I originally trained as a geologist, and gained a PhD in mineralogy and geochemistry (studying lignites in Northern Ireland) a long time ago. It’s been useful in many ways, not least to give me some insight into the relative magnitudes of things, be they time scales (3.8 billion years since life first evolved), physical scales (the difference between a microgram and a milligram) or the severity of earthquakes. The Richter Scale1 of earthquake intensity is logarithmic, meaning that for each increase in scale of one, the shaking, in short, will be ten times bigger. A Richter scale nine earthquake is 100 times ‘bigger’ than a Richter scale seven earthquake (which is already considered a major earthquake).
A recent article2 has pointed out in graphic detail that the Pacific North West of the US and Canada is now very likely to suffer a ‘really big’ earthquake: it may range up to and possibly beyond a magnitude nine earthquake, with continental-scale catastrophic consequences. This counts as one of those ‘known unknowns’ that the futurologists like to prognosticate upon.
The Cascadia subduction zone is a portion of the continental margin where the Pacific ocean seafloor is slowly sliding under the continental crust. As the oceanic plate descends into the hot Mantle of the interior of our planet, it heats up and melts, creating lava-lamp like blobs of hot molten rock that rise up through the Earth’s crust and create volcanoes - like Mount Saint Helens. However, the portion of the Cascadia subduction zone between California and Vancouver Island in Canada (approximately) is stuck - and has been for the last 316 years, since 9pm on 26 January 1700. The scientists worked this out from the oral histories of native Americans, from dating the demise of groves of long-dead trees and from the arrival in Japan of an ‘orphan tsunami’ - one that arrived onshore with no previous shaking - on the morning of the 27 January 1700 - having taken 10 hours to cross the Pacific.
Looking back at seafloor sediments, the scientists have worked out that the subduction zone moves on average every 243 years (creating underwater landslides that leave a signature trace in the sedimentary record). This portion of the seafloor subduction zone has been stuck since 1700AD, with stress building up since that time. The zone is now 66 years beyond its average date of release: Scientists suggest that’ ‘the odds of [a] big Cascadia earthquake [8.0 - 8.6] happening in the next fifty years are roughly one in three; The odds of [a] very big one [8.7 - 9.2] are roughly one in ten.’2 The problem is, the longer that the release of tension does not happen, the worse that the final outcome will be.
According to the Mercalli Scale3, at Richter scale events around eight, few, if any, (masonry) structures remain standing, bridges are destroyed, broad fissures occur in the ground, underground pipelines are completely out of service, earth slumps and land slips in soft ground, while rails are greatly bent. At this level of destruction, structures built of reinforced concrete will certainly be safer than others built of flimsier stuff such as brick and wood. However, at Richter scale events much beyond eight (that is to say, in all scenarios for the eventual release of the Cascadia subduction zone), objects are thrown upwards in the air, waves are seen on the ground surface, and damage is ‘total.’ The forecast in this situation is for total devastation.
After the initial shockwaves and seismic destruction, landslides (30,000 are expected in Seattle alone) and fires will then take their toll. On top of this, a very major tsunami is expected to hit the coasts of the US (and of Japan), ranging in size from 6m to 30m in height. Japan’s magnitude nine Tohoku earthquake of 2011 killed 18,000 people and cost around US$220bn - we all remember the horrifying images of the tsunami, relentlessly advancing and destroying everything in its way. The Sumatra - Andaman earthquake and ‘Boxing Day tsunami’ of 20044 caused by a 9.1 - 9.3 scale earthquake, killed 230,000.
Around 70,000 people live in currently designated ‘inundation zones’ on the US northwest coast. Many more people visit them (for example the popular beaches of Oregon, where 150,000 might visit on a summer weekend). The Federal Emergency Management Agency (FEMA) has suggested that it may take at least 1 - 2 months to supply electricity and up to a year to restore water supplies to the previously well-populated areas hit by the disaster.
Enough doom and gloom, for now. The fact is, we know that this disaster will befall us (like the present-day inhabitants of the slopes of Vesuvius and of the city of Naples), but we are content to live in these places and do next to nothing. We humans: how odd we are.
1 www.sms-tsunami-warning.com/pages/richter-scale
2 www.newyorker.com/magazine/2015/07/20/the-really-big-one
3 https://en.wikipedia.org/wiki/Mercalli_intensity_scale
4 https://en.wikipedia.org/wiki/2004_Indian_Ocean_earthquake_and_tsunami
