Stop saying “overdue”

25 October 2021

The last week has had its share of local earthquake news, even though there weren’t any earthquakes nearby. It all centered around the release of volume 3, the last part, of the massive HayWired Scenario report, conveniently timed for 21 October, anniversary of the 1868 Hayward earthquake (not to be confused with the 17 October earthquake of 1989).

HayWired is a virtual magnitude-7 earthquake, complete with aftershocks, that represents a typical Big One on the Hayward fault. Seismologists created it as accurately as their science permits, then asked emergency responders, social scientists, planning agencies, structural engineers and other specialists what they think would happen to the Bay area and how they would handle it. Volume 3, “Societal Consequences,” presents all their answers, as accurate as their expertise permits.

In brief, the consequences would be dreadful. Ace reporter Ron Lin of the Los Angeles Times wrote an able summary that I’ll just point you to rather than write my own. Besides, I covered some of the same ground a few months ago.

The East Bay Times, to its credit, also ran Lin’s story, and two days later it issued a wake-up editorial, “Prepare today for next major Bay Area earthquake,” aimed at goosing its readers into action against the threat. It’s a bit overdone, starting with the opening paragraph: “Gulp.” I don’t really mind that, but the editors went on to say something sloppy that I will focus on today:

“We know that the last major earthquake on the Hayward Fault occurred in 1868 — 153 years ago. We also know that, on average, dating back to the year 1134, the fault produces a major earthquake roughly every 150 years. So, yes, we’re overdue.”

No, we are not overdue. Scientists don’t use that word because it’s a deep error in thinking. Something that’s overdue is late, behind schedule, and earthquakes don’t follow a schedule. I don’t like scaring people with inaccurate statements.

Ron Lin, to his credit, stopped short of using the O-word:

“The Hayward fault is one of California’s fastest moving, and on average, it produces a major earthquake about once every 150 to 160 years, give or take seven or eight decades. It has been 153 years since the last major quake — a magnitude 6.8 — on the Hayward fault.”

Instead, he included the uncertainty around that irresistibly tempting “average.” That was helpful, but he didn’t come up with the best word.

Even the U.S. Geological Survey creeps close to the wrong word in its excellent Fact Sheet 2018-3052 titled “The Hayward fault — Is it due for a repeat of the powerful 1868 earthquake?” It sidles up to this D-word, and by implication the O-word, by saying that “the interval between successive quakes has varied from 95 to 183 years, averaging 150 years, and it is now more than 150 years since the 1868 earthquake….” and trails off with that coy string of dots. The sentence leads with the uncertainty, which is good, but the conclusion it implies is not correct. The graphic it goes with is useful in showing the raw numbers behind the average:

There’s a rhythm to this timeline, but not a good beat. Here’s a longer timeline, currently the best we have, that presents the uncertainty of the radiocarbon-based dates in it:

Source: USGS

Those snappy stars are actually smeared into blurs. For instance, the date of that event “in the year 1134” that the newspaper cited is uncertain by over a hundred years.

Maybe I’ve made it clearer what frosts me (and most seismologists) about the O-word. Now the correct, best word for the situation on the Hayward fault is this: the fault is ready for a major earthquake. It’s primed, loaded, set to go. This is scientifically correct because we’ve measured the actual motions of the crust around the fault and know that since 1868 it has accumulated enough strain energy (the kind in a stretched rubber band) to be released in a HayWired-sized earthquake. “Ready” is not as scary as “overdue,” but sit with it and the word is pretty motivating just the same. Are YOU ready?

The prospect of reading the whole HayWired Scenario report is intimidating. I recommend Chapter R as a good summary that will guide you to specific chapters where you can dive deeper.

Load casts in the Shephard Creek Formation

11 October 2021

I see that I haven’t given my photos of this interesting feature their own page. They show some dramatic load casts on the south side of Shepherd Canyon, near the east end of Escher Drive. A load cast is made when heavy material, like a mudflow, crosses soft sediment and sinks its feet into it.

My most memorable lesson about features of this kind happened in 2008 during a visit to Point Reyes. The path leading to the lighthouse passes a chaotic scene that most visitors ignore.

