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.
