How long should we wait for an overdue earthquake on the San Andreas?

http://rocksandwater.net/blog/2016/07/wrightwood-recurrence/

by cossatot • 10 years ago

72 89 10 years ago

lordnacho• 10 years ago

What does an historic earthquake look like in the layers of rock? How do they know when they occurred?

cossatot5• 10 years ago

The science of finding out the (pre-)historic earthquake history is called 'paleoseismology'. You can read about it from a variety of sources.

In most instances the earthquakes are dated by finding carbon-bearing material in deposits that look like they were created during the earthquake, and doing radiocarbon dating of them. The ages may be bracketed by radiocarbon dating of sediments that were clearly broken by the earthquake (and are therefore older) and ones that overlie the broken rocks (and are therefore younger).

In a few other instances, earthquakes can be dated by tree-ring dating; for example during big subduction zone earthquakes, the coast often subsides by a meter or two, which will put some forested beaches under sea level (or at least high tide level) and kill all of the trees that can't deal with salt water.

In others, the dates of sediment layers broken or caused by earthquakes can be dated through other means, or correlated with climate events of known time.

Some example of deformed and undeformed sediments in the Wrightwood trench site on the San Andreas is here[0,1]. [2] is from the nearby San Jacinto fault. [3] has a lot of images. [4] is a good introduction to the science with good pictures.

[0] http://www.bssaonline.org/content/97/4/1054/F14.large.jpg [1]: http://www.bssaonline.org/content/92/7/2726/F11.large.jpg [2]: http://lithosphere.gsapubs.org/content/7/3/211/F6.large.jpg [3]: http://earthquake.usgs.gov/research/external/reports/04HQGR0... [4]: http://isites.harvard.edu/fs/docs/icb.topic988563.files/EPS2...

sintaxi• 10 years ago

If it doesn't happen this year, just wait longer.

stillworks• 10 years ago

winning by induction

DavidSJ• 10 years ago

Subduction actually.

sveme• 10 years ago

Some fault lines seem to accumulate energy, so the longer the waiting period between two earthquakes, the stronger it gets. Is it the same for San Andreas, so that more frequent earthquakes would actually be beneficial as they would be easier to handle structurally?

tqkxzugoaupvwqr• 10 years ago

I’m not an expert and I have the same understanding. Futuristic earthquake mitigation approaches talk about inducing small, non-destructive earthquakes in a controlled way to reduce the magnitude. The goal is to completely manage earthquakes.

iamcreasy• 10 years ago

That's insanely cool!

sbuttgereit• 10 years ago

Oh sure... now you want to frack. ;-)

ScottBurson• 10 years ago

Not as good an idea as it sounds. The problem is that the largest earthquakes release massive amounts of energy. Each point on the Richter scale corresponds to about a factor of 30 in energy. So to release the energy of one magnitude-7 quake would take some 900 magnitude-5 quakes. And a 5 in a populated area can still do a little damage. Can you imagine how people would feel about having one of these a week for 20 years?

rwmj• 10 years ago

There's a ton of data on the very regularly occurring (hourly) small earthquakes around the world available from USGS: http://earthquake.usgs.gov/earthquakes/ I'd like to see whether this analysis works on this rich data, not on the very sparse data set of San Andreas earthquakes.

ejcx• 10 years ago

So, we aren't only overdue but a quake seems imminent? The upper bound of time between quakes was very slim.

This is a stupid and selfish question, but what does a quake on the San Andres fault mean for San Francisco? My knowledge of all of this is based on 4th grade science.

celticninja3• 10 years ago

Bad news. Whilst building codes may be better than in the past, due to population density the risks of a high number of deaths is significant.

abalos• 10 years ago

I would imagine a very significant portion if the Marina district just turning into rubble unfortunately. I'm hoping this doesn't happen until buildings have a chance to improve even more.

peterbraden• 10 years ago

This is fear mongering. The tectonic structure of the SA fault means that the upper bound for magnitude is limited, and building codes are really good.

