Archive for the ‘The Hayward fault’ Category

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.

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.

Lake Temescal, the west side

12 April 2021

For one of Oakland’s most rugged places, the west side of Lake Temescal doesn’t expose a lot of rock. But what’s there is unusual for Oakland, and interesting.

I’m talking about this ridge — tectonically, a shutter ridge — across the lake from the swimming beach.

The map of Lake Temescal Regional Park shows two trails there, the low one along the water and the high one up in the woods. An even higher trail, not marked on the map, is off limits and doesn’t expose much rock anyway.

I often wish I’d lived here in the 19th century when Oakland was new. It was in 1868 when Anthony Chabot acquired a steep little canyon back of the hills and built a dam to supply the young city with dependable water service. His technique, perfected in the gold fields of the Sierra, was to take a high-pressure water hose and wash down the sand and gravel from the hillsides to build the dam. I wish I could have inspected the scrubbed slopes at that time, but there were almost no trained geologists in the whole state, let alone me.

The canyon has a flat floor now, after decades of sedimentation, but you can see from the high trail that it’s still steep and narrow.

And the action didn’t end when Chabot finished the dam. He built a control tower in the new reservoir, but a landslide soon took it out. I’m guessing that was probably near the sluicegate where the beach house is today, and I’m guessing that the rainy winters of 1868-69 and 1869-70 plus afterslip and aftershocks of the big 1868 earthquake on the Hayward fault had something to do with it.

Speaking of which, two major strands of the Hayward fault run right through the reservoir. They’re helpfully shown on the map above. This is where the main strand crosses the dam. (Don’t worry, the massive dam will not fail even under the largest possible quake on this fault.)

The dam itself was raised and then lowered during the next few decades, and presumably the lake rose and fell too. Meanwhile trees and brush moved in upon the slopes where the Ohlone had previously maintained grassy meadows, and the rocks decayed and soil built up.

And the rocks themselves embody the complicated history of starting out in a vigorous subduction zone, being deeply buried and exhumed probably more than once, then being torn up and shoved around by the San Andreas fault system of which the Hayward fault is part.

All that is to say that Lake Temescal is a dynamic area at all time scales. It’s more complex than the small-scale geologic maps can show, even though it’s complex enough on that map.


KJfm, Franciscan melange; ch, chert block; af, artificial fill; sc, silica-carbonate rock; sp, serpentinite; Jsv, Leona volcanics; fs, Franciscan sandstone; KJkm, Knoxville Formation

The west side is mapped as melange, which is basically a mess of mashed-up sandstone with big blocks of other rocks, each with its own separate history, suspended in it. The little blip labeled “ch” is this block of chert at the top of Hill Road. So when I visited the west side trails last week, I expected to find things I didn’t expect. And most of the rock along the trails appears to be nondescript sandstone — I say appears because since hammering is forbidden, it’s hard to find a fresh surface. But lo and behold, along the high path coming down from Broadway Terrace, there’s the telltale gleam of blueschist in the exposed subsoil.

The color really comes out when you wet a piece.

Other apparently high-grade metamorphic rocks include this hard, glittering schist. Not having access to thin sections and petrographic microscopes, I can’t check for the presence of jadeite, which has been reported in blocks from this melange.

And over at the north end of the train is a distinctive outcrop of another schist. Hard rock supports slow-growing lichens, and the species differ depending on the rock’s chemistry.

Underneath the lichens, the rock is a bluish-gray mixture with a strongly folded texture, both signs of a rock that’s been through a lot of distortion at high pressure and temperature. These photos are from exposures by the lawn. The first shows the folding and the second shows fault-related crushing on the left side. The crushed material is called gouge, and bits of it are common in this sector.

My authority on Oakland’s Franciscan rocks, John Wakabayashi, holds that the west side of the lake hides the same ancient major thrust fault — a megathrust, in fact — that’s famously exposed in El Cerrito above the recycling center, where high-grade rocks have been pushed above lower-grade rocks. Unfortunately the fault itself appears to be in “a brush-filled gully with no exposure,” so it may be a while before we ever learn more.

But I did learn something more last week, about the beach house.

It is made with the local rock, namely the Leona volcanics. Whether the stones came from the hill just upslope to the east, the one that collapsed in the 1870s landslide, or from across the freeway in the great Tunnel Road cut during the 1930s, I do not know. I’m guessing the former, but I would be happy to be wrong if it means I can be certain. Putting the building and the roadcut in the same photo, there is a resemblance.

The fun thing is, both locations can be closely inspected. That sounds like a good afternoon project.

