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How deep could the next Cascadia earthquake rupture be?
How deep could the next Cascadia earthquake rupture be?
How large, deep, and damaging a future earthquake will be depends on factors such as energy release that must be constrained by precise observations of previous earthquakes in the same area. But such data are rarely available. Instead, computer models of earthquakes guided by the laws of physics can provide us with estimates of potential ground shaking for a future event.
In the second fully dynamic earthquake study of its kind, I designed two‐dimensional earthquake simulations for the Cascadia fault below the northwestern United States coast and test different hypotheses for how stress may be accumulating at depth along this fault. Our models focus on a portion of the fault referred to as the “gap.” The gap physically separates a shallow region that slips during large earthquakes from a deeper region that experiences intermittent slip between large earthquakes. A gap region similar to that in Cascadia is also found in Japan, Mexico, and around other active faults worldwide.
I found that ruptures were able to extend to deeper regions at faster speeds (supershear rupture) given the current understanding of stress levels and earthquake fault friction in the gap.
While this work represents only a first step toward understanding how stresses and friction influence how the Cascadia fault might slip, it lays the foundation for modeling more complex physics that can help scientists better predict shaking from seismic waves.
What might geodetic data predict for big earthquakes in Cascadia?
What might geodetic data predict for big earthquakes in Cascadia?
Given the lack of seismic recordings of megathrust earthquakes, and the uncertainty regarding the extent to which the offshore portion of the Cascadia megathrust is truly locked, how can we generate physically reasonable models to predict upper plate motion during a future earthquake? How does the creeping region of the megathrust offshore Oregon influence margin-wide (magnitude > 8.5) rupture?
I am exploring these questions by generating heterogeneous stress distributions informed by geodetic coupling models to represent a range of physically-motivated earthquake scenarios.
Major takeaways from this study are 1) how sensitive the stress drop amplitude in the creeping portion of the megathrust fault is to controlling large ruptures; 2) paleoseismic data from the last rupture (i.e., 1700 A.D. ) most likely can't uniquely constrain the downdip rupture limit; and 3) better resolution is needed offshore to improve tsunami warning and model predictions! This study is the first fully dynamic and 3-D earthquake model of the Cascadia subduction zone.
The above figure is one such 3-D model where the coseismic uplift and subsidence are plotted assuming a particular shear stress distribution that reflects a scenario where the shallow region of the fault is not accumulating strain through time (creeping, low stress-drop). Rupture is spontaneously initiated in the northern Cascadia region. While the goal is not to exclusively reproduce subsidence measurements from 1700 A.D., they are plotted for reference.
3-D perspective of the wavefield and fault slip-rate during an earthquake initiated in northern Cascadia. I generate physically consistent rupture models using the open-source software SeisSol.
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