The Hayward earthquake: 1868-2018

15 October 2018

There’ll be lots of press this week about the anniversary of the “Great San Francisco Earthquake” — the original one, 150 years ago on 21 October 1868, caused by a big rip in the Hayward fault just before 8 in the morning. (Also the annual ShakeOut exercise, this year on 18 October at 10:18 am.) Behind the press stories there’s lots of sound info for you, and I’ll put a list of good links at the end of this post. Rather than write another standard thing about the earthquake, I’ll focus on what I’ve been studying lately, which is the problem of earthquake landslides.

Contemporary accounts of the 1868 quake tended to focus on homes wrecked and buildings ruined, but several geological manifestations were widely recorded: the ground cracked open from Oakland all the way to present-day Fremont, some of the cracks spewed muddy water, streams ran high afterward, and many new springs appeared near the crack.

Only in more distant places like the San Mateo Peninsula did people mention rockfalls and landslides — which tells me those were the most widespread forms of damage that day. Landslides were everywhere, even in places where buildings weren’t bothered. And though our buildings are even stronger today, the landscape is weaker than ever.

Landslides were everywhere in the East Bay hills in 1868 because that’s always what happens. No one made mention of them because they were unremarkable. The hills were empty countryside, a mix of spent rangeland and razed redwood groves. What matter was a landslide in that waste? But nowadays . . . when the next large Hayward fault earthquake comes, landslides will cause widespread and costly misery — in the high bedrock hills as well as the low gravel hills of the Fan and along the fault.

We don’t see the scars of the 1868 slides today because after a few decades, they fade out. Here’s a small slump in Sibley Regional Volcanic Preserve that I photographed in 2005, 2009 and 2017.

I think in another 12 years it will be pretty subtle.

The high hills owe their basic shape to earthquakes and landslides. Their sides are steep slopes, swept clean and straight as the Hayward fault raises the heights by about a millimeter a year, one earthquake at a time. So they’re always primed to slide.

USGS landslide researcher David Keefer estimated in a 1984 paper that earthquakes as small as magnitude 4 are large enough to cause rockfalls. A magnitude 6.8 event, the size of the 1868 quake, would cause up to 1000 rockfalls in the affected area.

And up to 1000 slumps like the one at Sibley.

And up to 1000 debris slides, what people usually call mudslides. The majority of these will start along roadcuts, which inherently destabilize hillslopes . . .

. . . and near hillside homes, which do the same.

The city will be overwhelmed. The roads will take months to clear, years to fix.

All these photos show rainfall landslides, or sites where one could happen. They’re the usual kind. They happen where the flow of water, both on the ground and below it, unbalances the slope and makes it fail. They especially tend to form on the sides and floors of valleys, where the slopes converge.

Earthquake landslides, though, tend to start on ridgetops and promontories, landforms that focus seismic energy toward their tops. They also aren’t confined to the wet season. Sites that aren’t prone to rainy-season landslides may instead be the preferred targets of earthquake slides. If we’re lucky during the next big quake, as we were in 1868, the ground will be dry. If we aren’t, and the ground is waterlogged, well, heaven help us because we’ll get both kinds of landslide at once.

This danger is built in with the tectonically active setting that makes Oakland so beautiful. We have to prevent what we can, and cope with what we can’t.

Here’s that list of Hayward fault resources. Actually, the whole list is on one page, from the U.S. Geological Survey:

In that list I want to single out “What to Expect in a Big Urban Earthquake,” a phone-friendly “geonarrative” aimed squarely at people who live in cities — that’s us.

The Dunsmuir-Chabot trail

1 October 2018

The most remote part of Oakland will be opened to public access fairly soon, when the East Bay Regional Parks District finally gets around to constructing a trail between Dunsmuir Ridge and Lake Chabot. I made my way into part of the route a few weeks back. It’s interesting and inviting territory, set above an untouched oak-filled stream valley with Fairmont Ridge beyond.

The land has divided ownership, with parcels belonging to the city, EBMUD and the East Bay Regional Parks District. They’re all public agencies, so the bureaucracy must have been difficult. Here’s the setting, as laid out in a 2009 EBRPD map.

The solid blue line is a trail in Anthony Chabot Regional Park that’s been closed for years. The dashed blue line is the proposed Dunsmuir Heights to Chabot Regional Trail.

This summer the EBRPD board was shown this map of the approximate route. Again, the solid line is an existing (deteriorated) roadway and the dashed part will be built from scratch. The photo at the top of this post is the view from the EBMUD water tank at top center. The part I’ll be showing is between there and the “P” mark at the city golf course.

