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.
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.
