Geologist explores Seal Cove’s fault from afar

Michael Welland is a geologist and the author of Sand, the never-ending story published by the University of California Press. Sand, last month, was awarded the 2010 John Burroughs Medal for a distinguished book of natural history.

He also writes a blog, Through the Sandglass, in which I found this interesting post about the San Gregorio fault which lies just offshore and, in fact, comes on shore briefly at Seal Cove.

I asked Michael if he would like to write an introduction to his post for Montara Fog and he kindly agreed.

California is a spectacularly beautiful state, with a dramatically dynamic coast and a torrid geological past (and present) – I really enjoy visiting (and, indeed, I lived and worked there many moons ago). The basis for last year’s trip was that the Long Beach Aquarium had very kindly asked to give one of their evening talks, based on my book; I had a great time preparing it, customising the topic for the marine realm, the relationships between marine life and sand, and California in particular – I learned a lot. We decided to base a trip around this event (and, of course, sand) and started off by driving out to the Kelso Dunes, coming back to LA via Palm Springs (which we’d never visited – once is good!), Long Beach, then Santa Barbara, the Oceano Dunes (and the best clam chowder we’ve ever had), Monterey, Berkeley and Point Reyes with friends, Santa Cruz, your peninsula and then San Francisco airport. I wrote several posts for the blog derived from this trip; the Monterey Canyon is an extraordinary feature and the Monterey Bay Aquarium Research Institute does a great job of making all kinds of information available, so I continue to check what’s going on there.

Now, geologists love images – maps, photos, satellite images (as you can probably tell from all the graphics on my blog). Google Earth is amazing – you can literally sit anywhere in the world and investigate any part of our our planet’s surface; I suspect that geologists everywhere spend large amounts of time simply browsing around – I certainly do. On the geology-related blogs (the gloriously named “geoblogosphere”) there are regular competitions (with no prizes) to identify the location of an anonymous Google Earth image. So there I was, looking into the extraordinary new detailed imagery of the floor of the Monterey Canyon, then browsing northwards to see what else the sea floor had to offer. That’s when I came across the superb structures that I wrote about – after a fair amount of research, but the online resources of the US Geological Survey and State and academic institutions are fantastic. Within a few minutes I was looking at a detailed geological map of Seal Cove and Half Moon Bay, reading the field trip guide to your part of the coast – and learning about the San Gregorio Fault.

It’s interesting that this is a major fault that is not one of the better-known or well understood ones. One of the problems, as I see it, with understanding the San Gregorio fault is that so much of it is offshore. It’s named (as I’m sure you know) after the small community of San Gregorio further south from Half Moon Bay, where the fault runs northwards out to sea before reappearing at the Half Moon Bay airport. To the south of San Gregorio, it cuts across land east of the coast, around Pescadero, and then heads southward offshore again, cutting across Monterey Bay and becoming (according to the geological map of California) the Palo Colorado-San Gregorio fault. This is made up of several strands that reappear on land again back of Point Sur. Where the fault does occur onshore, it’s not easy to trace exactly, what with vegetation, erosion, and man’s activities obscuring the landscape. What struck me was how much clearer the features are when they occur on the sea floor.

All these fault systems, even if their dominant movement is sideways (lateral or “strike-slip”), there are also significant displacements up and down (hence the topographic scarp west of Half Moon Bay airport). And, because the shape of the faults is never linear and simple, when they move they invariably force the rocks on either side of them into folds and multiple small faults – incredibly complex and messy geology. Such folds, on land, associated with the San Gregorio Fault, appear on the geological map that I showed, both west of the airport and further south along the coast, but when they occur offshore they are so dramatically obvious, swept clean by ocean currents. However, these features really don’t have any implications in terms of what the fault might do in the future, they simply help reconstruct what has happened in the past – in this case, the geologically very recent past, and, as the source that I quoted shows, the historical past.

It was that report, written in 1997, describing major earthquake events only 600 years ago, that made me comment on the mobile home park that I had spotted on Google Earth. Mobile homes are hardly the most robust and secure places to be in an earthquake. And then I received your e-mail about the major construction proposal right next door. Now, I’m a geologist, and geologists tend to enjoy our planet’s dynamic activity (at Point Reyes, I put one foot on either side of the San Andreas Fault and urged it to bring it on). But we certainly don’t wish these events on anyone – recent earthquakes around the world have shown the appalling human suffering that results. What geologists do often feel is that the threats and the risks of such events are too often ignored for one reason or another, largely through the less admirable side of human nature. The fact is, of course, that we can’t predict with any useful precision when an earthquake is going to occur; but this doesn’t mean that we can’t identify the risks and provide data that allows those risks to be reduced. Planning policy and construction standards are the basic means of reducing those risks – just look at the example of Santiago where many buildings were constructed to withstand an earthquake, while many elsewhere were clearly, and tragically, not (and Haiti doesn’t even bear thinking about in that context). Now, I’m, as I say, no expert, and I’m not in any way familiar with policy and planning in California, so have absolutely no business sticking my nose in any further than to comment that, if building over an active fault zone is not necessary, why do it?

You can read Michael Welland’s post about the Seal Cove fault on his blog.

To learn more about Michael and his book, Sand, check out these two interviews:

The first is from the geology page at

This second interview is from Clastic Detritus, a blog about–what else–sedimentary geology.