Why didn’t anyone tell me San Francisco use to be the largest sand dune ecosystem…

[Pedestrian]

Originally Posted by jd3189

As a hilly city, I always was conflicted on whether that made SF more suspectable or defensive to earthquakes than the flatter cities on the east coast. Now learning that much of SF was originally sand dunes gets me thinking that this also contributed to the severity of the 1906 earthquake.

From those risks to sea level rise, San Francisco is probably not in a good position as natural disasters increase in the coming years. The situation is worse than for LA or SD. At least those cities have a good chuck of their land away from the ocean or another body of water ( ex. The Bay).

Here's an attempt to show the ground shaking in SF with a 1906 type event. Telegraph, Russian, Potrero and Nob Hills and the city's central spine (Mt. Davidson/Twin Peaks) stand out as the places to be.


https://www.researchgate.net/figure/...fig4_327832392

[ChrisLA]

Originally Posted by jd3189

From those risks to sea level rise, San Francisco is probably not in a good position as natural disasters increase in the coming years. The situation is worse than for LA or SD. At least those cities have a good chuck of their land away from the ocean or another body of water ( ex. The Bay).

I believe Marina Del Rey is vulnerable as well, but San Bernardino in spite of it being far inland and quite dry is probably the most vulnerable to liquefaction. There is a lot ground water that sits just below the heart of the city, it will rise in a big earthquake and could be very devastating for this city.

[edale]

Originally Posted by jd3189

As a hilly city, I always was conflicted on whether that made SF more suspectable or defensive to earthquakes than the flatter cities on the east coast.

Wait, what? SF (and all of CA) is more susceptible to earthquakes because it's positioned near several major fault lines. Whether an area is hilly or flat has no bearing on whether it's susceptible to earthquakes. They happen in mountainous areas and flat areas alike. Obviously the east coast has much less seismic activity, even though small earthquakes occasionally do happen. The one that hit Philly and DC a decade or so ago caused considerable damage to the Washington Monument.

[Centropolis]

Originally Posted by dave8721

There is an old dune system (now grassed over) that covers almost half the state of Nebraska. The Nebraska Sandhills:
https://www.google.com/maps/place/Ne...!4d-99.9018131

came here for the nebraska dune content.

[Centropolis]

To the original point…you can really feel how the sunset was graded flat down towards the pacific. Must be interesting to dig around in back yards.

[Centropolis]

Half on topic but I have Superfund sites on massive glacial sand eskers that are 100 ft + deep over bedrock in Illinois that rise above ground surface and could conceivably defoliate under dry climatological conditions and create fairly large dunefields between St. Louis and Chicago.

[Pedestrian]

Originally Posted by edale

Wait, what? SF (and all of CA) is more susceptible to earthquakes because it's positioned near several major fault lines. Whether an area is hilly or flat has no bearing on whether it's susceptible to earthquakes. They happen in mountainous areas and flat areas alike. Obviously the east coast has much less seismic activity, even though small earthquakes occasionally do happen. The one that hit Philly and DC a decade or so ago caused considerable damage to the Washington Monument.

The soil conditions matter VERY MUCH how intense the shaking is during a quake and fault proximity being equal, the shaking and therefore the damage can go from nearly nothing to severe just because of the soil.

The map I posted above shows this very clearly (as does the historical experience of 1906): With a San Andreas quake such as 1906, there would be minimal shaking and little damage on the bedrock at the top of Mt. Davidson or Telegraph Hill but severe damage in the Marina and Mission Districts, just as there was. While it's not so much a question of hilly or flat, the fact is (and I'm sure was being alluded to) that hilly terrain in coastal California is mostly rock whereas the valleys between are sand or mud or alluvial soil.

[jd3189]

^^^ Yeah, that’s what I was alluding to. If the hills are made of solid rock and highly packed soil, it should be more resistant to earthquakes ( based on my understanding of physics, but I could be wrong). If the composition is loose soil or sand, lower chance anything will stand on that after a bit of shaking.

Despite California having a bunch of faults running along its length, the West Coast is more limiting in scale than the East Coast when it comes to seismic activity. If a quake happens at Ridgecrest, people in LA, Bakersfield, San Bernardino, and Las Vegas may feel it. When a quake happened around Virginia ten years ago, the whole eastern seaboard felt it. From what I just researched, soil and rocks in the East are older and since there are less faults there to block dispersal of the seismic waves, they can go much farther out from the epicenter.