Triple creek junction

17 March 2015

A while ago I featured the casting ponds at McCrea Memorial Park, in the valley of upper Lion Creek. I didn’t poke further downstream at the time, but since then I have. The creek runs alongside a pair of small concrete “trout ponds,” cunningly made with cobble-lined runnels that would send a lifegiving trickle through them if there was enough streamflow, which there isn’t, to sustain fish, which there aren’t any of.

leonack-troutponds

The lower pond did have enough water in it to interest a mallard couple. The male kept watch on me as the female gorged on duckweed.

To the left of that photo, just over a low ridge and behind a fence, the iron-stained waters of Sulfur Mine creek exit a pipe.

sulfurmineck-outlet

Farther down, it joins Lion Creek and the combined watercourse enters a tunnel beneath the Warren Freeway. I haven’t gone into it, but people clearly do.

sulfurmine-lion-ck-junction

If you cross the freeway on the little-used pedestrian overpass, you might expect to find the creek on the other side. Instead you’ll see Horseshoe Creek coming out of its canyon in Leona Heights Park and entering its own final conduit.

horseshoeck-mouth

Where the two creeks meet must once have been a cheerful place. Today it must be a black and dismal one, visited only by rare daredevils. If the tunnels aren’t screened, perhaps they can crawl all the way down to where Lion Creek reemerges, at Lake Aliso at Mills College, in a wretched feat of urban spelunking.

The 2015 California Earthquake Forecast

11 March 2015

The U.S. Geological Survey issued a major update to its statewide earthquake forecast yesterday, the Third Uniform California Earthquake Rupture Forecast (UCERF3). No surprise, the news media boiled it down a little too far for my taste. For our side of the bay, there wasn’t a lot of change in my mind. Click the image below for a larger and wider image showing the whole Bay area. The whole report is online.

UCERF3lilmap

The Hayward fault is now officially considered to have a much higher risk of a very large, magnitude 7.5 or so, earthquake. (The figure in parentheses shows the percentage of the change in assigned risk.) This is because now we’ve added the possibility that not only the whole fault, but its neighboring faults (Rodgers Creek on the north and Calaveras on the south) could join it in an oversize rupture. Scientists (and I) have known about this change for a while because we follow the literature, but the new forecast is a formal admission.

The headlines and radio blurbs have been easy to misinterpret. The Tribune this morning was typical: “CALIFORNIA’S CHANCES OF ‘BIG ONE’ GROWING.” But look at it this way. The state of the Earth’s crust is hardly different from what it was seven years ago when the USGS issued its previous forecast. It’s we who have changed—our knowledge and models have progressed. We have a better idea of California’s chances of a “big one.”

The people who will study this in detail are doing things like setting earthquake insurance rates and designing large structures. For the rest of us, there is no change. We still live in earthquake country. We still need to work on our personal readiness. The largest events still will be rare. Better for Oaklanders to prepare for the smaller but still destructive magnitude-6 earthquakes, like the one in Napa last year. We will experience more than ten times as many of those, and they are worrisome enough.

What I like about the new forecast is that it isn’t really a forecast. The system has grown in sophistication and flexibility to the point that it’s really a modeler’s sandbox, a software environment that can handle surprises, new information and complexities better than ever. Talk to a seismologist and they’ll instantly agree that earthquakes pretty much always take us by surprise. The giant Tohoku earthquake, which happened four years ago today, took seismologists by surprise. You name it, the quake was a surprise. It will be many decades, maybe centuries, before this state of affairs ends.

We can’t deal with the situation using simple, linear computer models based on one idea of Earth’s behavior. The third UCERF is a supple, fine-grained instrument that takes advantage of many significant advances during the last decade. When I told a USGS quake guy yesterday how much I admired the new model, his eyes twinkled. They’re proud of this.

Landslides of Outlook hill

5 March 2015

I’ve been surveying the low hill between Mills College and Holy Redeemer College, home of the Millsmont and Eastmont Hills neighborhoods. Its western face has no bedrock, either on the geologic map or in my experience. Here’s the relevant portion of the geologic map.

outlookmap

Its crest is supposedly Jurassic basalt, which would be part of the Franciscan assemblage. But the Hayward fault runs right along its length, and I lean toward calling it a pressure ridge. Long story short, it is squeezed up, shattered, and oversteepened, and these make it prone to landslides. Here are some, starting with the notable example at the top of 64th Avenue. This is its toe . . .

