Portland lies in a shallow basin with northwest-facing faults where it meets the rugged terrain of the Coast Range. These errors tend to experience both compression and lateral movement due to the combination of tectonic forces. Long term fault movements often leave distinct, linear marks in the landscape, although the amazing moisture and vegetation in this area darkens things out pretty well. Still, some of these flaws can be seen in the landscape once you know what to look like.
These include the Gales Creek fault 35 kilometers west of Portland. A Portland State University team, led by Alison Horst, set out to dig a small trench over this flaw to investigate its history. Since it can be seen in the landscape, it was believed to be an active bug, but little beyond that was known. By digging over the surface fault, the sediment and soil patterns can reveal movement during past earthquakes. Find a few pieces of organic matter there, and carbon dating can even tell you when it happened.
Read between the lines
After digging over the fault, several layers in the ground jumped towards the researchers. Changing colors and sediment content highlighted unusual patterns, such as a top layer of soil buried under the current top soil. This can happen when a fault movement opens up a space that is filled with sediment. One layer also contained a jumble of cobblestones that appear to have been shaken by the earthquake.
Enlarge /. Here is a wall of the trench above the fault with the stratification relationships identified by the researchers.
There were also subtle vertical defects perturbing these layers. And this is where the geological deduction starts. If a fault breaks through one layer but stops at the base of the next layer, that top layer must have been added after the fault event. Using these spatial relationships, the researchers identified three different earthquakes. The carbon dating of lumps of forest charcoal between each of these earthquakes puts them around 1,000 years ago, 4,200 years ago, and 8,800 years ago. On average, there were around 4,000 years between these major earthquakes.
It also appeared that the fault had moved a meter during an earthquake. This all roughly agrees with estimates based on things like horizontal displacement in streams that exceed the error. These estimates assume an average earthquake event of around 3,000 years and a movement of 1.7 meters. If the entire 73-kilometer fault moved during one of these earthquakes, it would have a magnitude of 7.1-7.4. It is possible that in each event only parts of the fault moved, resulting in a smaller size. This could be tested by digging more trenches like this one and looking for differences between them.
The insight here is that this fault is very active and is capable of a magnitude 7 earthquake. And it's not the only flaw in this area. Severe earthquakes can be so rare that the real risk is greater than the events we have witnessed and that we must discover the history and learn from it. For Portland, this includes more than just the big offshore earthquake that everyone is worried about.
Bulletin of the Seismological Society of America, 2020. DOI: 10.1785 / 0120190291 (Via DOIs).