Tag Archives: earthquake

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!

Earthquake in Nepal: Science And Response

It has been a while since I have made some scientific comments about an earthquake so for those of you who are not aware, I am an earthquake geologist and part of my day job is research and public outreach related to earthquakes.

Needless to say the earthquake over the weekend in Nepal got my attention and my response may be a bit surprising – no surprises here.

If you have been following the coverage you know that the most commonly reported magnitude measurement puts it at 7.8 although another slightly different magnitude measurement scale gives is a value of 8.1. For the record those of us in the business don’t spend a lot of time fretting the differences between the scales. Let’s just say that they all measure the event in slightly different ways and each has it’s advantages and disadvantages. The bottom line is that it is a big earthquake.

The most interesting scientific result to me is the finite fault model. This is a method of reconstructing the behavior of the earthquake as the fault breaks and it is interesting to note that the fault started breaking on the west end and broke to the east. In addition, the larger fault offsets were in the eastern portion and both of these circumstances would have increased the damage in the Kathmandu area. In addition, at 15 km deep it was relatively shallow and therefore more destructive. In addition, Kathmandu sits in a basin with soft sediments which would also amplify the shaking for a couple of different reasons. The circumstances of this quake were not in its favor.

You may have caught in the news coverage that there was a slightly larger earthquake in this area back in 1934 so these events do occur on a regular basis. There have been some other smaller earthquakes in the area but this event does overlap with both the 1934 event and the previous substantial event back in A.D. 1255. The Earth Observatory of Singapore has a nice page with a lot of technical information about the event. There is also a set of slides from IRIS (available in a PDF file) that gives a great overview of the event.

The bottom line is that this is a plate boundary where India is colliding with and going under Asia — very large earthquakes are to be expected. The Himalaya are being pushed up and Southeast Asia is getting squeezed out the side. In fact, this event moved Kathmandu about 10 feet south and raised the central Himalaya a little bit and Mt. Everest is most likely a bit taller, but think in terms of an inch, not feet. The majority of the motion is horizontal and this was a pulse in the constant shortening of Asia that is bringing Beijing closer to New Delhi. [Update: My initial uplift calculations were a bit off and Mt. Everest appears to be on the far side of the flexture line and is actually now an inch shorter.]

Aftershocks will continue for a while but with a couple in the magnitude 6 range and a good number of magnitude 5 events everything is looking typical. The big question is triggering and whether the stress redistribution of this event will make another large earthquake more – or less – likely. It would seem that triggering another earthquake is likely but it is best to think on the scale of decades to centuries and not a few days, months or years.

So what Presbyterian news has come out related to this?

First, branches are reporting on the status of their workers in that area: The missionaries from the neighboring Mizoram Synod are reported safe and have declined evacuation and will continue working there. Similarly, the Presbyterian Church in Ireland and the Church of Scotland have reported that their workers are safe and furthermore that all the workers with their partner agency, the United Mission to Nepal, are safe. And the Moderator of the Free Presbyterian Church of Northern Ireland returned from Nepal the day of the earthquake and the church reports that members and workers with the Free Presbyterian Church of Nepal are shaken but safe.

Second, work has kicked into high gear across the Presbyterian family to solicit relief aid for the country. A quick rundown:

I will keep updating that list as I hear of more branches who are reaching out with aid for that country.

As we look ahead prayers for the country are certainly in order. The death toll has passed 5,000 and based on the building styles and the disruption of communication with smaller villages I think the Prime Minister’s estimate of 10,000 dead is unfortunately a real possibility. Thank you for your prayers and however you can help out in this disaster.