The scene was an offshore basin, probably like the offshore Monterey Canyon today, where every now and then an undersea debris flow, full of gravel and sand, fell rudely upon nice quiet beds of deep-sea clay or soft mud. The results included scour marks . . .

and rip-up clasts, hunks of (easily eroded) clay swept into the flow . . .

and downward-pushing load casts accompanied by upward-pointing flame structures.

It was a great pleasure to come upon high-quality load casts in the Oakland Hills, in the mudstones of the Shephard Creek Formation, and lead group walks past them. Here’s the overall scene, photographed in February 2016. A thick layer of massive (i.e., unbedded) sandstone overlies thin-bedded shale and mudstone.

Near the base of the sandstone, on the lower right side, are these well-exposed load casts.

The previous June, I took a closeup of the underside.

Unfortunately, a Google Maps image from January 2021 appears to show that this feature has crumbled off the roadcut. That’s how geology goes in the Oakland Hills, and that’s one reason I constantly take photos. I also tell myself, in consolation, that new examples could appear on any given day.

The classic 1995 text Sedimentographica has good photos of these and many more features of sedimentary rocks. I treasure my hard copy, but maybe the publisher’s online version will outlast it. That one, YOU can enjoy.

Merritt Canyon

27 September 2021

Like all true Oaklanders, I keep coming back to Lake Merritt. In this visit, I’ll muse about the many times in the recent geologic past when there was no Lake Merritt here.

If we assume, as I do, that the uplifted block of bedrock making up most of Lake Merritt’s watershed is about 1 million years old, then this little arm of the Bay has a fairly deep ice age history. A million years rather neatly fits the period of Pleistocene time after the Mid-Pleistocene Transition, when for imperfectly understood reasons glacial cycles changed from roughly 40,000 years to 100,000 years in length.

Let us then stipulate that this part of town has gone through about ten full glacial cycles. Each time the world’s ice sheets and ice caps grew, the sea level fell by a hundred meters or so — three or four hundred feet! Here’s a recently published set of sea level data for the last nine cycles. Various lines of evidence agree, though never in exact detail, about the timing and magnitude of the changes. That’s what paleoclimate science looks like.

From Rachel Spratt & Lorraine Lisiecki, 2016, A Late Pleistocene sea level stack, Clim Past 12(4)

Each time the sea fell, all of San Francisco Bay slowly drained dry and the coastline withdrew out past the Farallon Islands. Every stream that could do so cut into the newly exposed ground, digging gulches, ravines and canyons into the young sediment where before they were prevented by the high sea level. That’s when Lake Merritt became temporarily Merritt Canyon, with Merritt Creek surging along its bottom.

Each time, Merritt Creek dug out all the gravel and mud that had filled the basin of Lake Merritt and shoved it straight out into the Bay, where the drainage ran south and then around the end of San Bruno Mountain into the Pacific. That’s right — the Golden Gate didn’t exist. Instead, the Bay area streams and the great Central Valley rivers drained through what’s called the Colma Gap.

Here’s an illustration from a publication I refer to often, Sandy Figuers’ “Groundwater study and water supply history of the East Bay Plain,” that shows the typical drainage pattern of those times.

Courtesy State Water Resources Control Board

It’s very interesting: the river ran east of the Potrero Hills in Richmond, east of Yerba Buena Island, and all the way down to around San Mateo. And Merritt Creek, right in the center of that map, pushed a big fan of alluvium — river gravel and sand — into the basin that rivaled the fans of the other major East Bay streams. It went right under Alameda. That’s because Alameda wasn’t there.

This configuration of the Bay lasted from about 630,000 years ago, when the great freshwater Lake Clyde that once filled the Central Valley broke through the hills and cut Carquinez Strait, until the last warm interglacial period about 125,000 years ago. The sea at that time rose even higher than it is today, and motion on the San Andreas fault closed the Colma Gap. That’s when another gap opened up farther north on the fault, which became the Golden Gate.