Arroyo Viejito

6 January 2020

Some of Oakland’s most interesting land is also its most inaccessible; I’m speaking of our streambeds. And on the whole, the largest remaining stretches of wild streambed belong to Arroyo Viejo. Just to orient you, here’s the Arroyo Viejo watershed, as it’s mapped today by the Alameda County Flood Control District. The red stripe, which I added, represents the Hayward fault. (I’ll return to that.)

Here’s a zoom-in to the lower right corner, showing the upper part of Arroyo Viejo and the valley of a defunct little stream that I’m calling Arroyo Viejito.

The peculiar feature that caught my eye several years ago is how Arroyo Viejito runs parallel to Arroyo Viejo, very close to it, with a distinct rocky ridge between the two streams. Today the two valleys are very different, and a century’s worth of maps hints at what happened. Here’s the 1897 topo map showing the two streams, underneath the word “Viejo.”

In 1915, the area was more accurately mapped, and the two streams are shown as extremely close together at one point.

Everything changed after this. The country club was expanded and the adjoining land was subdivided and developed into the very exclusive Sequoyah district starting in the early 1920s. At that time Golf Links Road was pushed through to what would become the Grass Valley district in the 1950s, and Arroyo Viejito was diverted into the large stream at their closest approach and a sewer line inserted into the abandoned valley. It was very handy for the developers. As of 1947, the little stream had vanished and the land lay open for a new wave of luxury homes.

As of 1980 the buildout around Arroyo Viejito was complete.

The sewer line is accompanied by a maintenance road that is now a nice place for the locals to walk, and it connects with the little-visited creek trail at the north edge of the zoo’s property. I featured this area, in passing, three years ago in Ramble 3.

The reason these two streams ran so close together is related to the Hayward fault. It’s been dragging the lower, western half of Arroyo Viejo north, and for the last few hundred thousand years the stream has stretched out along the fault line before turning toward the Bay. Models of landscape evolution suggest that the headwater streams have been getting squeezed, aligning themselves and crowding together.

The combination of an especially large earthquake and a major flood could cause Arroyo Viejito to break through the narrow waist and join Arroyo Viejo farther upstream, abandoning the stretch with the sewer line and leaving the ridge standing there for a few more thousand years until it erodes away. But impatient developers have short-circuited all of that, and now the little stream is defunct, its former catchment part of a sterile golf course.

As I said, it was the ridge between the two streams that caught my eye and dared me to set foot on it. It’s in the middle of this Google Earth view looking west.

Its sides are very steep; it’s like an island. One day I found that it has a tiny trail running along its top, and signs of an old road and excavations. My guess is that the ridge was dug up for fill material when the sewer line was put in. The high-resolution lidar data acquired along the Hayward fault a few years back covers the west half of the ridge, and the resulting digital elevation model (with the trees and buildings stripped away) shows these features plainly.

Lately I’ve visited this ridge and the stream valleys of both Arroyos, in search of access and ultimately in search of rocks. Access beyond what I’ve already mentioned is difficult, and I have paid dearly for it in poison-oak rash. But I shall return.

The bedrock map looks like this, but I am suspicious of all of it given the difficulty of access and the paucity of outcrops. One big goal of mine has been to inspect the stream bed where bedrock might be exposed, for some real ground truth. I suspect that geologists, while doing their best, have resorted to drawing lines based on the topography.

The green zone marked KJk is shale and conglomerate of the Knoxville Formation, and that’s what I’ve always found in the eastern chunk of it. This shale is just west of Golf Links road where it crosses the creek.

And the conglomerate is abundant as loose boulders (not bedrock) downstream. It’s beautiful stuff.

But I have found none of it yet in the western section. Instead, everywhere I’ve looked the rock is either coarse sandstone shot with calcite veins, interpreted as the very oldest part of the Knoxville . . .

. . . or familiar rocks of the Leona volcanics (Jsv).

This includes up on the little ridge and down in the Arroyo Viejo streambed.

I still have a good bit of territory to visit, though. The streambed will have to wait until dry season, when I can poke around this weird-ass lime-cemented breccia.

And there’s more ridge to check out. Outcrops like this are so crusted with lichen that I might need to bring a rock hammer for some very careful, unobtrusive chipping.

There are some other charms in this northernmost stretch of Knowland Park. Every time I’ve visited there are fresh deer bones, indicating a mountain lion’s sphere of influence. And the cries of exotic animals occasionally drift down from the zoo’s hilltop center.

No other place in the world exactly like that.