Up in Portland and Seattle, the risk for high numbers of causalties is far greater due to the subduction fault rather than the transverse fault.

ghaff6• 10 years ago

Note that this article is specifically about the south end of the San Andreas near LA.

brador2• 10 years ago

Question: what happens to land rights after a quake? Since the quake moves land around, stretching in parts and destroying land into cracks in others. Do property lines get redrawn? Who owns any newly created land space? Is it fair game for anyone to claim?

celticninja3• 10 years ago

It doest really work like that, it's not like new land is created by pushing out into the sea, but even if it was the property owners boundary is likely the sea, therefore the land is incorporated within their area.

slazaro• 10 years ago

But what if you have a fence that gets shifted around, and no longer matches the boundaries in the property maps?

brador2• 10 years ago

Say there's a crack or fissure that opens up, creating new "space". Someone then fills that crack with soil making new "land". Or say the land under a fence moves. Since land ownership diagrams contain specific measurements, are they redrawn? Replaced? Accepted? Measurements ignored and features used instead (apple tree to 7th fence post)?

If my property deed says my yard is 20 metres wide, but post earthquake it's 21 because a 1 metre wide split occured in it what happens when I sell? Is accuracy thrown out the window?

JshWright• 10 years ago

It very much does work like that though... The San Andreas fault is a "strike-slip" fault. The plates are (effectively) moving past each other, in opposite directions.

Imagine you owned a rectangular piece of land, with the fault running right up through the middle of it. After an earthquake, the western half of your property is now shifted a non-trivial distance to the north relative to the eastern half. What happens to the property line? Does it now have a 'kink' in it? Does it remain in the same place relative to the 'ideal' geodesic plane (meaning various landmark like fenceposts, etc, are now in a different place relative to the property line).

It's certainly an interesting question...

noobermin• 10 years ago

Question: what happens to your IP when it ends up on a torrent and the world copies it all over?

Human institutions do not win against nature/reality. Not that we shouldn't try, but we have to remember that our institutions are limited ultimately by the laws of physics and mathematics/statistics.

whoopdedo• 10 years ago

You should ask the residents of Hollister.

http://www.bldgblog.com/2015/04/the-town-that-creep-built/

Animats• 10 years ago

California has the Cullen Earthquake Act to deal with that.[1] That's a consolidation law, so that all the claims in an affected area can be resolved in one court proceeding.

[1] http://www.leginfo.ca.gov/cgi-bin/displaycode?section=ccp&gr...

ZenoArrow3• 10 years ago

What I can't understand is that after the 1906 San Francisco earthquake, where "Over 80% of the city was destroyed by the earthquake and fire" ( https://en.wikipedia.org/wiki/1906_San_Francisco_earthquake ), why on earth was it rebuilt? Surely that's a sign to move the new city elsewhere?

onion2k2• 10 years ago

You're only looking at the downside risk. There are huge upsides to the area - notably the San Francisco Bay is a natural harbor, which makes it a really good place to put a port. The promise of wealth from trade meant people would live them pretty much regardless of the downsides.

ZenoArrow3• 10 years ago

Fine, but what's likely to be cheaper, building an artificial harbour away from the fault line or rebuilding a damaged city every 100 years or so?

babuskov• 10 years ago

why on earth was it rebuilt?

They probably thought: Great, now we have 100 years until the next one. ;)

ghaff6• 10 years ago

Do what makes economic sense today. Then it will be someone else's problem.

Thankfully, we never make decisions like that with respect to software :-)

ghaff6• 10 years ago

And Tokyo in 1923. But really the entire Rim of Fire.

We build things in areas that are relatively dangerous geologically because there are other advantages to those areas. San Francisco has one of the best harbors on the West Coast and access to a lot of inland resources.

But just another reason not to concentrate a good chunk of the US software industry in the Bay area I guess.

rurban• 10 years ago

Fun fact: why did they decide to build a huge nuclear power station close to the fault, and 6m above sea level so that a tsunami will have Fukushima like effects?

