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Thursday, 03 October 2013 13:10

100313n3By Jim Knowles  • San Leandro Times

Let’s do the arithmetic.

The Hayward Fault that runs along the East Bay hills erupts about every 140 years. The last big one on the Hayward Fault was in 1868. That’s 145 years ago.

“So we’re due,” said Dr. Mitchell Craig from Cal State East Bay.

The Hayward Fault earthquakes don’t hit exactly on the 140-year mark. You can give or take 20 or 30 years. But it’s a sure bet it won’t be long before the next one.

Craig gave a talk about the 1868 earthquake at the San Leandro Museum last month. That quake is part of the reason the county seat is in Oakland, not San Leandro.

The county courthouse in San Leandro was destroyed by the earthquake on Oct. 21, 1868. The county clerk, J.W. Josselyn, died when he was hit by falling debris as he ran from the building.

A couple of years later, the new courthouse was built in Brooklyn, a suburb of Oakland at the time.

100313n4The 1868 earthquake, years before the San Francisco earthquake in 1906, was the first big quake that Europeans in California had seen at the time and is estimated at 6.8 to 7.0 on the Richter scale.

Cracks appeared in the earth from San Leandro all the way down to Warm Springs.

“The cracks in the roads made it difficult to drive a stage from Hayward (or Haywards, as it was called at the time) to San Jose,” Craig said.

The earthquake opened up a large crack next to a hotel in downtown Hayward. People put a weight on a string and lowered it into the crack in the earth but couldn’t find the bottom, Craig said.

The cracks on the earth’s surface are only a secondary result of an earthquake. The primary movement is in the bedrock, six miles deep, Craig said. So the cracks on the ground are nothing compared to what’s going on deep down in the earth.

“The cracks in the earth are a very minor subsidiary rupture,” Craig said. “Sometimes there are no cracks at all.”

Another thing happened on that day in 1868.

“San Leandro Creek suddenly started flowing,” Craig said.

Possibly cracks in the ground allowed underground water to come to the surface. Or water escaped through minor fractures in Lake Chabot Dam. It’s never been determined for certain.

But the earth doesn’t always crack because it’s a little elastic. Geologists study something called Elastic Rebound Theory where the bedrock flexes. That theory is the main understanding earthquakes and what they can do, Craig said.

Building laws today wouldn’t allow the unreinforced masonry buildings, such as the old courthouse in San Leandro. The Field Act in 1933 was one of the first laws that required earthquake resistant construction, especially in school buildings.

The law came in the wake of the 6.5 magnitude Long Beach earthquake in 1933, that destroyed or damaged schools in Southern California. Fortunately, the quake hit at about 6 p.m. on a Friday after the schools had emptied out.

The Alquist-Priolo Act of 1972 banned building on active fault lines and required the California Geologic Survey to map the traces where the faults cut the surface and a buffer zone.

Several faults run north-south through the Bay Area, the main one being the San Andreas Fault. The main cause of the faults and earthquakes is that two plates of the Earth’s crust are moving against each other, the Pacific Plate and the North American Plate.

The Hayward Fault extends from Warm Springs to San Pablo Bay. Springs are associated with faults and they make an area more attractive for human habitation. Fortunately, in 1868 this was an agricultural area and not heavily populated like it is today.

“A lot of the early settlement was along the fault because there’s good water along the fault,” Craig said.

We know a lot more about faults today than they did in 1868, but we still don’t know exactly when the next big earthquake of a 7 on the Richter scale will happen. But bedrock in the hills is more stable than down in the flats, and land filled in near the bay is the least stable.

“Bay fill is really susceptible to shaking,” Craig said.

CAPTION 1: Dr. Mitchell Craig, at left, gave a talk recently at the San Leandro Museum about the last big earthquake on the Hayward Fault on October 21, 1868.

PHOTO BY JIM KNOWLES

CAPTION 2: The county courthouse in San Leandro was destroyed by the earthquake on Oct. 21, 1868.

PHOTO COURTESY BANCROFT LIBRARY


 

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