Earthquakes in Oregon: The Dog That Hasn’t Barked?

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Oreon hasn't experienced much in the way of recent earthquake activity - this map shows tremors 1973-2012. Image by USGS.

Oregon hasn’t experienced much in the way of recent earthquake activity – this map shows tremors 1973-2012. Image by USGS.

Our tour of the earthquake history and potential of the US began with the challenge of an almost earthquake-less Florida. There’s no rhyme or reason to the order of our continent-hopping (other than a vague intent to keep it interesting) so next up is an altogether different proposition — the Pacific coastal state of Oregon.

Like Florida, Oregon can lie very quiet indeed (the largest tremor in the seven days prior to publication of this article was a tiddler of just M2.6, an order of magnitude smaller than a tremor that made news headlines in the UK the same week) and even that was no ‘proper’ earthquake but caused by an explosion (probably, though not confirmable on the available information, a mining incident).

And, like the Sunshine State, a map of seismicity for Oregon since 1973 shows nothing significant.

Tectonic Setting: The Cascadia Subduction Zone

It’s probably fair to say that Oregon is better known for its volcanoes than its earthquakes, with 14 big beasts listed in the Smithsonian Institution/National Museum of Natural History’s Global Volcanism Program map, including the notorious Mount St Helens. But the presence of this north-south trending line of volcanoes is itself testimony to the existence of a tectonic boundary — and such boundaries are the sources of many earthquakes worldwide.

Mount Hood, Cascadia.

Mount Hood is one of several volcanoes associated with the Cascadia subduction zone. Image by Howcheng.

The margin of two tectonic plates runs offshore along western North America from northern California to Vancouver Island. The continental North American plate to the east is converging with the oceanic Juan de Fuca plate, a remnant of the larger, mainly-subducted Farallon plate.

The margin between the two is known as the Cascadia subduction zone, and is unusual for a number of reasons. The angle of subduction is relatively low (because the crust is relatively young); there is no obvious offshore trench (it exists but has been largely filled with sediment); and, as seismologist Robert Yeats notes, “has the lowest instrumental seismicity of any convergent margin around the Pacific Ocean”.

It’s this last factor which is probably the most significant in terms of earthquakes. But before we look at this in detail, let’s take a look at the state’s earthquake history.

Historic Earthquakes in Oregon

Large earthquakes in Oregon since 1900

Since 1900, just two (related) earthquakes of M6.0 or more have struck Oregon – remarkably few compared to surrounding states. Image by USGS.

The USGS begins its earthquake history of Oregon in 1873 with a description of damage resulting from an earthquake in northern California. Four years later came the first significant recorded shock in Oregon itself, but a review of the earthquake history suggests that most residents are probably more likely to be affected by shaking form tremors centred in surrounding states as in their own.

There’s a little mystery surrounding some of the earlier tremors:

The USGS lists an M6.8 in Oregon’s historic earthquakes list which doesn’t get a mention in its page on historic earthquakes in Oregon. But a review of historical earthquake information suggests that sources do agree that small-to-medium-sized earthquakes in the state, even though not everyday occurrences, are not unheard-of.

There’s limited published information on the causes of such tremors, but it seems likely that most are associated with normal faulting in and around the Rockies and, as the USGS earthquake history indicates, may cause some damage to buildings. So historically there’s nothing to worry about. Is there?

The Real Big One: The Cascadia Earthquake of 1700

Instrumental recording of earthquakes is a relatively recent phenomenon and tells only the later part of a long, long story. Written and oral traditions predate it — and it was written evidence from Japan which puzzled historian and scientists for years with its teasing references to a great tsunami which struck on 26 January 1700 — but without an associated earthquake.

It wasn’t until the later years of the twentieth century that scientists uncovered geological evidence for a major earthquake which took place at some point along the Cascadia subduction zone. This, it appears, is the long-lost parent of the so-called ‘orphan tsunami’.

By ‘major’ we mean a magnitude which may have been as large as M9.2. That’s not just big — it’s bigger than either the 2011 Japanese earthquake (M9.0) or the 2004 Boxing Day earthquake (M9.1). According to the Pacific Northwest Seismic Network: “The earthquake released about 1500 times the seismic energy than the 2001 M 6.8 Nisqually Earthquake [in neighbouring Washington]”.

We don’t know for certain where the earthquake epicentre was (in other words, the point at which the fracture was initiated) but at Decoded we’ve placed it in Oregon because that is the middle of the Cascadia subduction zone which ruptured along its entire length — over 1000 km.

Earthquakes: The Past Informing the Future

But whether it in fact did occur off Oregon, or northern California, or Washington, or whether it was one major shock or, indeed, a series of rapidly-occurring smaller ones, the result was the same: a major earthquake and associated tsunami that would have caused devastation to existing settlements.

Tsunami deposits provide clues to past earthquakes.

Tsunami deposits were used to trace ancient major earthquakes. Image courtesy of Atwater et al.

The disturbing thing about the earthquake of 1700 is that it demonstrates the potential for a future earthquake of similar size.

Once geologists realised that such an event was not only possible but had actually taken place, they focussed their attentions on the geological history of the coastline of the western states. “Those geologic records,” notes Brian Atwood, one of those who reunited the parent earthquake with its orphan tsunami, “soon gave Cascadia a recognized history of great earthquakes.”

Robert Yeats describes research pointing to the existence of 19 major earthquakes that ruptured the length of the Cascadia zone in the past 10,000 years, alongside several smaller ones. Analysis suggests that the larger events might occur once every 500 years and the smaller ones (which could still be very large indeed in US terms) every 260 years.

It doesn’t take much of a mathematical genius to see that a major earthquake along at least part of the Cascadia subduction zone, is well overdue. Yeats reports probabilities of a smaller tremor as 15% in 50 years.

Oregon: Too Close to a Sleeping Giant?

In The Orphan Tsunami, Brian Atwater and his colleagues don’t mince their words. “The fault that broke in 1700 has been reloading for future Cascadia earthquakes,” they warn. “If the fault behaves as it has the last few thousand years, the earthquakes will happen sporadically at intervals ranging from a few centuries to a millennium.” It’s scary stuff.

Not just Oregonians but their neighbours north and south may find the thought troubling, but they can take comfort from the fact that knowledge is power — and the knowledge that the Cascadia zone is faced by a probable major earthquake at some point in the future provides the civil authorities with the opportunity to prepare for it.

The earthquake will surely come… we can just hope that the west coast of the US will be ready when it does.

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