The Oregon Surveyor Sept/Oct 2018

26 The Oregon Surveyor | Vol. 41, No. 5 The Big One high temperatures, nearly 1500 degrees Celsius at 100 km depth, it can melt ever so slightly. These physical changes cause the anoma- lous regions to bemore buoyant—melted hot rock is less dense than solid cooler rock. It’s this buoyancy that we believe is affecting how the fault above behaves. The hot, partially molten region pushes upwards on what’s above, similar to how a helium balloon might rise up against a sheet draped over it. We believe this in- creases the forces between the two plates, causing them to be more strongly cou- pled and thus more fully locked. A general prediction for where, but not when Our results provide new insights into how this subduction zone, and possibly oth- ers, behaves over geologic time frames of millions of years. Unfortunately our results can’t predict when the next large Cascadia megathrust earthquake will oc- cur. This will require more research and dense active monitoring of the subduc- tion zone, both onshore and offshore, using seismic and GPS-like stations to capture short-term phenomena. Our work does suggest that a large event ismore likely to start in either the northern or southern sections of the fault, where the plates are more fully locked, and gives a possible reason for why that may be the case. It remains important for the public and policymakers to stay informed about the potential risk involved in cohabiting with a subduction zone fault and to sup- port programs such as Earthquake Early Warning that seek to expand our mon- itoring capabilities and mitigate loss in the event of a large rupture.  x Regions where seismic waves moved more slowly, on average, are redder, while the areas where they moved more quickly are bluer. The slower anomalous areas 150 km beneath the Earth’s surface corresponded to where the colliding plates are more locked and where tremor is more common. continued T