Archive for the ‘the hayward fault’ Category

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.

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.

Lake Aliso

30 December 2014

Mills College occupies a geologically interesting part of town. It owes its stimulating geomorphology to the confluence of three streams under the influence of the Hayward fault. I plan to write several posts about it. Here’s the segment of the geologic map that includes the campus.

mills-college-geomap

The main strand of the fault runs just left of the “Jb” symbol. The narrow lobe of tan, symbolizing Pleistocene alluvium, is where Lion Creek turns from its southward course and cuts across a low ridge of Jurassic basalt (Jb) to cross the fault. I have to say that I haven’t yet found any basalt there, so treat the map with caution. After every large earthquake, whenever and wherever the ground is uplifted the creek, momentarily dammed, gathers its strength and cuts its way through to maintain its right of way. But a flat spot in the streamcourse persists above the fault trace, and there may be a tectonic element at play there too, downdropping the spot in a sag basin. In any case, that wet spot is where the college’s administrators erected a dam to create Lake Aliso, a picturesque basin that was also useful (1) as a water supply for landscaping purposes and (2) for regulating the creek in an attractive state of flow, neither flood nor trickle, as it traverses the campus.

Old photos show the lake as a fine place for boating and pageants, but sediment has inevitably filled it in. Today it’s trying to return to marsh, and from there it aims to retire as a nice meadow.

lake-aliso

But we made the lake, and we can maintain it with enough money and machinery. Here’s Lion Creek, such as it was, at the lake’s inlet, which must date from the building of freeways I-580 and Warren.

lake-aliso-inlet

My visit was a few weeks before the December rains but after November’s whistle-wetting, so the water was cloudy with fresh sediment and possibly some of that ugly runoff from the old sulfur mine. Right now Lion Creek should be closer to roaring.

The other end of the lake is an earthen dam, including this spillway.

lake-aliso-outlet

It demonstrates one of the basics of managing streams of any size: If you block a stream, it will silt up its bed on the high end and start eroding its bed on the low end. Another way to think of it is that when we mess with a stream, it usually backfires in the long run. The guidance of a licensed geologist with some expertise in hydrology can help forestall the inevitable.

There is some loose rock around, most of it looking like this.

lake-aliso-rock

Although they may just be landfill, I assign these stones to the “Jsv” unit—the metamorphosed volcanic rocks that make up the high hills.

The Wild Oakland walk on the Hayward fault

9 November 2014

Saturday I led a walk for the members and friends of Wild Oakland to show off one of Oakland’s most striking places to encounter the Hayward fault. There was a nice turnout, about 60 people. I was glad to see so much interest. I hope that this post will enable those people, as well as all of you readers, to visit in person and learn more.

Here’s the route we took. It was just over 3 miles, although the altitude gain in the middle made some people bail out. Next time I’ll try to have alternative routes for their benefit.

WildOakwalkmap

The numbers refer to the stops during the walk. The asterisks refer to direct evidence of the fault’s activity, both on and off the day’s route.

Next is the same map with topography added. The thrust of the day’s exercise was to tour some distinctive features that the Hayward fault has left on the landscape.

WildOakwalkmapterrain

Stop 1, where we started, is where Arroyo Viejo does its abrupt 90-degree turn on its way from the hills to the bay. The right-lateral Hayward fault has dragged the Bay side of the landscape to the northwest, and the creek has had to bend in response.

WildOakwalk11-14-1

It’s a vivid example of how plate tectonics works in California, caught between the Pacific and North America plates. As the Pacific plate moves northwestward, pulled in that direction by subduction zones off Japan and Siberia and Alaska, it moves sideways—right-laterally—with respect to North America. That distorts the courses of streams that cross the boundary between the two plates. That plate boundary is a wide zone with three main sets of major faults running along it. The Hayward fault is in the middle set.

At Stop 2 I was able to point out a good example of creep offset, where the curbs on both sides of Encina Avenue have been cracked and shifted by creep (slow motion, less than 10 millimeters per year, without earthquakes) on the fault.

WildOakwalk11-14-2

Stop 3 gave us a decent elevated view of the fault zone from the Oakland Zoo grounds through that offset valley of Arroyo Viejo. (Here’s an earlier post showing the other direction.)

At Stop 4 I pointed out another probable example of creep offset, and everybody turned on cue to look at it.

WildOakwalk11-14-3

Stop 5 was at a highly disturbed bit of ground on Ney Avenue. The scarp crossing the road appears to be the head of a landslide right on the active trace of the fault.

WildOakwalk11-14-3a

Stop 6 was on a hilltop in the King Estates Open Space with a high view over the fault zone and the rest of the Bay area. The set of smooth-topped ridges extending into the valley of Arroyo Viejo have been cut off to form shoulders, and the dramatic shutter ridge on the right is the landform that has forced the stream to run sideways around it instead of straight to the Bay as it would prefer. (Here’s an earlier post about this same view.)

WildOakwalk11-14-4

I am quite taken with King Estates, and I believe it to be the largest piece left of the East Bay hills’ original landscape of grasslands. As the rains come this winter, I hope some Wild Oaklanders will poke around and examine it closely. For most or all of the people who came, it was their first visit.

click for 900-pixel version

Along the way is this nice little example of a landslide.

WildOakwalk11-14-5

The King Estates hills are mapped as alluvium of an earlier generation than the Pleistocene alluvium that makes up East Oakland’s low hills. I wonder two things about them. Are they a pressure ridge, pushed up by compression across the Hayward fault? (I noted that the fault’s motion is 90 percent strike-slip and 10 percent compression.) And what is to be learned from the blend of stones that practically forms a pavement on the hills?

WildOakwalk11-14-6

Stop 7 was a view into the watersheds of the three creeks above the fault: Rifle Range Branch, Country Club Branch, and Arroyo Viejo. And Stop 8 was in the valley of Country Club Branch, so close to its neighboring streams but so well separated from them by elevated divides. I blame the fault, which keeps jolting the countryside out of the equilibrium it seeks.

For dessert, I present a portion of Jim Lienkaemper’s 1992 map of the fault, which has annotations about the detailed evidence along it.

WildOakwalk11-14-features

The features marked G are geomorphic—things geologists notice in the landscape. Those marked C are hard evidence of creep—offset curbs (rc), sidewalks (rs) and fences (rf), and at Stop 2, echelon cracks (ec) across the road that have been erased by road repairs since 1991 when the map was compiled. You can download the whole map and consult the updated version from 2008 if you like.


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