64th-beunaventura-slide

. . . and this is the view from its head, at Delmont Avenue.

64-buena-slide-top

Another is above Outlook Avenue, south of 76th Avenue. As you walk along its base, you’ll see bits of concrete from the homes that once stood here.

outlook-76-slide

Above it, on Hillmont Drive, there is a gap in the houses that offers a nice view. I have no business saying whether a landslide is responsible.

outlook-76-slide-top

Between these two obvious slides are some fine hillsides. This one, below Simson Street, makes a lovely backdrop to the Eastmont mall and, it seems, a nice informal park for the residents.

simson-field

It isn’t really vacant—all of the lots that subdivide it are extremely long for some reason. I think that spaces like this, shared without fuss by the landowners around it, are very precious.

A peek inside the Fan on Piedmont Avenue

19 February 2015

Construction is going on at the lot formerly occupied by a well-behaved motorcycle club, at 4225 Piedmont Avenue by the Kona Club. What caught my eye is that it offered a clean cut into the stuff that constitutes lobe 2 of the Fan.

I’ve referred to the Fan often over the years, but I haven’t formally introduced it. Here it is on the Oakland geologic map.

the-Fan

It’s a former alluvial fan that was last active during the Pleistocene, which has been dissected by several younger streams. There’s nothing else quite like it in the East Bay, and I think of it as the Fan with a capital F. I divide it into eight separate lobes. Lobe 2 has two separate parts, Pill Hill and Montgomery/Thermal hill. Anyway, I keep an eye on it because it’s rarely exposed. Only excavations and a few stream banks display it.

Here’s what it looks like from a distance.

piedmont4225-1

There’s indistinct bedding that slopes down to the left. The material is gravely clayey sand that’s quite firm and well behaved. Here’s a closeup of a gravely layer; the stones are large pebble size (about 50-60 millimeters) and represent the Franciscan rocks just uphill in the Piedmont block.

piedmont4225-2

Farther over, the wall of the excavation has been carved with a backhoe, and the clayey matrix is so strong that most of the stones have been cut in two, even the tough black argillite.

piedmont4225-3

This is alluvium—sediment carried and laid down by streams. The rock clasts are rounded, showing that they’ve been carried in a stream for some distance, although most of the rocks are sandstone that doesn’t endure long. The hardest chert pebbles are still pretty rugged.

Down on the ground was this very typical Franciscan chert boulder, shattered by the builders after enduring for more than a hundred million years.

piedmont4225-4

The lot will become a nice set of dwellings. The builders are blogging about the job, complete with cool drone shots.

Seminary Creek at Mills College

14 February 2015

The third creek running through Mills College is Seminary Creek. It gets its name not just from Mills, but from the Beulah Heights district that forms its headwaters. On the 1897 USGS topo map it’s the dashed blue line, signifying an intermittent stream, running due south to East Creek (now named Lion Creek).

mills-SemCk-topo

Nowadays it’s almost entirely culverted. Not only that, it’s been kidnapped! The Oakland Museum of California’s creek map shows the creek as being redirected away from Lion Creek. Today it runs just north of Seminary Avenue to a channel called East Creek Slough.

lion-seminarycreekmap

What glory the creek still has today is evident only on the Mills College grounds. It ducks between lobes 6 (Maxwell Park) and 7 (Mills) of the Fan next to MacArthur Boulevard, but a close look at the topography suggests that a landslide or earthquake could easily have made it spill to the east of lobe 7 (past the triangle marked “30” on the stream map). Be all that as it may, the creek daylights just where MacArthur curves west.

mills-SemCk-intro

It then wanders a couple hundred yards through a nice quiet forest, mostly eucalyptus. Other than putting a parking lot over half its course, the college appears to have left Seminary Creek alone.

mills-SemCk

When it hits MacArthur, at 57th Avenue, Seminary Creek disappears. Here’s the last sight of it, from the MacArthur side. Here’s where the stream got kidnapped.

mills-SemCk-outro

I’ve taken photos of the creek farther downstream, but it’s a nondescript ditch and now I feel sorry for it. So I’ll spare you.