A couple more maps to help you see what’s here. First is the bare land as shown in Google Maps terrain view.

The centerpiece of the trail’s route is the valley in the lower right quadrant. Note its depth and steepness. The permanent stream in that valley has no formal name, so I will hereby dub it Chabot Creek. And finally here’s the same area in the 1947 topographic map, which shows the old roads that will become the trail.

If you look at the upper part of Chabot Creek valley, you’ll see it turns sharply from southeast to southwest as you go downhill. On the Google map, though, the streamcourse is interrupted by a flat area. That’s landfill made of waste from the Cypress Viaduct, which collapsed in the 1989 earthquake. It’s sterile and weedy, but the view is nice.

At its edge is a curious structure, visible in Google Earth, that turns out to be a spillway, made for the event of a large rainstorm during a very wet winter. Presumably the landfill was capped with a layer of clay to stop any contaminants from leaching into Chabot Creek, and thus the site would fill with rainwater quickly and have a risk of spillover.

If you looked closely at the second map, you saw the intriguing pointers to an “old foundation” and a “1936 WPA rock chimney.” The chimney is a massive stone fireplace, suitable for a hunting lodge. But the building it once occupied is gone. Foundations around it show that it was a group facility of some kind. I’m hoping that local historians can say more about it in the comments.

A stone in the entryway is carved with the date 1935, so the map is slightly in error.

In any case, the stonework is indeed classic WPA masonry, of the same vintage, material and durability as the Woodminster Cascades in Joaquin Miller Park.

And speaking of stone, what are the rocks like around here, you ask. Here’s what’s mapped in the area.

“Jsv” is the Leona volcanics, “KJk” is the sedimentary Knoxville Formation, and the blue field is Franciscan melange, the same body of rock underlying Knowland Park. When I visited, I checked out a roadcut right where the Franciscan and Knoxville meet and found an assortment of rocks.

All of these are appropriate for the Franciscan, but the brown sandstone could just as easily be from the Knoxville. A return visit is in order during the upcoming wet season, when the ground is firmer. I hope to see more signs of the Franciscan “knockers” so well exposed in Knowland Park — and on the golf course, like this blueschist knocker cropping out in the rough.

The plans for the Dunsmuir Heights to Chabot Trail are supposed to come up for public comment this fall, with the work to be completed by 2021.

Boulders of the Transbay Transit Center

17 September 2018

San Francisco’s new transit center — with the 5-acre garden park on top — is worth a leisurely visit, no matter where in the Bay area you live. Naturally I had to see it too, because a reader sent me photos of a large, alluring boulder that’s part of the complex. He couldn’t decide what rock type it is. “Gneiss,” I typed back, refraining from adding “Nice gneiss.”

There are different ways to ornament a large structure with boulders, seems to me. You could make them identical, for that unified look. The FDR Memorial in Washington DC does that well with slabs of red granite. You could make them vary, like Ruth Asawa’s landmark “Garden of Remembrance” at SF State.

The transit center and the park on its roof (to which Salesforce has purchased naming rights for the next 25 years) are studded with about a dozen large boulders, from 4 to 6 feet tall. The designer’s scheme for them mixes unity with variety.

The variety lies in the personality of the stones and the mix of rock types. The unity lies in their source and their surfaces. Let me show you some personalities. The first two are up in the park and the rest are at ground level.

Oh, I should warn the sensitive that all of the boulders have had an opening carved into them to hold a steel sign either pointing to or announcing that you’re in “Salesforce Park.”

As for the rock types, these are mostly gabbro (basically, quartz-free granite), some with differing degrees of metamorphism. But each one is distinctive in details that geologists appreciate: petrology, deformation, texture. A selection:

Some of these photos show the texture that made me say “gneiss” — a banded appearance with dark and light layers. With closer inspection, though, they aren’t layered enough. They’re just a little smooshed, not stretched out like taffy.

The other thing I mentioned is the similarity of the boulders. First, they are similar in their provenance — where they come from. The only information I’ve been able to glean is that they were sourced in “the Pacific Northwest.” That doesn’t sound like much to go on, but Oregon and Washington are mostly volcanic, and these gabbros are not volcanic; they’re once-molten rocks that cooled slow and deep.

Only three areas have such rocks: around Medford in southern Oregon, in the Blue Hills of northeast Oregon, and in north-central Washington. I favor Washington, and my evidence is in the second similarity: their surfaces.

I notice two things about these boulders in general. First is that their shapes are all natural — they’re field stones, not chunks broken in a quarry, and they show some degree of rounding.

