Living in the Pacific Northwest, it’s impossible for me not to witness the destruction wrought by the Indian Ocean Earthquake and the more recent Japanese Earthquake and not think about the monster lurking just off my own coast.
Subduction zone earthquakes represent a tremendous threat. They tend to be very powerful, and since all subduction zones are undersea, they almost invariably spawn tremendous tsunamis. Most of the subduction zones are around the Pacific ring of fire, as the continental plates slowly press the Pacific Ocean into a smaller and smaller area.
One of the geologically most violent subduction zone faults is the Cascadia fault, where the diminutive Juan de Fuca plate dives under the North American plate 90 miles off the shore of Oregon and Washington. The geologic record of this fault shows that it ruptures every 300 to 600 years. We know that the last time this fault ruptured was January 26, 1700. East coast Japanese cities record a tsunami on that date, which was not accompanied by an earthquake. Analysis of trees killed by coastal subsidence confirms that the last growing season they had was 1699.
Inhabitants of the Northwest are blithely complacent of this threat for the most part, because the earliest settlements by Europeans was a scant two hundred years ago, when Jacob Astor founded Astoria in 1811.
In the years since the Indian Ocean Mega thrust quake, the Discovery channel has generated some significant ratings by inflating the threat that such a quake poses to the Pacific Northwest. The premise is that the Cascadia fault will generate a 9.0 quake and that the fault will rupture all along its thousand mile length. They then extrapolate what a 9.0 Earthquake would do to the local cities in the Northwest, along what we locals recognize as the I-5 corridor. The video footage was shocking, buildings flattened, people running for their lives, infrastructure destroyed.
Consequently, a lot of angst has built up as residents of Portland and the Seattle metro area all the way to Vancouver BC contemplate their beautiful cities ruined by a 9.0 earthquake. Sadly, none of it has anything to do with reality.
This always seemed to ring kind of hollow to me. I was in the big Philippine earthquake in 1990, when an 7.8 near Cabanatuan cut loose and nearly destroyed Baguio City. I was less than 100 miles from the epicenter, and what we experienced was about a 4.5 to a 5.0 – strong, but no real damage. I’ve said for a long time that the 9.0 off the coast of Oregon would not trash Portland, and I’ve received a lot of derision for my position. I guess people enjoy being scared. So I did some research, and the numbers are on my side.
The formula that relates Earthquake magnitude to ground motion is a =1300*(e0.67*M)*(D+25)-1.6 where a is acceleration in cm/sec2, M is the earthquake magnitude and D is the distance in kilometers. For those of you wanting to follow along with your own spreadsheets, in excel this is:
Where B2 is the cell with Magnitude and C2 is the cell with distance.
We can then convert this acceleration to g units, so we can relate it to the Mercalli earthquake scale. We do this by dividing the acceleration by 980.
Now, if we plot this in magnitude vs distance we get a graph that looks like this:
Note that the Y axis is plotted logarithmically. This makes sense, because the intensity falls off exponentially as a function of distance. I imposed the Mercalli definition points over this. By the Mercalli scale, you don’t start seeing substantial damage until it reaches VII or VIII. If we examine the 9.0 curve, we see that the maximum distance for any sort of substantial damage is about 120 km, which would be well short of the I-5 corridor. At this distance, the effect would be the same as standing at the epicenter of a 5.0 scale earthquake. Note, however, that this chart doesn’t take into consideration geologic conductivity or soil composition. Liquefaction of loose soils may extend the radius of destruction beyond what is described here.
We’re so used to news stories that deal with the damage that can be done by a 6.0 Earthquake. It is kind of scary, but we have to remember that in the news industry, bad news is good news. A 6.0 earthquake plays well when you can get footage of houses collapsed and thrown off their foundations, bridges destroyed, etc. What the news doesn’t show you is that 5 miles away, they put everything back on the shelves of the stores and life continued more or less as usual.
Standing in downtown Portland when the Cascadia fault ruptures will be about like being 10 miles away from the epicenter of a 5.0 Earthquake. It will wake you up, but you won’t have to dodge falling buildings.
The real threat of Cascadia is the tsunami it will generate. That will devastate the Coast, but fortunately there are no metropolitan centers in the extreme danger zone for such an event. There are ways to prepare for disaster on the coast to save lives.