Interestingly, the old map showed the creek as intermittent, but I’ve never seen it dry even after our three-years drought. As I mentioned about Chimes Creek, I think undergrounding Seminary Creek has kept it from evaporating.

Chimes Creek and the Hayward fault

7 February 2015

Chimes Creek is the second of the three streams in Mills College. It is said to get its name in reference to the college’s church bells. The sound would have traveled up the creek bed to the meadows behind Millsmont ridge. Today the freeway noise drowns them out. Here’s how it looked to the mapmakers of the U.S. Geological Survey in 1897—it’s represented by the dashed blue line in the middle. Below that is the same patch of land in Google Maps as of today.

chimescreekmap1897
chimescreekmapnow

The land has been changed substantially in the last 118 years, but the creek continues to drain its catchment. Let’s look at the changes from the top down:

  • The headwaters have been filled and paved and are now occupied by Viewcrest Drive.
  • The Leona Quarry removed all the overburden below a short stretch of the upper creek, exposing bare rock.
  • The flats beneath have been leveled and developed, and the creek is culverted.
  • Seminary Avenue has been widened and straightened, putting more of the creek underground.

All of these changes have added to the runoff seeking to enter the creek while constricting its course. A stream will respond by running higher and faster and eroding its banks.

I haven’t yet visited the highest part of the catchment. Here’s a look at it down Altamont Avenue.

chimescreek1

The original creekbed is high above the left edge of the quarry, and the creek ran toward the lowest part of the foreground. Next is the view one block over, at Delmont Avenue and Hillmont Drive looking north. The creek comes out of its culvert behind the houses on the left.

chimescreek2

I should note that the Hayward fault is mapped running right up the valley to this spot. That’s an important detail that no one seems to acknowledge. For my purposes in this post, it means that Chimes Creek is probably cutting downward through fault gouge, the finely ground material that faults make all over California.

Farther downstream, this is looking across the creek valley at Nairobi Place. The sides are quite high here because the stream cuts downward rather strongly.

chimescreek3

The presence of the Hayward fault also explains why the right (opposite) bank of the creek valley is elevated above its surroundings—it’s not a levee, but rather a pressure ridge. Farther downstream along Oakdale Avenue, the valley is at its deepest.

chimescreek4

The lots along Hillmont Drive, across the creek, are being undermined as the invigorated stream does its work.

chimescreek5

I’ve love a good look at this material, but I’ll probably never get the chance. The geologic map shows this area as the northernmost splinter of the San Leandro Gabbro.

The creek enters a culvert under Seminary Avenue here . . .

chimescreek6

. . . and emerges here on the grounds of Mills College for a couple hundred feet. Then it enters its last culvert and joins Lion Creek underground.

chimescreek-mills

The Chimes Creek Neighbors site has thorough documentation of the human squabbling over this much put-upon watercourse. The neighbors know it as a permanent creek, although the 1897 map showed it as intermittent except for its lower reach on the Mills College campus. I suspect that the land-use changes of the last century have turned it into a permanent and more powerful stream.

Some East Oakland stones

31 January 2015

Oakland’s rocks aren’t all in the ground. They’re in our yards and homes, too. Here are a few presented in the order I found them lately.

There’s a house on 60th Avenue that stopped me in my tracks, its walls studded with stones. A neighbor down the block told me “Oh yes, those are wonderful! We looked at that house when we were buying in this neighborhood. The owner’s daughter runs a preschool across the way.” Click that one for an 1100-pixel version.

morse-at-60th-450

A few weeks later I visited Best Avenue, high in Maxwell Park, and was arrested by the front yard here. Sometimes rocks, like people, look best with painted faces.

bestrox2

A block away is another property treated by the same decorator.

bestrox1

And just yesterday this basket of painted stones seized me by the eyes. It’s at the Free Oakland UP gallery and workshop, in the Loard’s plaza at Coolidge and MacArthur.

oakland-up-rox


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