At the end of September last year, a swarm of small but disconcerting earthquakes occurred under the Salton Sea, in California. While the individual earthquakes didn’t ever rise over magnitude 4.3, the USGS temporarily raised their risk of a large earthquake on the San Andreas fault (the anticipated Big One) from 1/10000 to 1/100.
Now, 1/100 is not a particularly high risk, and it reduced daily afterwards until the USGS reported on October 6 that the risk had gone back down to baseline levels. There is something slightly uneasy about these events, however, especially since that earthquake will likely leave Los Angeles without fresh water for months. It’s even interesting that the baseline likelihood is even so low—the structure of the faults in southern California is so ridiculous and interconnected that there’s at least one other fault that could trigger the San Andreas earthquake if it slips first. Recent models even suggest that the entire San Andreas fault would fail all at once, as opposed to just individual sections as was previously assumed. A model from 2014 also suggests that there’s a roughly 50% chance of a magnitude 7 or higher earthquake happening in each of San Francisco and L.A. in the next 30 years.
The overarching feeling that I’ve gotten from reading about earthquake predictions for cities in seismically active areas is that it really isn’t a question of if these earthquakes will happen in the next few decades but when, and what can be expected, and how to be ready. Incredibly detailed reports have been released by most cities that expect significant earthquakes to happen soon, so the good new is that at the very least people are thinking long and hard about how well at-risk cities will perform when actually subjected to these events. The bad news is that realistically there’s still a lot to be done before anyone can rest easy.
Easily the two reports that I’ve read the most of are from North Vancouver and San Francisco, each detailing models for what will happen in earthquakes that have a decent chance of happening relatively soon. It appears that they used the same sets of criteria, as well, as they consider events that, in the next 50 years, have a likelihood of 10% (called “expected” in the SPUR report from San Fran) and of 2% (called “extreme”). SPUR detailed another category, however, of “routine” that have a 70% chance of striking. Completely coincidentally, as well, their expected categories are similarly sized earthquakes, with a magnitude 7.3 anticipated in the District of North Vancouver and one of 7.2 in San Francisco.
(side note: the SPUR report I read is from 2009, as well. The 2014 model reported by the USGS gives a somewhat more pessimistic view of the sort of earthquake that may occur, so the predictions made may not even be accurate.)
The contents aren’t exactly super promising either—while certain things (like the airport in San Fran or roads in Vancouver) will be fine, most other pieces of infrastructure will be in considerably worse shape than what is considered ideal if adequate preparations are made. That is, these cities aren’t exactly ready for these scenarios that have an already somewhat worrying chance of occurring. An example of one of the worst discrepancies between expected and preferred outcome is in the lead time before hospitals in San Francisco will be operational again. Hospitals need to be available immediately after any natural disaster, and that is show here, but it might take up to 3 years for hospitals to recover in current circumstances. To be honest, I suspect that this means that the analysts here are expecting them to be completely destroyed in the process, and will need to be rebuilt in their entirety.
http://www.seaosc.org/resources/Documents/PreparingForTheBigOne.CivilEngineeringMagazine.03.24.15.pdfI hadn’t realized when I first started researching these articles that the earthquakes that people are worried about are by no means small, but aren’t exactly in the range of the most massive ever seen on the planet. The biggest concern in Vancouver isn’t the megathrust that might also happen in the next few decades, which it would still be damaging, but the amount of shaking is greatly reduced by the 300 kilometres between the fault and the coast. While it’s possible to get an 8.0 or higher earthquake in California, the likelihood is low enough that it isn’t being considered as the biggest risk. Instead, either place experiencing a magnitude 7ish earthquake is the issue.
This fact actually makes sense, if the Wikipedia page for the deadliest and costliest earthquakes ever is to be believed. Most of the entries on these lists aren’t magnitude 8 or 9, but instead lie within the 7-7.9 range or even less, because the amount of damage done is so heavily dependant on the particular location of the epicentre and both human and natural geography around it. In fact, the 1960 Chile and 1964 Alaska earthquakes don’t even appear on either list, despite being the two most massive earthquakes ever recorded. This is probably because both occurred in areas that are not as heavily populated, so they didn’t cause as much damage. (As much, meaning still billions of dollars worth of damage and thousands of deaths, but this pales in comparison to some of those on these lists).
Another interesting tidbit of information is that none of L.A., San Fran, Vancouver or Seattle are actually under any real risk of being struck by tsunamis. California doesn’t have any faults that are in the ocean, so tsunamis can’t be created, while Seattle is shielded by the Puget Sound and Vancouver is conveniently placed behind Vancouver Island, should one be created by a large disturbance in the Pacific. The west side of Vancouver Island is still screwed though, so if you live in Tofino figure out where you’re going before you only have 20 minutes to do so.
There's an actual ton of information that I would like to include but realistically can't for space considerations, so first off I highly recommend that you read (or at least skim) the Wiki entry about the 2004 India earthquake.
And if you live in an area that is at serious risk of an earthquake, please read this from the Government of Canada.
And then here's the general link dump of the sources: