Earthquake advice for Oaklanders 2: The ground

14 September 2020

In my last post I talked about the Hayward fault itself, the actual crack in the ground. I argued that the extra hazard of living right on the fault doesn’t add much to the risk, which is already high for other reasons. Those reasons are mainly two: the size of likely earthquakes and the type of ground that they shake. This post goes into the ground and how the ground can fail during earthquakes.

The ground is a BIG factor in how the same quake affects different places, as we learned from the 1989 Loma Prieta earthquake. Vibrations from an event way down below San Jose damaged a number of buildings in downtown Oakland, some fatally, but none of them collapsed outright. By contrast, the same earthquake, at the same distance, singled out one type of ground — landfill in West Oakland, formerly wetlands of Bay mud — and brought down the Cypress Street Viaduct, crushing 42 people to death. Soft ground slows down seismic waves, which pushes their energy into a smaller space, and lowers their frequency. In a word, soft ground amplifies shaking.

The Oakland flats consist of alluvium and have no solid rock anywhere near the surface. They’re deep dishes of mud, sand and clay, saturated with groundwater. When you shake this kind of material, the water in it starts disrupting the sediment into a slurry. Think of the times you’ve played near the water at a sandy beach, patting the damp sand until it turns wet, shiny and loose. The water transmits the energy of your hand very efficiently and drives apart the sand particles.

A moderate quake in alluvial ground will start triggering little eruptions of sandy water — sand boils.



Sand boils formed by the 2011 Virginia earthquake; photo by Mark Carter, US Geological Survey

Larger quakes raise the risk that the ground will fail en masse, losing its strength in the phenomenon called liquefaction. If this happens near a vertical boundary, like a streambank or a harbor channel, the ground may spread out sideways (lateral spread). Not just buildings but anything on or in the ground, like pipelines and railroad beds, can fail during liquefaction.

To help guide building designers and city planners, maps showing liquefaction-prone areas are published by the US Geological Survey and by the California Geological Survey. Here’s a sample of the state’s map, which shows the suspect areas in green. These are not areas of guaranteed damage, but zones where we should be aware of the likelihood of damage.

Notice that flat, wet areas aren’t limited to the coast — green areas extend up every stream valley, even in Montclair.

The high ground has its own geologic vulnerabilities. First, of course, is that it’s already prone to landslides: the slopes are steep, our rocks are generally soft and like to disintegrate into soil, and their bedding is steeply tilted, which helps rainfall to infiltrate them. The map above shows landslide-prone areas in blue, in the upper right corner east of the Hayward fault (the black line). Strong shaking will trigger landslides in these areas — not everywhere, but anywhere.

Notice that there’s blue immediately west of the fault too, in Piedmont and west Montclair. The blue areas are smaller there because the rocks are different: harder Franciscan rocks as opposed to softer Great Valley Sequence rocks.

The second geological factor that affects how this ground responds to earthquakes is its topography. Steep ridges tend to direct earthquake waves upward in a sort of whip-crack effect, so that the highest ridgetops shake most violently. This is an area of active research, because it’s complicated.

Finally there’s the middle ground, the low hills that sweep from Pill Hill through Grand Lake and Haddon Hill and Maxwell Park to Evergreen Cemetery. These hills have no bedrock, but they’re made of alluvium that’s older and firmer than the flats. The problems lie around their edges, where the slopes are steepest and prone to landslides, like those at Jungle Hill and McKillop Road and Wallace Street, among others.

If you live in parts of Oakland that consist of these types of ground, I won’t say “you are warned,” because I’m not licensed to practice geology, but you are hereby informed. These facts are part of what makes up your overall picture.

My next post will get into earthquakes themselves, and how they’ll affect Oakland.

Earthquake advice for Oaklanders 1: The fault

31 August 2020

The topic of this set of posts arises from the terrible wildfire season we’re experiencing in 2020, on top of the terrible pandemic, on top of the unemployment crisis, and other more distant events that may be affecting our friends and family. We’re all learning a lot about these topics because directly or indirectly they personally affect each one of us.

When we think about them, we each come from our own place, and my place is geology. I find myself thinking about the ways these new disastrous events feel compared to the old familiar geological risks that have been on my mind all along.

So I thought I’d talk about earthquakes and their risks more deeply and more frankly than I usually do on this blog. I’m doing this because the catastrophes of 2020 have given us all a lot to think about, and if your perspective on those things has evolved — for instance, the way you react to masks — maybe you can think about earthquakes from a different perspective too. Let me stipulate right off that this is only my earthquake advice, and that I speak for no one else and do not pretend to supplant the US Geological Survey, the state Office of Emergency Services or any other authority, although I endorse everything they say.

Oakland is a special case when it comes to earthquakes. Here the risks are different from most other cities because the Hayward fault runs through the whole city, including some important parts. And the long-anticipated large earthquakes — not just one Big One like the 1868 earthquake but about ten times as many damaging Pretty Big Ones — will hit the whole city hard, not just along the fault.

Every so often, someone writes to me with a question about buying or living at a homesite on the Hayward fault. For them it’s always about the fault itself. Something about a rip in the ground on a million-dollar piece of property brings out these sharply felt concerns about “The Fault.”

I’m not a licensed geologist, so what I can say is legally limited. I always say that first. Practicing geology in California without a California geologist’s license is a crime. If someone really wants specifics, they need to engage the correct professional — architects, contractors, geotechnical engineers, home inspectors, lawyers — and all of those experts are legally limited too. The expertise of specialists is partial and does not add up to certainty or wisdom.

The ways I try to help questioners are to provide data about the hazard and correct errors in their thinking about the risk. Here’s an example: a person wrote me with a question about a house that was apparently right on the fault. They’d looked up all the maps they could find, but wanted newer maps, maybe unpublished maps, that showed the fault’s location more precisely. They were asking for more knowledge about the hazard, namely a rip in the ground that damages the house.

Before answering, I had to step back, because better maps aren’t available (if they even exist) and aren’t necessary for making a decision. The thing about the fault is that we don’t precisely know where the rip in the ground is, except in a few well-defined places. And even where we do, the fault is not just a sharp line on the ground but a zone, sometimes tens of meters wide, that will warp and crumble when the fault gives way. In other places, like the south end of Redwood Heights, we aren’t sure where it even is.

Earthquakes happen deep underground, where most of the energy is, and the surface where we live is near the outer edge of most quakes. Up here along that outer boundary, being “on the fault” gets fuzzy. Every large earthquake is different. You can’t count on the ground breaking in the same place each time.

The state’s official solution to this uncertainty is the Alquist-Priolo Act, under which the authorities do their best to map active faults and then establish a wider zone around the faults (usually 50 feet) that effectively has the same hazard. Insurance companies, for instance, rely on these. So my answer to questions about better fault maps is that (1) this is as good as they get and (2) this is as good as they need to be.

There is one way to get more certainty, which is to look at the ground very carefully, and call in an expert to confirm any suspicions that arise. That’s because the Hayward fault doesn’t only rip the ground; it also pulls the ground, very slowly, all the time — the process called aseismic creep, or just “creep” for short. If creep on the fault is already pulling the house apart, the signs may be there, although deep landslides can create the same signs.

This is about the best one can do in gauging the hazard for a particular site. As for the risk, well, being in an Alquist-Priolo zone presents a relatively high risk and being directly affected by creep presents a risk that is absolutely high, not just relatively high. But like I said, the next rip in the ground may come somewhere else, maybe across the street, so the risk is still not 100 percent certainty.

This is where I step even further back, back to where knowledge may edge into perspective. From here, all natural threats are alike at their core. I can tell you this: Earthquakes are inevitable threats, but homebuyers roll the dice and most of them come out winners, because they survive without experiencing a Big One and their houses sell at a profit, and in the meantime they have enjoyed years of pleasurable life in their homes. Even a house directly on the fault, if it’s well sited and well made, will not kill you in a major earthquake.

I think that’s the attitude most people around here have, a California attitude. It strikes outsiders as odd, as I recorded in a story at the end of this post from 2017.

But life and death are not the only risk considerations. There’s also injury, damage and inconvenience, matters that mix concern with money. I’ll get into that topic in posts to come.

The armored shore

17 August 2020

Oakland’s shore is not what it used to be, not at all. The only hint of how it was is Arrowhead Marsh, part of the Martin Luther King Shoreline Park: a broad wetland laced with tidal creeks and vegetation for all gradations of water from fresh to salt.

And yet even Arrowhead Marsh is reputed to be an accident, formed when Anthony Chabot’s dam, under construction in the hills, had a failure that washed huge amounts of sediment down San Leandro Creek. To fans of nature this history, true or not, is scant comfort.

The geologic map (in this case the map of non-bedrock deposits in USGS Open-file Report 2006-1307) shows that every foot of Oakland’s original shore has been erased and built upon, with the small possible exception of the western tip of Adams Point.

Radio Beach at Oakland’s north edge, our nearest thing to a natural beach, exists on landfill.

Everything in the Oakland Harbor complex, from the Outer Harbor to the military grounds to the airport, is on landfill — “made land” as the old-timers called it.

Coast Guard Island is an artificial pile of Bay sediment, built with dredging spoils.

Alameda doubled its land area by dredging and piling. It’s all there on the map.

East Creek Point, the little peninsula of made land directly east of Alameda on the shore of San Leandro Bay, is a good place to contemplate this great undertaking, its aftermath and its possible futures.

Sitting here in 1852, when Oakland was first incorporated, this scene was a luxuriant marsh like that shown in the 1857 Bache map of San Antonio Creek where the Harbor is today. (The photos in this post are extra big, so click for the full experience.)

From here the view shows nothing of what once was, other than the water and the distant mountains across the Bay. The scene was a planar surface of green and blue, with added brown mudflats at low tide. There were no sharp lines between land and sea; none of the trees on the Corica Park golf course or anywhere else on Bay Farm Island; no brown hill covering a former landfill; no palms and wharves and homes — no ground at all — on the Alameda side; no riprap boulders on this side.

All the land in this view is “reclaimed” from marsh or built outright. Sandy mud was dug up from the Bay floor, crushed rock freighted down from quarries, sand and gravel carried in from excavations elsewhere, construction waste dumped on the shore. Trenches drained the marshes, removing the water to create low-lying land for development.

Even much of the water is artificial, in that it exists by virtue of dredging the Tidal Canal. From initial planning to dedication, the massive project took 28 years, every dollar intermittently funded by Congress. Here’s the canal’s east end from the High Street Bridge. An extension dug across San Leandro Bay is called Airport Channel.

Like the rest of the shore, the canal is heavily armored from end to end.

But what we have reclaimed, nature works to claim back. Around the bend from East Creek Point, the gentle Bay surf winnows out the finer grit and exposes the construction debris holding up the weedy shoreland.

The made land is wearing down, and rising sea level will accelerate the decay. There are two ways to deal with that, and both will be used as the century proceeds: more armoring, and returning the sharp shoreline to gentle transitional marsh. This restored marsh is a start. Similar projects are under way in Alameda, especially in the former military zones.

The “made water,” for its part, is filling up. The harbor needs regular dredging to stay open. That will never end as long as water flows and sediment is carried to the sea.

Finally, the made land is prone to its own problems. In this example near the west end of the High Street Bridge, the ground has settled around the water main in the last 80-plus years. Eventually something will need to be done.

It turns out that the Tidal Canal didn’t work as planned. It was supposed to allow the tide to flush through the Estuary and save the work of dredging the ship channels. That didn’t happen, but now we can’t fill it back up. What an alternate history we would have if it hadn’t been built.

In search of McAdam’s quarry

3 August 2020

Alexander McAdam (1854?-1920s) was a minor character in Oakland’s history who left a highly visible mark in our cityscape. A Canadian farmboy who was orphaned at a young age, he came to California after apprenticing as a wheelwright, and after eight years he saved enough money to buy a farm “at the head of Thirteenth avenue,” according to a short biography by James Guinn in 1907. “He was successful in this occupation, but in the meantime had discovered a sandstone quarry on his property. Upon the sale of his farm he acquired considerable financial returns. Stone from it has been used in many of the largest buildings of Oakland, among them being the Unitarian Church, the last buildings of the deaf and dumb asylum, numerous retaining walls, and for many other purposes.”

This caught my eye because I have long thought that Oakland’s rocks were exclusively used as crushed stone. Yet here in the First Unitarian Church, ashlar blocks of genuine Oakland sandstone form the dignified cladding of this important cultural monument and civic institution, built in the early 1890s under the energetic leadership of a leading Progressive of his time, Rev. Charles Wendte.

Rev. Wendte oversaw the building project from his home across the street. The stone cladding was the costliest item in the project, and he singled it out in his memoirs: “Our employment of stone led to vexatious complications. Quarrymen were unable to deliver this material in sufficient quantities, workmen struck for higher pay in handling it. Contracts were broken or remade.”

I had to track down this stone somehow. The documentary clues are slim, and any signs of the quarry appear to be lost. But first, there is the stone itself.

It’s a fairly sound stone of an even consistency with a warm grayish-brown color and massive (i.e., absent) bedding. The block serving as a lintel over the doorway probably broke during the 1906 earthquake, when most of the cladding along Castro Street and the top of the tower collapsed. (The tower was rebuilt without any stone, a smart move.)

A closer look shows that the stone actually varies (although some of that may be substitute stone from another source, as Wendte’s wording suggests), and that a century of exposure has caused a fair amount of spalling. No wonder there were quality problems during construction.

A still closer look reveals it as a medium-grained wacke (“wacky”): a sandstone with grains no larger than a millimeter and a large component of minerals that are not quartz. The black grains are mostly biotite mica; without a microscope I’m limited in what more I can say.

It’s familiar to me. It’s not the Franciscan sandstone produced by the dozen or so quarries in and around Piedmont. I can rule that out categorically. It’s from the high hills on the far side of the Hayward fault.

All of this is consistent with the documentary evidence placing the source in Montclair. The “head of Thirteenth avenue” is where Park Boulevard, the former 13th Avenue in Brooklyn Township, meets Mountain Boulevard. It’s the intersection at the bottom of this excerpt from the 1897 topo map.

To orient (or disorient) you, here’s the same area today.

The “XII Report of the State Mineralogist,” published in 1894, said the following about McAdam’s quarry: “It is in Medos Cañon, back of Piedmont, and is a small quarry, producing sandstone for rubble and ashler [sic]. It is not worked regularly.” The official who wrote that description, a busy guy on a quick visit to cover the whole county, wrote down “Medos Cañon” when someone said “Medau’s canyon,” meaning the valley of present-day central Montclair where the dairy farm of John H. Medau once lay. I believe that if the site had been in Shepherd Canyon, his informant would have said so as that name was in wide use at the time.

All of this means that the quarry could have been a good exposure of the Redwood Canyon Formation, a wacke of Late Cretaceous age, that forms part of the east side of Montclair’s valley along the Hayward Fault. It’s the unit marked “Kr” on the geologic map, below. The lithological description of the unit, and the composition data from Jim Case’s 1963 Ph.D. thesis, are close enough to the stone in the church.

But also likely is the Shepard Creek Formation (Ksc) and even the Oakland Conglomerate (Ko), when you consider that the units are only subtly different, variable in composition and not well mapped despite the best efforts of competent geologists.

In any case, I had a good time visiting these rock units along the Montclair Railroad Trail the other day. There’s a lovely outcrop of the Redwood Canyon Formation above the trail along the route of the recently upgraded powerline, southwest of the word “grade” on the map. That warty weathered surface, reminiscent of the Incredible Hulk’s hide, is one of this unit’s distinctive features.

But the rock there’s not a good match.

Neither is the rock in the landslide at the upper end of the trail.

And just for good measure, here’s a chunk of sandstone from the Oakland Conglomerate. The material is coarser and wacke-er, but again under the 10X hand lens it’s not like the church’s stone.

Nowhere in this area, in many years of visits, have I seen a body of rock big enough and sound enough to support a quarry capable of producing ashlars — not on this side of the Hayward fault. The nearest quarry site is down Park Boulevard where the Zion Lutheran Church sits today, the former Heyland/Diamond Cañon/Bates & Borland quarry on the side of Dimond Canyon. But that produced crushed Franciscan sandstone, something quite unlike McAdam’s stone.

I can only conclude that McAdam found a lucky hillock on his farm and made the most of it, one that’s been obliterated during the waves of development since 1890. And the site of his farm is, as we say, poorly constrained. Even his life dates are fuzzy. But his accomplishments include making a profit from farming, acquiring a large home in Temescal, serving two terms on the City Council in the nineteen-oughts, and equipping an important building with a handsome exterior (despite the vexation he caused Rev. Wendte). I can’t confirm when he died or where he’s buried, so this building surely is his monument.

While I was researching this post, the papers covered a lovely story about how archeologists used advanced geochemistry to pin down the source of Stonehenge’s biggest stones, a peculiar sandstone known in Britain as sarsen. The New York Times version was my favorite writeup, and the hardcore details are in Science Advances in an open-access paper.