The Oregon Surveyor Sept/Oct 2018

Professional Land Surveyors of Oregon | www.plso.org 25 The Big One builds. Eventually that stress can be re- leased rapidly as an earthquake, with the magnitude depending on how large the patch of fault that ruptures is. Geologists have recently been able to de- ploy hundreds of GPS monitors across Cascadia to record the subtle ground deformations that result from the plates’ inability to slide past each other. Just like historic seismicity, plate locking is more common in the northern and southern parts of Cascadia. Geologists are also now able to observe difficult-to-detect seismic rumblings known as tremor. These events occur over the time span of several minutes up to weeks, taking much longer than a typical earth- quake. They don’t cause large ground motions even though they can release significant amounts of energy. Research- ers have only discovered these signals in the last 15 years, but permanent seismic stations have helped build a robust cat- alog of events. Tremor, too, seems to be more concentrated along the northern and southern parts of the fault. What would cause this situation, with the area beneath Oregon relatively less ac- tive by all these measures? To explain we had to look deep, over 100 kilometers be- low the surface, into the Earth’s mantle. Imaging the Earth using distant quakes Physicians use electromagnetic waves to “see” internal structures like bones with- out needing to open up a human patient to view them directly. Geologists image the Earth in much the same way. Instead of X-rays, we use seismic energy radiat- ing out from distant magnitude 6.0-plus earthquakes to help us “see” features we physically just can’t get to. This energy travels like soundwaves through the struc- tures of the Earth. When rock is hotter or partially molten by even a tiny amount, seismic waves slow down. By measur- ing the arrival times of seismic waves, we create 3D images showing how fast or slow the seismic waves travel through specific parts of the Earth. To see these signals, we need records from seismic monitoring stations. More sensors provide better resolution and a clearer image—but gathering more data can be problematic when half the area you’re interested in is underwater. To address this challenge, we were part of a team of scientists that deployed hun- dreds of seismometers on the ocean floor off the western U.S. over the span of four years, starting in 2011. This ex- periment, the Cascadia Initiative, was the first ever to cover an entire tectonic plate with instruments at a spacing of roughly 50 kilometers. What we found are two anomalous re- gions beneath the fault where seismic waves travel slower than expected. These anomalies are large, about 150 kilome- ters in diameter, and show up beneath the northern and southern sections of the fault. Remember, that’s where research- ers have already observed increased activity: the seismicity, locking, and trem- or. Interestingly, the anomalies are not present beneath the central part of the fault, under Oregon, where we see a de- crease in activity. So what exactly are these anomalies? The tectonic plates float on the Earth’s rocky mantle layer. Where the mantle is slowly rising over millions of years, the rock decompresses. Since it’s at such Green dots and blue triangles show locations of seismic monitoring stations. Ocean bottom seismometers waiting to be deployed during the Cascadia Inititive. continues T