Fourth Down on the Fault

Tension is building up, not just on the field of Memorial Stadium, as the Pac-10 season heats up for the No. 4-ranked Bears, but under the field between the tectonic forces of the Hayward Fault.

According to geologists, there is a one in three chance of a major earthquake along the Hayward Fault in the next 30 years, and it is widely acknowledged that the fault is

potentially the most dangerous in the Bay Area.

Memorial Stadium, which was constructed in 1923, sits directly on the Hayward Fault. The fault runs north to south through the football field, literally from goalpost to goalpost, roughly dividing the stadium in half.

"In a catastrophic earthquake, you could see significant damages, though whether it could have a partial or complete collapse is hard to judge," said Filip Filippou, UC Berkeley civil engineering professor and chair of the campus' Seismic Review Board.

The stadium is rated "poor" in seismic performance from the 1997 analysis of the campus' Seismic Action Plan for Facilities Enhancement and Renewal (SAFER) Program, meaning that the structure is expected to experience major damage and threaten the lives of its

occupants in the event of an earthquake.

According to UC spokeswoman Marie Felde, the campus' seismic safety goal is to address the buildings that have the greatest risk for loss of life, including the structural strength of the buildings and how often they are occupied.

In the case of Memorial Stadium, it is unlikely that there is a large risk to spectators at the football games. The stadium is only occupied for a few hours on the average six gamedays per year.

"Currently, I would venture the educated guess that if somebody were sitting in the stadium and an earthquake were to happen, not very much can happen to them, nothing very catastrophic," Filippou said.

Filippou points out, though, that there is danger for occupants of the facilities that are housed underneath the stadium.

The stadium's biggest danger, the old press box, which was rated "very poor" in the analysis, was removed in 2002. Now the danger lies in the western part of the stadium where the football team's facilities and offices are located.

"Memorial Stadium has basically two sides, the hillside, which is sitting on the ground, and the western side, the downhill side, which is a large edifice filled with offices and recreation rooms and all sorts of facilities," Filippou said.

Structurally, the two halves are expected to react very differently in the event of an earthquake.

"The eastern half of the stadium is carved into the hillside, and has no deficiency. The seismic deficiencies are only in the most western two-thirds that is a nonductal concrete superstructure," said David Friedman, a structural engineer at Forell/Elsesser Engineers, Inc., which is working on the retrofit project.

Because the concrete in this area is relatively brittle, there could be a partial or complete collapse of the structure in the event of a catastrophic earthquake, according to Filippou.

Since Memorial Stadium lies directly on the fault line, it could experience not only strong shaking, but also a tear along the fault line during an earthquake.

"Sometimes it is not known where the fault runs, but with the Hayward Fault, it is very well known because the creeping motion of the fault is so obvious," said UC Berkeley geology professor Roland Bürgmann.

Evidence of Earth's movement below the stadium is even obvious to the eye of any stadium-goer-on the southern side of the stadium, behind the International House, there is about 13 inches of separation in the concrete at the top outside wall of the stadium, joined by a strip of metal.

This displacement is because of a seismic creep, according to Bürgmann, which is a slip on the fault in the absence of an earthquake. Measurements taken by global positioning satellite technology show that the fault is slipping about 1 centimeter per year.

"Every year, we're building up another centimeter of strain. The next earthquake may have 1 to 2 meters of slip," Bürgmann said.

Because the stadium lies directly above where this 1 to 2 meters of slip could occur, the stadium has a possibility of tearing apart in a quake, providing a unique challenge to structural engineers.

"The stadium is a rigid structure, and the fault has a potential to rupture. If a mile of the earth's crust is going to tear, it is likely that 30 inches of concrete will tear," Friedman said.

According to Friedman, a proposed solution is to prefracture some elements of the stadium structure to create a large seismic joint.

"This aspect of the retrofit is to take portions of the bowl and segment them in a way, to prefracture them, and segment these zones on mats that move independently, so that if the fault does in fact rupture, it could withstand as much as a 6-foot horizontal offset," Friedman said.

The mat footing on which the rest of the structure sits would be thick and rigid to support the structure, with a weakened plane adjacent to it. Friedman says that if the fault ruptures, the mat would float over the rupture.

"It's hard to design a structure to withstand a 6-foot split, usually seismic joints are for inches. We don't want the stadium to tear, so we can retrofit in a way that the structure will rupture where we want it to," Friedman said.

Another seismic concern is the "fault normal," or nearfield effect, which Friedman defines as "perpendicular forces in addition to the sinusoidal rolling waves we usually think of with earthquakes. This pulse is a shock more than the oscillating force that is normally designed for."

To strengthen the western half of the stadium to withstand very high seismic forces, Friedman suggests new foundations and new concrete sheer walls.

"(These are) fairly conventional solutions." Friedman said.

Filippou suggests that the columns on the western side be wrapped in carbon fiber or steel jackets.

Knowing where the fault lies is only part of the information needed in the seismic evaluation of the fault below the stadium.

According to Bürgmann, the main goal for

geologists is to estimate the probability of earthquake by identifying when the last quake occurred, using trenching studies.

Geologists have determined the history of the fault by digging down towards the fault where the earth has been offset, then estimating when and how much slip occurred by examining fractures in the layers of different ages using maps, photography and radiocarbon analyses.

"In the case of the Hayward Fault, people have been able to go back five or six earthquakes to find out how much time passed between each rupture," Bürgmann said.

According to Bürgmann, the result of these studies show that the last earthquake occurred in 1868 on the segment of the fault between Fremont and Berkeley.

"They are finding that the time since the last earthquake is similar to the time between earthquakes in the past. The hazard is especially high since it's been so long since the last earthquake," Bürgmann said.

The issue of the seismic stability of Memorial Stadium has come to the university's seismic review committee two or three times in the last few years, but ground has yet to be broken.

"The university has a relatively well established prioritization process that takes into account the number of occupants and the amount of time the building is occupied," Filippou said.

Eventually, the stability of the stadium must be addressed because it threatens lives.

"Even if we had one death, it would be a disaster," Filippou said.

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