Archive for March, 2011

Oakland gets a (small) tsunami

18 March 2011

tsunamis

Oakland is fairly safe from tsunamis. Not as safe as San Ramon, but safer than most Pacific ports. First of all, we sit rather far from any dangerous subduction zones. When you track the subduction zones of the Pacific’s “ring of fire,” you get a continuous zone from Chile up to Puerto Vallarta, then jump to Cape Mendocino where the Cascadia subduction zone begins. We’re in between, and huge thrust-type quakes are not tectonically possible here.

A giant quake in Cascadia or the Aleutian Islands subduction zones would send waves down this way, possibly up to 2–3 meters high (this is the height out at sea; as they hit shore they would rise much higher). That’s where our second line of defense comes in: the Golden Gate. Anything coming through there would first be throttled by the narrow strait, then would spread out once it enters the bay. The people who prepared the Alameda County tsunami inundation maps also considered a few local extreme earthquakes that may have components of thrust. These could raise local tsunamis, but not of the horror-inducing size we saw in Japan.

I don’t want to minimize the possibilities, but the Sendai scenario cannot occur here. The harbor, however, is liable to damage even in a modest event, as we saw in Santa Cruz. Ships will bang into docks and each other; a less obvious hazard is that swift currents may pluck ships off their anchors and wash mud into our carefully dredged channels. You won’t want to go down to Jack London Square and watch if there’s a big local quake or a great event in Alaska or Cascadia. But last week would have been OK. Did anyone see the water there?

Oakland groundwater

11 March 2011

groundwater

I’ve been taking National Ground Water Awareness Week to think a little more than usual about groundwater. My KQED Quest blog post yesterday, on Bay area groundwater, mainly focused on the South Bay. Oakland’s not a big groundwater town.

The rancheros and early Anglo settlers here all dug wells, of course. As I understand it, Dunsmuir House still has an operating well, the Pardee Home has a water tower that suggests the presence of a well, and some of the other old properties must have them too. Some long-standing Oakland industries probably have wells, and maybe the golf courses too. I don’t know a lot about it.

But municipal water service from Oakland’s earliest days exploited local surface water, starting with Temescal Creek and ending with San Leandro Creek (see the two dams post). Lion Creek supplied laundries in the area near Mills College once called Laundry Canyon. Today we’re all served by East Bay MUD with clean Sierra runoff from the Mokelumne River watershed. In Oakland, surface water rules.

Today groundwater is off the radar here. Sure, we have to clean it up where old gas-station tanks used to leak—this monitoring well is from one of those. It seems to me that the aquifer west of Chabot Dam, in the alluvial fan crossed by San Leandro Creek in far East Oakland, must have good potential, and so would Fruitvale and Temescal. San Francisco is opening up its formerly used aquifers to serve emergency purposes; we ought to look into that too. Why go to such expense to clean up the groundwater and not get some sustainable use out of it at the same time?

Pebbles and perils

3 March 2011

pebbles

I was writing a post on Pebble Beach for the KQED Quest site. The pebbles there are special because they represent a huge variety of source rocks, pieces of which ended up offshore in deep-sea rocks back in the Cretaceous. Those rocks were pushed up above the sea and eroded again, freeing the pebbles for several more rounds of polishing and recycling. Then they were sifted in the surf to isolate them in a pure deposit.

Oakland wasn’t lucky that way. The sediments here derive mostly from finer-grained rocks, and the big exception, the Oakland Conglomerate, didn’t include such a variety of gravel clasts. And even then, our sediments don’t seem to have enjoyed a vigorous winnowing and polishing in the Pacific waves. Rather than shiny pebbles, our shores are mostly clay and mud.

But not entirely. The Franciscan block that underlies Piedmont and neighboring parts of Oakland contains the grab-bag of seafloor rocks typical of melange, and the debris it has shed down Oakland’s creeks has made it to the bay. So have bits of the basalt in the high hills. One place that these pebbles appear is in a little spot along the shore of Lake Merritt.

Unfortunately the waves of Lake Merritt will never turn them into a dazzling display. However, neither will Oakland ever be ground into bits like the retreating San Mateo coastline. Our dwellings will never fall down the eroding coastal bluffs like they do in Daly City. I can live with that.


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