So when the next glacial age began, the whole drainage pattern of the Bay shifted dramatically. Also the winds: instead of blowing south through the Colma Gap, the ice age westerlies carried huge amounts of glacial sand through the Golden Gate and across San Francisco, across the dry grasslands of the Bay and onto the East Bay slopes. That’s when the big fields of sand dunes accumulated in San Francisco and in three places on this side of the Bay: in downtown Oakland, in Alameda and on Bay Farm Island.

The latest time that Merritt Canyon formed, Alameda sat in its way all of a sudden, and I think Merritt Creek must have drained west, down San Antonio Creek (today’s harbor estuary), not southward as shown on this figure from the same report.

Here’s part of a third figure from that report, showing the depth to bedrock in Oakland as determined in boreholes.

Merritt Canyon really stands out underground: over repeated ice-age cycles, as the Bay floor has gradually subsided, the earliest versions of the canyon now lie 600 feet below Lake Merritt.

There should be a record of successive incisions and fillings of the canyon preserved down there. It would take a concerted campaign of core drilling and seismic reflection profiling to map and characterize it, and if I were a billionaire like some people I won’t mention, I’d spend the money even though it would be a huge hassle to get the permits.

Reichert’s pit, the Fruit Vale White Cement Gravel Quarry

13 September 2021

Starting on 8 June 1871, an ad in the Oakland Daily Transcript touted “White Quartz Gravel / for Sidewalks, Garden Walks, and Carriage Drives, It Makes A Beautiful And Solid Walk!” and offered this recommendation:

Mayor N. W. Spaulding, in his recent message to the City Council, said: ‘The only macadam walks which have so far proved successful have been made from [the Fowler quarry or] the white cement gravel found in the vicinity of Fruit Vale. The latter appears to be preferable because it becomes more solidified than any other material heretofore used, being less affected by the agencies of the weather. It has been used in some localities in this city for the last eighteen months. The peculiarities which recommend this cement gravel are: that when it is exposed to the elements it becomes adhesive and firm, is comparatively free from mud in Winter and dust in Summer. This makes it a complete and permanent improvement. Sidewalks made from this material are estimated to cost about 35 cents per lineal foot for walks eighteen feet wide.’ The subscriber has now got his road through to the White Cement Gravel Quarry, and will furnish at short notice any amount of Gravel for the above purposes, by leaving orders at Gardiner & Hunt’s office, Broadway, between Eighth and Ninth Sts., Oakland, and at the Brooklyn Postoffice.”

It was signed “L. Reichert, Fruit Vale.”

This material seems quite out of place for Oakland, and its properties appear unlikely too. But I’ve tracked it to land that Reichert owned above the Dimond district, at the end of today’s Maple Avenue, where a “gravel bank” is noted on the 1878 Thompson & West map.

And we’ve been here! It’s in the land south of the LDS Temple that was ruined by the London Road landslide in 1970. And that explains the peculiarities of the material. It was fault gouge: bedrock crunched into powder by the Hayward fault.

I believe its self-cementing character comes from a significant content of calcium carbonate, which is present both in the Franciscan melange on the downhill side and in the serpentinite a little ways uphill.

Despite the mayor’s endorsement, business for the Fruit Vale Quartz Company seemed to be spotty. Business broker Andrew Baird, of San Francisco, took over for a short time in 1873 under his own name; then Reichert sold the “inexhaustible” gravel pit, and the 25-acre parcel it sat on, in July 1873 to Elias L. Beard, a prominent wheeler-dealer based in Mission San Jose. Beard is shown as the owner in later maps (misspelled Baird, probably because the adjoining parcel was owned by Julia C. Baird). The 1874 city directory lists L. Reichert Jr. as a teamster with the Fruit Vale Quartz Company — perhaps the founder’s son.

Baird tried again to sell the parcel in 1875, 1876 and 1878, the year that Beard went bankrupt and lost almost everything.

I have little idea what happened after that, except that the State Bureau of Mines annual report 38, published in 1906, recorded this as the “Packard Quarry,” of which the newspapers make no mention. And as of 1912, the land was in the hands of the Realty Syndicate, part of its enormous hillside empire. A decade later the land began to undergo the process of residential subdivision that endures to this day.