A Day on the Hayward Fault

paleoseismic trench
Jim Lienkaemper lays geofabric at the end of a trenching study. Geology Guide photo

The Hayward fault is a straight crack in the ground, about 100 kilometers long, that runs through the cities of millions of people on the east side of San Francisco Bay. Some day it will be the centerpiece of your evening news, no matter where you live, because on that day one side of it will suddenly move a few meters past the other side in a magnitude 7-plus earthquake.

I joined a group of geologists in June 1999 to take a tour of the Hayward fault.

We stopped at five significant places along it where recent work had uncovered new information, and the people who did the work told us about it in person.

The Coming Quake

Tour leader Sue Ellen Hirschfeld awakes each morning grateful that the big quake didn't come during the night, and she wonders if it will happen during the coming day. "This fault runs almost entirely through a highly developed area," she said at the start of the trip. "It's awesome when I think about what is going to happen here."

It was different from the usual geologic field trip. We saw interesting things and had fun too, but our discussions had a certain gravity. The coming Hayward fault earthquake will probably kill hundreds of people and cause damage worth perhaps $100 billion. What we're learning about the Hayward fault has profound consequences for people's lives and the region's economy.

The San Francisco Bay area has a written record dating to 1776, when the Spanish founded the mission of San Francisco Dolores.

Nobody recorded major earthquakes on the Hayward fault for 90 years, until a large one struck the East Bay on 21 October 1868. There have been none since. So the big question is when we might expect another one.

Dissecting the Hayward Fault

We can make some calculations based on the large-scale motion of crustal plates.

Those suggest that on the average, over geologic time, big quakes on the Hayward fault happen every few to several centuries. That's not good enough. We need to find evidence of actual earthquakes before 1776 and figure out when they occurred. We have to dig some trenches.

Our tour stopped at five trenching study sites. Trenching is a kind of dissection of the ground. A backhoe makes the incision, a ditch tens of meters long, a meter or so across and 3 to 5 meters deep. First braces are installed so the walls don't cave in, then people climb down and scrape the trench walls clean. They take samples, photograph everything, and make a detailed log of the various layers and features. Then the braces are removed and the hole is filled in. (See this photo tour of a paleoseismic trenching study.)

Stop 1: The Golf Course

Our first stop was at Mira Vista Golf Course north of Berkeley, a rare place along the Hayward fault where the ground has been pretty much left alone. Expensive homes surround it, and the views are terrific.

The Hayward fault runs right down the third fairway, where a foursome was playing as we watched discreetly. The firm that did the trenching work found traces of the last big earthquake there and determined that it happened around the year 1710, give or take 65 years.

The 1868 quake did not break the ground here, which is important knowledge too.

Stop 2: The Football Stadium

Next we went to Memorial Stadium on the University of California campus in Berkeley, built in 1923 directly on the fault. At that spot, Strawberry Creek comes down from the hills, takes a hard right turn at the fault line for 300 meters, then resumes its downhill course. Fault motion over the last 32,000 years did that.

One unusual thing about the Hayward fault is that it's not fully locked, like its neighbor the San Andreas fault, but rather is characterized along much of its length by aseismic creep—steady, slow motion of several millimeters per year. Creep can be seen where it pulls apart street curbs, sidewalks, and other permanent structures in the East Bay. Creep accounts for a small part of the total motion that takes place on the fault over geologic time.

The rest of that motion must happen in earthquakes. Nobody knows if that situation is temporary or typical.

Creep is pulling the stadium apart today, right down the middle. The university is strengthening the foundation as part of its SAFER Program. As we sat on the bleachers in section KK next to the gap in the stadium rim, UC Berkeley's Pat Williams told us, "It's not the highest-priority job, because with the stadium fully occupied only a few days per year the overall risk isn't as high as it is in other buildings. But on the other hand, the last large quake in this region, in 1989, just happened to occur when Candlestick Park's stadium was full for the World Series." Williams led us to a trenching study just beginning north of the stadium, hidden under some trees. It seemed that the steep hillside, near a roadway and full of tree roots, might not be a promising site.

Then we strolled a little ways south and walked up another small stream that jogs to the right where it crosses the fault. We had to be quiet, because a Buddhist monastery is right there.

Stop 3: The Busy Park

The third site is in Montclair Park, Oakland, just a couple miles from my house. A trench was dug there in 1996 on the third-base line of a kids' baseball field. A game was going on, and we sat in the bleachers while Jim Lienkaemper of the U.S. Geological Survey described what he had found. The 1868 quake tore the ground there, which had not been documented before. This trench demonstrated that the quake, once thought to be limited to the southern part of the Hayward fault, had offset the ground in some of the northern part too. In fact, there is some sentiment that the fault should be thought of as one segment, not two. That would mean that the whole thing could go at once.

On our way to the next trenching site, we stopped at a quiet residential street corner in Hayward where creep is pulling the curb apart. Sue Hirschfeld has been photographing the spot since 1971. We saw that a mark made just two years earlier had shifted by about the width of a fingernail.

(See it in my photo-tour, "The Hayward Fault in Hayward.")

Stop 4: The Old Folks' Home

The fourth site is in Union City near a stream that, again, takes a rightward jog where it crosses the fault. It's a lovely spot, and we took a few minutes to marvel at the huge old trees. The Masonic Home is a stone's throw away, a large private institution for elderly and infirm people.

The trenching here documented some 80 meters of slip over the last 8,000 years. About half of that appears to be creep, so the rest must be due to earthquake motion. The 1868 earthquake created an offset across the fault—the coseismic displacement—of nearly two meters. So there is room in the numbers for 20 quakes that size in that time, or once every 400 years on average. But like they say in the mutual-fund ads, that is no indication of actual performance. And the trip leaders argued throughout the day about what evidence helps us tell coseismic movements from aseismic creep. Much is uncertain in this kind of research.

Stop 5: The Historic Homestead

Our final stop was at Shinn Park and Arboretum, deep in the Fremont suburbs near the railroad tracks. The buildings on the site were here in 1868, and creep was damaging the old blacksmith's shed. It seemed like a perfect spot for a trenching study, and Keith Kelson of the geotechnical firm William Lettis & Associates reviewed his thinking for us. Human artifacts, like old nails and blacksmith's rubbish, would be "cultural evidence" that could date the sediments rather well, and the well-known 1868 coseismic movement should leave definite traces.

We agreed with Kelson that his thinking was sound. "But when we did the trenching," he said, "we were pretty much let down." He dug another trench at the site of the cook's shack, with no better luck. A lot depends on the soils, Lienkaemper said—they can preserve delicate features like those at Mira Vista, "or they can be great cloakers." Although the results weren't as good as he wanted, Kelson found some evidence of several large earthquakes during the last 2,000 years.

"BEFORE"

The day's events left me with many different impressions—the passion and insight of the scientists, the charm of the localities we visited, the stark evidence of powerful tectonic forces under our feet. Above all I had a palpable sense of tragedy to come. The line of the moving fault went through parks, streets, schools and stores, the old folks' home and the railroad track—a straight line through people's lives. When the fault lets loose, all those people will be knocked off the tracks of their lives. My tour of the East Bay was a gigantic snapshot, a living diorama, labeled "BEFORE."

PS: Then there's the Hayward fault quake of 1836, which was put on the books in the early 1900s. Only in 1997 did Touson Toppozada and Glenn Borchardt dig into the primary historical sources, in a sort of trenching study of the archives. They proved to everyone's satisfaction that the 1836 event was the quake that wasn't there—it was really down near Monterey. That threw the official 1990 assessment of earthquake hazards out of whack.