Archive for the ‘the hayward fault’ Category

The Hayward fault by the Oakland Zoo

31 August 2015

Most people don’t know this, I think, but the Hayward fault runs right through the Oakland Zoo. I won’t take you there this week; instead let’s look at the residential area just south of the zoo. Here’s the topography from Google Maps, tilted 40 degrees from north to bring out the grain of the land.


The fault runs from the intersection of Golf Links Road and I-580 at the top, between the words “Oakland” and “Zoo,” through the S in “Hood St,” and across the base of the truncated hillside at the bottom. The USGS map of the fault trace shows the specific features of the fault here.


You’ll see things marked in the area just south of the zoo, a length of the fault labeled “G1, sl” and a dotted oval marked “G1, df.” G1 means a geomorphic feature (a landform) of “strongly pronounced” character, the most clear-cut kind of evidence. The first item is a linear scarp and the second is a fault-related depression. They’re somewhat visible in Google Earth if you tilt the view and squint (800 pixels).


If you go there, all this is more apparent. Here’s a view west, toward the bay, down Hood Street across the odd level spot marking the fault. The cross street is Mark Street.


This is looking north up Mark Street toward the zoo. The depression is behind these houses, in their back yards (800 px).


And this is the view toward the Knowland Park hills back up Hood Street from the bend at its west end (800 px). It’s very odd for the steep slope of our foothills to be interrupted this way. Normally a valley like this would be carved by a stream, but none is evident.


In all of Oakland, there are only four places where geomorphic evidence of the fault is ranked “strongly pronounced.” Two of them are now obscured: one was a line of vegetation in Redwood Heights that quarrying has removed, and the other was at Sausal Creek where Park Boulevard crosses the Warren Freeway. The remaining high-grade feature is the valley south of the LDS Temple, which is inaccessible and highly disturbed by landsliding.

So this is Oakland’s clearest trace of what the fault has done to our landscape. To me it looks quite similar to the Jordan Road stretch of the fault.

Even with all that build-up and explanation, you don’t really see much here unless you know what you’re looking at. But this shot I took in 2005 looking south from the zoo parking lot shows the depression pretty well.


Next time you visit the zoo, take a peek. You might also see if the zoo parking lot is showing cracks from fault creep. the last time I looked, they had just repaved everything.

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.


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.


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 . . .


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


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.


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.


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.


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.


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