The Fault Line and the Dams 

Lake Temescal Dam sits directly atop the perilous Hayward Fault. How safe is it, and the nearby Tilden Park Dam at Lake Anza?

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The East Bay Regional Parks District was not asked by the division to do a large-scale evaluation of its dams for earthquake safety, Orrock said. Based on past reports, the district's dams are not considered in need of updates, he added.

Vanya said Lake Temescal Dam and Tilden Park Dam are inspected every other year for earthquake safety by the Division of Safety of Dams. But Orrock said that the dams are inspected annually by the agency. And both dams are listed by the division of dam safety as being in "satisfactory" condition, meaning that "acceptable performance is expected under all loading conditions," including earthquakes.

Orrock said that even though Lake Temescal Dam lies partly atop the Hayward Fault there is no indication that there has been movement or shifting in its structure. Geologists with the Division of Safety of Dams placed what are called "monument markers" at the dam — devices used to measure fault creep, often amounting to only millimeters per year. Orrock said the markers have not moved from where they were placed. If they had, and the movement had impacted the dam, that might indicate that the dam would need additional improvements to withstand an earthquake. "The instrumentation and physical inspections of the dam have never shown any issues related to seismic activity," he said.

But James Lienkaemper, a retired geologist with the U.S. Geological Survey who has studied creep along the fault, quarrels with that assessment. He said that he has measured creep in the spillway of the Lake Temescal Dam — on average, 4.4 milimeters per year — between 1976 to 2018. The creep does not cause significant damage to the dam or the spillway, he said.

When presented with this information, Orrock conceded that engineers with the Division of Safety of Dams have observed creep in the spillway of the dam, but not at a rate that gives the agency any cause for concern. The spillway begins as a concrete culvert, which then turns into a 94-inch steel pipe, and division employees have not observed an amount of creep in either of these areas that would cause them to think that the dam is unsound in any way, he said.

Dr. David Schwartz, emeritus geologist with the U.S. Geological Survey, who studies the fault and contributed to some of the documents in the HayWired Scenario, said that the agency had already worked with BART on how to address the creep that has been observed in the Antioch line BART tunnel where it crosses the fault, roughly a quarter mile north of the Lake Temescal Dam.

Even knowing that the Hayward Fault is not like other faults, it is still difficult to pinpoint exactly what will happen in a large quake.

"Let's say, for example that the next quake is a 6.8," Schwartz said. "If the fault ruptured completely and broke the surface all at once, there might be four or five feet of offset at the surface."

Alternatively, he said, there might just be some cracks at the surface at first, and then the creep will accelerate and create four or five feet of surface offset slowly over time, he said. This is what makes a fault like Hayward especially complicated to plan for and earthquakes particularly tricky to recover from.

Parts of Napa experienced this hardship first-hand in the aftermath of the 2014 South Napa quake, a 6.0 quake on the West Napa Fault, which is considered to be part of the Calaveras Fault. That fault also experiences creep between earthquakes. After the quake damaged a road, crews repaired it, only to discover it needed repairs again five days later due to continuing shifting and settling of the ground, Schwartz said. He believes communities along the Hayward Fault could be destined for the same complications.

"In some ways, it's alarmist, but in other ways, I think we really don't know what is going to happen when we have that major event along the Hayward in a densely populated area," Schwartz said. "I think we are going to see a lot of surprises in terms of the amount and styles of damage."

But Orrock says that even if a quake caused intense ground movement and shaking, or even if huge cracks happened along the fault, the dam and its abutments are so thick that they wouldn't experience damage that would lead to dam failure. First off, it is an earthen dam, an older style of dam that tend to weather quakes better than concrete or hydrofill dams. Second, the dam itself is so thick — over 500 feet at its base — that even if the quake affected the ground in some way beneath it, it still wouldn't likely affect the dam. Though he noted no structure is guaranteed to be completely safe.


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