Comments on: The Novato Quarry terrane

By: rob

rob — Tue, 12 Aug 2008 02:27:46 +0000

although i’ve been exposed to this stuff when i was in college, reading “geography of the SF bay region” has kind of given me pause. our home here in the bay area seems so transient now. its hard to believe that just 20,000 years ago the edge of the pacific was way out in the farallons, and long before that all of this was underwater somewhere. not to mention that the bay itself has come and gone many times over the last couple of million years. kind of puts this recent round of climate change in perspective.

as for the hayward fault itself, i can only hang onto the slim hope that creep has relieved enough pressure such that we don’t have a massive earthquake anytime soon, but i know that’s just wishful thinking.

By: Rick

Rick — Mon, 11 Aug 2008 05:08:59 +0000

I found the profile of the Hayward fault on that USGS page chilling. Somehow seeing the profile gave me a profound impression of the huge slabs of rock that are grinding past each other, with the fault the near-vertical, miles-deep edge of two plates. I had the feeling of looking over the edge of a cliff. What a guaranteed time bomb.

Great shot and explanation, Andrew. Albany Hill always was a puzzle that you’ve answered and more, revealing it as part of a hidden string of hills across the north Bay. Sweet!

By: rob

rob — Sun, 10 Aug 2008 04:57:21 +0000

thanks! that’s pretty much exactly what i was looking for.

By: Andrew

Andrew — Sat, 09 Aug 2008 21:27:40 +0000

The USGS page on the Hayward fault has some state-of-the-art examples.

By: rob

rob — Sat, 09 Aug 2008 06:01:17 +0000

so are there maps or 3d visualizations available that show the structure of the rock formations underground? or is this something only the oil companies bother with? :)

By: Andrew

Andrew — Sat, 09 Aug 2008 02:40:57 +0000

We have pretty good ideas of what’s under the surface. There is some drilling, for water or oil&gas wells and so on, but our main tools are geophysical methods. There are seismic profiles that use acoustic signals to map the subsurface. There are magnetic surveys, conducted from airplanes and satellites, that can outline buried rock bodies that match surface exposures—for instance serpentinite has a strong magnetic signature. Gravity surveys similarly detect density variations due to changing rock types. And earthquake studies help us outline the shapes of active faults underground—fault planes aren’t always flat. We can study the whole thickness of the crust with these tools. The picture is still blurry, but much better than knowing nothing.

By: rob

rob — Fri, 08 Aug 2008 22:40:45 +0000

somewhat related to this post:

i started reading “geology of the san francisco bay region”, and i was wondering one thing about the rock maps. i think these only show the rock types at the surface, right? do we have any idea what the composition of the rocks are, say, 1000 feet down? 2500 feet down? 5000 feet down? or is it just too expensive to find out (because you’d have to take core samples all over the place?) or, is it just obvious from what’s poking up in different places plus knowledge of plate tectonics?