes called tremor, which are too small for humans to feel.
In a new study published in the journal Geophysical Research Letters, I reveal how much plate motion is accommodated by these events in the Cascadia Subduction Zone.
Using GPS satellites to observe plate motion
Knowing where and how much slip occurs during these events helps scientists understand how they may influence the location and timing of a future large earthquake.
Measuring slip on a plate interface is not as easy as pulling out a yard stick, however.
The plate interface lies below Earth’s surface, so to find out how much slip occurs during these events, we needed to look at what we can see—the ground beneath our feet. This is where satellites come in.
In this study, I used satellite GPS observations to determine how much ground motion occurs during each event. I then calculated how much slip had to occur along the plate interface at depth to account for that ground motion. I tallied up ground motion during these events to find the cumulative effect of all episodic tremor and slip events across the Cascadia Subduction Zone over the last 15-25 years, averaged over time. Applying this systematic approach across the region revealed that not all parts of the subduction zone are behaving the same.
A highly variable system
GPS observations and other data collected over the past few decades show that the Juan de Fuca and North American plates are moving toward each other at 40 millimeters per year in the northern part of the subduction zone near Seattle, and 31 millimeters per year in the southern part near Cape Mendocino, CA. The rate of motion between the two plates defines the “slip budget”, or the total amount of slip that must be accommodated everywhere on the plate interface. I compare this total to the amount released in episodic tremor and slip to understand its role in the overall accommodation of slip on the plate interface.
Episodic tremor and slip events accommodate a highly variable amount of slip along the length of the subduction zone. This has implications for how stress is distributed along the plate interface, and thus where future large earthquakes may nucleate.
In some areas, the slow-slip events account for all of the measured plate convergence. In the very southern part of the subduction zone, slow slip actually releases more slip than the expected convergence rate of the two plates. This might mean that the plates are moving together faster than previous estimates in this region. In other areas of the interface, the slow-slip events accommodate only a fraction of the convergence motion of the two plates—one-fourth or less of the motion in some places. This means that a lot of the motion between the two plates must be released in another way, most likely as steady creep of the plates past one another but potentially also in future earthquakes.
Identifying regions at risk
Previous work on the plate interface in this region revealed the locations where the plate is locked—that is, where friction prevents slip between the two plates (Schmalzle et al., 2014). Large earthquakes occur in these locked sections when the lock is abruptly broken. My results show that slow slip generally occurs in a region offset from the locked section of the plate interface. This means that at present, these events are less likely to trigger large earthquakes than if they were located right at the edge of the locked zone.
The main region of episodic tremor and slip in Cascadia is in an area with no locking. This means that the full slip budget is accommodated by episodic tremor and slip. In the majority of the subduction zone where episodic tremor and slip takes up less than the full slip budget, the plate interface is creeping along at a slower rate between these events.
It is possible that over time the episodic tremor and slip events will migrate closer to the locked zone over time. If this were to occur, it may indicate that the next big earthquake is on the horizon. It is also possible that slow-slip events will become larger or more frequent when a large earthquake is imminent. It is therefore important to monitor episodic tremor and slip in Cascadia over time. The method we applied here can be used to detect these changes and therefore remains an important tool in earthquake hazard monitoring.
Further reading
Bartlow, N. M. (2020). A long‐term view of Episodic Tremor and Slip in Cascadia. Geophysical Research Letters, 47, e2019GL085303. https://doi.org/10.1029/2019GL085303
Schmalzle, G. M., McCaffrey, R., & Creager, K. C. (2014). Central Cascadia subduction zone creep. Geochemistry, Geophysics, Geosystems, 15(4), 1515-1532.
