Archive for September, 2014

The Las Aromas serpentinite

26 September 2014

The southern edge of the Piedmont Pines neighborhood butts against Joaquin Miller Park, and there’s no reason to visit if you don’t happen to live there—except if you like serpentinite. The street named Las Aromas dips into the belt of serpentinite that extends up and over the ridge to Serpentine Prairie, and the rock type is well exposed. One resident has done a lot of landscaping in this inhospitable rock type.


The scaly structure is typical of serpentinite, and here it’s fairly close to the Hayward fault. Farther uphill is this exposure, where the grain of the rock is parallel to the street.


Its blue-green color is striking. And there are a few spots where friction in the recent geologic past has turned it glossy. The rocks aren’t as spectacular as Serpentine Prairie itself, but they’re still noteworthy.


I never tire of our state rock. Speaking of Serpentine Prairie, today on KQED Science there’s a story on the ongoing effort to propagate the rare Presidio Clarkia. (I photographed it a few years ago before they fenced off the area.)

After Oakland’s Big One, expect a rush of water

8 September 2014

In the days since the South Napa earthquake of 24 August, the people and press appear to be astonished as the local streams have filled with water. The Chronicle published a good summary yesterday. But this always happens with a decent-sized earthquake. It will happen here. You can expect to see this stretch of Arroyo Viejo, in the middle of Hegenberger Expressway, full of freshly released groundwater.


There were widespread reports of this kind in 1857, after the the great Fort Tejon earthquake of 9 January:

On a range of hills, about fifteen miles from the coast, in the district of San Fernando, we understand that a surveying party have discovered quite a large stream making out of the mountain and down a cañon, where, to their knowledge and complete satisfaction, not to say to their sorrow, no water was running or could be found previous to the earthquake. By the letter from Tejón, it will be seen that a similar circumstance occurred in that vicinity. Los Angeles Star, 17 January 1857

Just back of my camp was the dry bed of a stream, where in heavy rains water had at one time run; in this bed two weeks before I had sunk a well some 20 feet hoping to find water, but at that depth the earth was so dry I gave it up as fruitless. Two days after the first or heavy shock a little stream of muddy water was running by my camp which continued to increase each day, until when we moved was quite a little rivulet: no doubt the result of some new fissure in the mountain. Letter of W. E. Greenwell, U.S. Coast Survey, 24 February 1857

The effect upon some of the artesian wells in this neighborhood was remarkable: for a moment the water ceased to flow from the pipes, and then gushed out in greater volume and with more power than usual; we have heard that the channels of other wells, which had become obstructed, and ceased to discharge water, have become re-opened and the subterranean current is now flowing out from the orifice. San Jose Telegraph, 13 January 1857

The water goes away after a few weeks. UCB geologist Michael Manga explained the phenomenon in a talk I attended in May of last year: The shaking settles the bedrock, which in turn forces the groundwater it contains upward. In effect, the rock’s permeability in the vertical direction increases as a result of basement consolidation. This is the same basic mechanism that creates quicksand. Studies after the 1999 Taiwan (Chi-Chi) earthquake showed that this water would be replaced in about 140 years—which is coincidentally the average interval between large quakes on the Hayward fault in the last thousand years. The effect takes place within a rupture length of the fault—that accounts for the response of wells in San Jose to a Southern California earthquake whose rupture ran about 360 kilometers, from Parkfield to Cajon Pass.

Oakland’s droughtproof lake

5 September 2014

I’ve been walking around town a lot this year, and our perennial streams still have running water even in the third year of severe drought. If we still had any natural lakes, I think they’d be suffering, just like our reservoirs. But we don’t. Whatever else happens, we have Lake Merritt.


Lake Merritt isn’t like other lakes: it’s an arm of the Bay. So let’s relish our luck and make the most of our inexhaustible, droughtproof “lake.”


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