We prepare—or don’t—based on our perceived risk.
Knowing how many PNW megathrust earthquakes have occurred in the past 10,000 years helps scientists analyze the frequency and probability of future occurrences. Communities often base their preparedness efforts around those probability numbers, so how many have there been since the end of the last ice age? Depending on the source, it can be confusing.
Starting at 40
A 2015 Oregonian article listed dates and details for 40 events going back 9,882 years (as of 2015) to event T18.
Getting to 41
Event T19 is estimated to have occurred 10,271 years ago (as of 2022) and is included in the 41 events shown in the 2012 research paper, Turbidite Event History—Methods and Implications for Holocene Paleoseismicity of the Cascadia Subduction Zone (pg 14/184)1.
Getting to 43
In 2016, OSU issued a press release regarding the addition of two events, T10b & T10f, bringing the total to 43 events. This change is explained in section 4.4.1 Holocene Turbidite Count of the research paper, The importance of site selection, sediment supply, and hydrodynamics: A case study of submarine paleoseismology on the northern Cascadia margin, Washington USA2.
Getting to 46
T2c, T9c, and T10e are the final events leading to all 46 shown in the images below1. Those final three earthquakes are currently undergoing further research and evaluation and have not yet been dated. OSU Professor Goldfinger discusses the 46 earthquakes in the new Netflix documentary, Earthstorm, episode 3.
As an aside, the documentary Mega Disaster: It’s Happened Before, It Will Happen Again, has two episodes (#2 & #4) that also discuss the CSZ.
Note* All 46 are used in the Log-Normal calculation leading to a “37% chance in 50 years” probability.
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1) Goldfinger, C., Nelson, C.H., Morey, A., Johnson, J.E., Gutierrez-Pastor, J., Eriksson, A.T., Karabanov, E., Patton, J., Gracia, E., Enkin, R., Dallimore, A., Dunhill, G., Vallier, T., 2012a. Turbidite Event History: Methods and Implications for Holocene Paleoseismicity of the Cascadia Subduction Zone. USGS Professional Paper 1661-F (184 pp).
2) Chris Goldfinger, Steve Galer, Jeffrey Beeson, Tark Hamilton, Bran Black, Chris Romsos, Jason Patton, C. Hans Nelson, Rachel Hausmann, Ann Morey, The importance of site selection, sediment supply, and hydrodynamics: A case study of submarine paleoseismology on the northern Cascadia margin, Washington USA, Marine Geology, Volume 384, 2017, Pages 4-46, ISSN 0025-3227, https://doi.org/10.1016/j.margeo.2016.06.008.