That's what will bring Silicon Valley down, not direct quake causes.

marcosscriven4• 10 years ago

Possibly a silly question, but...

I hear the term 'overdue' applied particularly to earthquakes, rather than any other naturally occurring event. Does this mean 'overdue' in the same sense that rolling a six is 'overdue' if you've not seen one after six rolls?

991219921• 10 years ago

No, It means that the tension between tectonic plates steadily builds up, so an earthquake is more likely.

marcosscriven4• 10 years ago

I see - the article though seems to talk about 'surface-breaking' earthquakes occurring every 100 years in that region, and that we're now at 159 years without one.

This idea of a time expectation doesn't seem to match your explanation (which seems more intuitive to me).

gradys• 10 years ago

My understanding is that the probability and expected magnitude of an earthquake actually increases over time as tectonic tension builds. So no, not like dice.

marcosscriven4• 10 years ago

I meant in the sense of probability, not literally like dice :)

otempomores• 10 years ago

Is there a way to milk tectonic tension?

avar• 10 years ago

It's like knowing that 1/2 dice rolls yields an even number, and after rolling twice in a row and getting odds making a big deal of the supposed statistical anomaly, because we got two odd numbers in a row we're really overdue for an even number, don't you think?

When volcanos/earthquakes etc. are said to occur every N years, that's averaged over geological periods. An event that occurs on average every 100 years over tens or hundreds of thousands of years could be occurring once every 10 years some millennium, or once every 250, 500 or 1000 years for some time span.

batbomb• 10 years ago

Please read up on binomial and Gaussian distributions.

cossatot5• 10 years ago

With dice, each roll is independent of the others. With earthquakes, the probability of an earthquake occurring in any given year is (almost always) dependent on how long it has been since the last event. That is represented by the hazard function in the post (lambda t). I say 'almost always' because for a few distributions the hazard rate is constant with time.

An interesting thing about the hazard function with earthquakes is that it isn't always monotonically increasing; I've done this analysis on some other faults that show a very high hazard in the years to decades after an earthquake, and then the hazard drops down and doesn't recover until the far right tail of the distribution. This reflects temporal clustering of earthquakes, which is something we're gradually becoming aware of.

sethammons• 10 years ago

I would compare it to filling water balloons and timing when they pop. Our current water balloon should have popped a while ago, but the tension/pressure is still building.

fit2rule2• 10 years ago

I lived in SoCal and worked in SFO for some decades and witnessed both the Northridge and the Oakland quakes first-hand, narrowly avoiding personal injury or even death in both cases (big things falling around me), and the conclusion I made after these events, which I eventually acted on, was simply put thus: GTFO!

I really didn't want to be there when the Big One happened, nor even some of the minor small ones, so I've moved to Central Europe where such things have a lower chance of happening .. in a way, I have a perverse desire to see The Big One happen from so far away, but I always dread the mind-game of trying to conceive what SoCal will be like, afterwards. I truly dread the idea, and I honestly can't understand how any society with scientific awareness of its surroundings can nevertheless commit to building so much expensive civilization and infrastructure in a known catastrophe zone. I guess its just a matter of momentum, but I sure wish the citizenry of the PNW and SoCal would prepare themselves for this event a bit better. It seems so defeatist to continue to build things in that area, knowing it could fall into the sea in a moment, and there is nothing we could do about it. California should just be a massive nature park stripped of all human involvement, just in case, but of course this is never going to happen, so if The Big One does happen in my lifetime, its just going to be a massive tragedy no matter what we humans could have done about it with all our science and stuff. Argh!

ghaff6• 10 years ago

I guess if I lived through two significant earthquakes in that way, I'd probably have had a thought along the lines of "This is a sign to move" as well.

It seems so defeatist to continue to build things in that area

Fatalistic is probably the better word. I can buy an old missile depot in the Midwest and stock it up with supplies and prepare myself for the zombie apocalypse or whatever.

