Portland Between Scylla and Charybdis

MtHoodOutlookOn my return home from work yesterday I was greeted by the cover of the of this week’s issue of the Presbyterian Outlook with a beautiful shot of Mt. Hood from the south. Not often we get a literal active volcano on the cover of the Outlook.

But it serves as a reminder for those of us going to Portland for the 222nd General Assembly of the Presbyterian Church (U.S.A.) that from a natural hazard point of view Portland lies between Scylla and Charybdis, between a rock and a hard place, or to be geologically specific the Cascades and the Cascadia Subduction Zone.

To be very specific, the geologic hazards are dominated by a chain of active volcanoes on the east and one of the world’s great mega-thrust subduction zones on the west. Think Mt. St. Helens (which is not that far away) and the 2011 Fukushima earthquake. In particular, the Cascadia earthquake potential got a lot of publicity from a New Yorker article almost a year ago.

If you sense a certain amount of interest and enthusiasm on my part it has to do with the fact that these are the types of things I deal with in my day job as a geologist who specializes in seismic hazard analysis.

So what are we looking at? Well, the US Geological Survey has put together a nice little schematic cross-section of the Pacific Northwest that goes right through Portland and Mt. Hood.

subductionBasically, the sea floor is going down under Pacific Northwest and as it goes down the rocks heat up, magma is produced and comes to the surface in the Cascade range. As far as the volcanoes are concerned, they are clearly active. In Mt. Hood’s case it appears that Lewis and Clark missed the last big eruption by a bit over a decade, but reports of smoke and clouds later in that century are considered to be small eruptions from the mountain.

The good news, is that based on the current volcanic hazard assessments for Mt. Hood the mountain is far enough away that the most energetic products of a future eruption – lateral blast, pyroclastic flows, lava flows and lahars – would probably not directly affect Portland. The city would almost certainly get covered in airfall ash however.

And in case you are wondering, Portland is not unique. Here is a diagram of the last 4000 years of volcanic history for the Cascade Range from Wikipedia (and yes, I can say professionally that this chart is pretty good).

Cascade_eruptions_during_the_last_4000_yearsSo that is Scylla – the rock. What about Charybdis? What lurks in the deep blue sea?

The answer is a subduction zone capable of generating great earthquakes and accompanying tsunamis. The zone is long, it is wide and it is shallow – perfect conditions for a giant earthquake of around magnitude 9. We know because, among other reasons, one of these hit in January 1700. The indigenous peoples have legends about the earth shaking and the sea rising and inundating their villages. And while those accounts and geologic evidence give a narrow date range, the exact date of the earthquake on January 26th comes from Japanese records of a surprise tsunami that arrived with no shaking felt on those islands. Overall, there is evidence of seven great earthquakes in the last 3,500 years with a recurrence interval of between 300 and 400 years. And yes, with the last event 316 years ago we have entered that interval.

But what are the chances in a one week interval? Pretty low for both.  Doing a rough calculation including not just the mega-thrust but also the local faults around Portland, I get a probability of about one chance in 10,000 of damaging shaking during GA. And yes, one fault, the East Bank Fault, runs very close to the convention center. But if you want to be prepared, the Portland Bureau of Emergency Management does have their Survival Guide to the Big One online.

As for Mt. Hood – enjoy the great view. While there has been some recent new earthquake activity, it is minor and is not accompanied by other signs of impending volcanic eruption. Any critical activity would come with enough warning for us to get out of town before something big happens.

Now, if you want to use any of this natural activity and hazard as an analogy, metaphor or allegory for what might happen at the meeting well that is left as an exercise for the reader.

And if you need a final assurance that major geologic activity has a low probability of occurrence, you can look for me in Portland right there with you.

Have fun!

Leave a Reply

Your email address will not be published. Required fields are marked *