To the west of Portland we see Dark Gray which is Basalt (if you've ever driven through the tunnel and up and over the west hills to the western suburbs of Portland, you have driven through the basalt region) and then there is a big light yellow oval that is labelled Tualatin Basin. What is the light yellow according to the legend? It is surface deposits. Long ago this was a basalt bowl. Then a giant flood came down the Columbia river gorge and dumped hundreds of feet of clay silt into the bowl. When you are standing in Beaverton, Hillsboro, Bethany, etc. you are standing hundreds of feet above the original valley floor.
Now look at the map again. The basin is pretty much all to the west of I-5. If you live in the basin you are screwed when the big earthquake hits (the one exception is that hill of basalt rising like an island in the middle of the basin - that is Cooper Mountain, it's no coincidence that the houses on top of Cooper Mountain are where the rich people live). What is going to happen is the earthquake waves will come in and bounce off the basalt on one side of the bowl and reverberate back to the basalt on the other side. The waves will go back and forth in the bowl and liquefy all of that old clay silt. Just like this:
https://www.usgs.gov/news/volcanoes-vineyards-new-geologic-m...
For instance, look at page 3 of the pamphlet:
https://pubs.usgs.gov/sim/3443/sim3443_pamphlet.pdf
To the west of Portland we see Dark Gray which is Basalt (if you've ever driven through the tunnel and up and over the west hills to the western suburbs of Portland, you have driven through the basalt region) and then there is a big light yellow oval that is labelled Tualatin Basin. What is the light yellow according to the legend? It is surface deposits. Long ago this was a basalt bowl. Then a giant flood came down the Columbia river gorge and dumped hundreds of feet of clay silt into the bowl. When you are standing in Beaverton, Hillsboro, Bethany, etc. you are standing hundreds of feet above the original valley floor.
Now look at the map again. The basin is pretty much all to the west of I-5. If you live in the basin you are screwed when the big earthquake hits (the one exception is that hill of basalt rising like an island in the middle of the basin - that is Cooper Mountain, it's no coincidence that the houses on top of Cooper Mountain are where the rich people live). What is going to happen is the earthquake waves will come in and bounce off the basalt on one side of the bowl and reverberate back to the basalt on the other side. The waves will go back and forth in the bowl and liquefy all of that old clay silt. Just like this:
https://www.youtube.com/watch?v=3NSXmTfASM0