Boulders don’t just round themselves; it took a very vigorous environment to make these. Something like the enormous snowmelt flows that once ran down all the rivers of the Sierra Nevada, leaving streambeds like this, in the upper Stanislaus River. The largest rock here is the size of an SUV.

However, the Sierra rocks are scrubbed fresh and smooth, and that’s the second thing about the Transbay Transit Center boulders: they were all tumbled to a rough rather than polished state, and their surfaces have since then been exposed at the Earth’s surface for a particular amount of time, not long enough to decay into clay minerals or crumble apart, yet long enough to acquire rusty colors from iron oxides. A short time, geologically speaking, measured in thousands of years.

This surface staining takes the form of red-brown streaks and spots, as in the photos above, and an all-over patina in some cases.

Here are two different versions of the altered surface, a crust a millimeter or so thick, formed on a coarse-grained gabbro, and a thin bronze sheen well developed on a fine-grained version. In both photos the underlying rock is exposed by chipping.

All this evidence points me to a scenario in which a deeply exposed body of gabbro was broken into large pieces, which were tumbled briefly and left piled upon each other, perhaps in a steep talus slope along the foot of a cliff. There the wind and weather gave them their delicate earth-tone finish. I picture a locality along the Columbia River in north-central Washington that was inundated, over and over, by the colossal Ice Age Floods that ended (for now) about 13,000 years ago.

These boulders are cool. Give them a pat as you take in San Francisco’s newest public park. The NL bus takes you right to it, a world-class ride.

The rocks of Mulholland Hill

3 September 2018

Over the last couple years, I’ve been more and more tempted by Mulholland Hill, the ridge shared by Moraga and Orinda that dominates its area and shelters the former village of Rheem. Tempted because I crave summits, but also tempted because its rocks, named the Mulholland Formation, are interpreted as the youngest in the region.

The Mulholland Formation is mapped in two shades of light tan on the geologic map, due east of Oakland. It extends from downtown Orinda past Moraga and into the watershed lands to the south; a finger of it (not shown) sticks beyond the rest across Cull Canyon and all the way to Crow Canyon.

Mulholland Hill sits in the northern part. Much of it is preserved as open space, and that’s where I went to see its rocks.

But first, what does it look like? This 2016 view east from the ridge above Wilder Valley shows Mulholland Hill’s level top just in front of Mount Diablo; the grassy ridge dominating the view is another hill that overlooks Lost Valley.

This February 2018 view north from Redwood Ridge shows Mulholland Hill against the horizon left of center, dotted with homes and trees.

And here are two closer views, the first looking southeast from 1204 Hill:

And the second looking northwest from Alta Mesa Drive in Moraga last week.

Here’s a closer look at the geologic map between downtown Orinda at top left and downtown Moraga at bottom right, showing the north half of the Mulholland Formation that underlies Mulholland Hill.

The formation is divided into upper and lower parts (Tmlu and Tmll respectively). Notice how the lower part flanks the upper part on both sides. That’s because the whole thing is folded like a taco, so the older rocks wrap around the younger rocks — a configuration called a syncline. The upper rocks have more sand and gravel in them and resist erosion better than the muddier lower rocks.

The paved trail is the middle part of Donald Road; you can get to the open space on Donald Road from north or south. I came up from the south and recommend that unless you’re in a hurry.

Along the way you may see cattle. Moraga originated as a cattle ranch in the 1840s, so these represent an old tradition. For all I know, Moragans still fill their household freezers with artisanal grass-fed Moraga beef.

Get off the pavement to see bedrock poking through the soil. It’s coarse sandstone with a fair share of pebbles.

Elsewhere it’s full-fledged conglomerate, mostly pebbles that represent a variety of different rock types.

These rocks are interpreted as freshwater deposits, laid down by a vigorous river draining hilly terrain. Nearby exposures of this unit contain horse bones and teeth and plant leaves that fix its age around early Pliocene time, some 5 million years ago.

These rocks are pretty tightly folded. This detail from the geologic map shows the direction and angle that the rock beds dip into the hill. You can see that over a short distance, their orientation changes by roughly 90 degrees. As surely as folding a taco, that would push the central belt of rocks upward. The red line with the arrows indicates the syncline’s axis and sense. (An opposite fold, with the arrows pointing away, would be an anticline.)

But by all means, look around from the top of Mulholland Hill. Depending on the weather and the direction, the vista can be stern, like this view of Round Top and the Oakland Hills,

or grand, like this view of Las Trampas and Rocky Ridges, with Bollinger Canyon between them,

or just splendid.

A fine place to visit. It’s also prime raptor habitat — but if you’re a birdwatcher you probably already know about it. I’m tempted to return.