Last week, a magnitude 4.0 earthquake occurred between Mollala and Silverton, about 35 miles south of Portland. I made a joke on LinkedIn that I was too busy designing a seismic upgrade to notice until I read this article from Temblor the following day:
Increased seismic activity in Oregon highlighted by tremor and a M=4.0 quake
These are some of my takeaways:
- We might be overdue for a moderate earthquake. Quakes with magnitudes in the 5’s have been occurring near the Portland area roughly every 20 or 30 years since we’ve been keeping track of them. The last one, the “Spring Break Quake”, happened in 1993.
- Moderate earthquakes can be damaging; for example, the M5.6 earthquake in 1993 knocked down chimneys and caused part of a brick wall to collapse at Mollalla High School.
- We have the potential in the Portland area for shallow earthquakes that could have magnitudes in the 6’s. These could be quite damaging and even deadly. If the Portland Hills Fault were to shift, for example, It’s possible ground accelerations near the fault in Portland could exceed the accelerations we would experience during a 9.0 subduction zone earthquake (although shaking wouldn’t last as long).
- The likelihood of a damaging local earthquake in the next 50 years is likely lower than the odds of a subduction zone megathrust earthquake occurring (approx. 15-20 percent likelihood of a subduction zone earthquake significantly affecting Portland in the next 50 years), but it’s another scenario to consider.
- “Episodic Tremor” has come alive again recently deep in the Subduction Zone, as discussed in the Temblor article noted above. A reminder for us that things are happening below us…