Or I can live somewhere I enjoy, and has good opportunities, etc. and not worry too much about events I don't have a lot of control over or a lot of insight into their probabilities.

Of course, there are situations between those two extremes and it's not unreasonable IMO to include "the Big One might hit" into a calculus for evaluating moving to or staying on the West coast. (As with living in areas with a lot of potential for floods, hurricanes, and tornadoes.)

Actually turns out Las Vegas, for example, scores pretty well on non-disaster metrics. One reason a lot of datacenters are there--in addition to evaporative cooling.

fit2rule2• 10 years ago

Its not like I didn't have a great time living in California, because I truly did, and in many ways its a delightful civilization in which to participate.

Perhaps its just a cultural comfort I seek, in which the citizenry at least seem aware and prepared for total and utter disaster, any minute now, and live accordingly. I guess its exhausting to live that way, so if it happens, it happens; maybe that liberal and - lets just get out and say it, "Zen-"like mindset truly is just a part of the California love ..

w0utert• 10 years ago

> I honestly can't understand how any society with scientific awareness of its surroundings can nevertheless commit to building so much expensive civilization and infrastructure in a known catastrophe zone.

The same thing can be said about many other places on earth where people decided to settle and build a society. Near volcano's, below sea level, between rivers that could flood the whole area, in arid regions that barely support agriculture, very high up in the mountains where it's cold and dangerous, in regions infested by parasites and disease-carrying insects, etc.

I think you should not try to approach these things from a rational/calculating perspective, that's not how societies work. Why societies cluster at certain locations is determined by so many different factors that building a highly advanced, densily populated, and expensive to (re-) build society close to a faultline should IMO be seen as a 'historical accident that, in the case of SF, had a high probability of happening'.

ac29• 10 years ago

People often assume the 1989 earthquake was particularly dangerous. While it caused quite a bit of property damage (about $6 billion according to Wikipedia), there were only 63 deaths and 3,757 injuries. The previous major earthquake in the region was in 1906, and there haven't been any since 1989.

Given that, it is extremely unlikely to die or be injured by earthquakes in the Bay Area. On a list of most likely ways to die, it would be near the bottom.

edit: I should mention I lived in the Bay Area in 1989 and experienced it first hand. I still live here and of all the reasons I might move, earthquakes isnt even on the list.

qrendel• 10 years ago

I was under the impression that the bigger danger was the expected eventual earthquake of the Cascadia Subduction Zone, potentially leading to tens of thousands killed or injured and millions more displaced.[1][2]

[1] http://www.newyorker.com/magazine/2015/07/20/the-really-big-...

[2] https://en.wikipedia.org/wiki/Cascadia_subduction_zone#Earth...

cossatot5• 10 years ago

The damage from a Cascadia rupture is going to be more widespread but the ground shaking in the metro areas may be less than for the San Andreas because the San Andreas hits the surface quite close to populated areas (LA also has more people that WA and OR combined). The intensity of ground shaking decreases with distance due to geometric spreading and energy absorption by the intervening rock mass. Cascading ruptures are also every 500 years (or so) vs every 100 years, so the hazard per year is less. But yes, both are major concerns.

int_19h2• 10 years ago

Also, speaking of other faults, WA also has the smaller, but much more shallow, Seattle fault. Because it's so much closer to the surface, and right under the city, it can potentially do more localized damage than Cascadia.

(There are other dangerous faults like that in Puget Sound region, but Seattle fault gets special attention because of how many densely populated Seattle metro areas sit literally right on top of it.)

Cortexia2• 10 years ago

People always say Hayward Fault is more troubling for California because so much is built directly ON the fault.

Cortexia2• 10 years ago

Hopefully San Andreas will wait another 10 years, because by that time there will be no Nuclear Reactors operating in California. We are down to only ONE, and it will shut down within the next 6 years. PG&E is getting out of Nukes.

How long should we wait for an overdue earthquake on the San Andreas?

Related Stories