Monday, July 28, 2014
PLACERVILLE, CALIFORNIA
99 CENTS

Big quakes a Delta worry

THE MAP shows the known faults in the Bay Area.

Map of known faults in the Bay area.

By
April 7, 2011 |

The Delta Stewardship Council members were all ears when three experts from the U.S. Geological Survey Earthquake Center in Menlo Park reviewed the current level of understanding of earthquake forecasting for the Delta region at the Jan. 27-28 meeting. This was before the 9.0 magnitude earthquake on March 11 near the east coast of Honshu, Japan.

The focus of this report was primarily on risk to the Delta levees.

All members of the council were present for the session on earthquake studies on the second day of the monthly meeting: Chairman Phil Isenberg, Council members Randy Fiorini, Don Nottoli, Patrick Johnston, Gloria Gray, Hank Nordhoff and Felicia Marcus.

Earthquake basics

The three scientists — geophysicist Jon Fletcher, and geologists Russell Graymer and David Schwartz — showed several maps of earthquake activity in and around the Delta, both historical and current. They also demonstrated the effect of shaking intensity along the Hayward fault near Pleasanton with an animated video.

Schwartz told the council, “The planet is literally cracked up.” Under land and under water around the world there are plates in motion. The Pacific and North American plates that meet over East Bay move about 40 mm a year. The plates are not monolithic, but are made up of a series of faults. Faults serve as stress relievers for energy building up underneath.

The USGS estimates that several million earthquakes occur in the world each year, but many go undetected because they occur in remote areas or have very small magnitudes. The USGS now locates about 50 earthquakes each day, or 20,000 a year.

Earthquake measurements

The most common measurement used is the Richter scale. For each point on the Richter scale, the amount of energy released by the earthquake increases 32 times. For example a 5.0 magnitude earthquake releases 32 times the amount of energy as a 4.0 magnitude earthquake, which is 32 times as powerful as a 3.0 earthquake. The energy release measurement is the best indication of the destructive power of an earthquake. An earthquake measured at magnitude 6.7 or greater is considered a large earthquake.

“Stress shadow” lifts

Between 1850, when regular records began to be kept in California, and 1906, the Bay Area region literally rocked and rolled with earthquakes in the 5.6 to 6.3 range, said Schwartz.

The 1906 San Francisco earthquake was measured at 7.8. Since then, there have been some moderate earthquakes in the area. The largest was the 1989 Loma Prieta earthquake, 70 miles south of the Bay Area, measured at 6.9.

Schwartz said that the energy released by the 1906 San Francisco quake released energy throughout the area, much like releasing air from a balloon. The scientific name for this relatively quiet period is “stress shadow.”

There were fewer and smaller magnitude earthquakes during the “stress shadow” period after 1906.

The 20th century was the time of rapid population growth in the region. It was also the time that supporting infrastructure was put in place.

Earthquake forecast

With pressure building up in the earth’s crust over the last century, the USGS forecasts a 62 percent probability for at least one magnitude 6.7 or greater earthquake by 2032. USGS scientists expect there will be a series of large events on individual faults rather than one major earthquake similar to the one in 1906 San Francisco.

Earthquake types

Schwartz said that most faults in the Bay Area have a strike-slip motion, in which blocks of earth shift horizontally past one another. The slip rate is the average rate of movement over years.

Thrust faults have a vertical motion, with one side moving up over the other. Thrust faults often do not reach the Earth’s surface, making them difficult to find. The 1994 Northridge earthquake occurred on a previously undiscovered, or blind thrust fault, as was the 1989 Loma Prieta earthquake.

There are thrust faults in the Delta, but the USGS expects the primary sources of strong ground motion in the Delta within the next 100 years will be from the large plate boundary in the East Bay. The East Bay fault group includes the Hayward, northern Calaveras, Greenville, and Concord-Green Valley strike-slip faults and the Mt. Diablo blind thrust fault.

Ground motion

In the 1906 earthquake ground motion caused levee failure and the failure of a bridge and hotel in the Delta. Factors that measure the intensity of ground motion is measured by different factors: distance from the epicenter, type of ground (rock, silt, sand, peat, soil), and geologic formation (flat or enclosed).

If the ground is relatively flat, the shear waves (seismic body waves that shake the ground back and forth) can pass through. In a basin, the waves get trapped and bounce back and forth for an extended period. Mexico City is built on a dry lakebed. In 1985, ground shaking from the 8.1 earthquake that occurred around 200 miles from the city was amplified, resulting in losses to tall buildings of about $40 billion and at least 8,000 fatalities.

Schwartz said that one-story wood frame buildings are fairly safe in an earthquake.

This was before the 9.0 magnitude earthquake on March 11 near the east coast of Honshu, Japan.

Levees

There are 1,100 miles of levees in the Delta. Drawing on the Delta Risk Management Study completed by USGS in 2005, the scientists “grade most of the Delta levees as highly vulnerable where a 6.5 earthquake…could cause failure that breaches the levee.”

The goal over the next few years is to get much more understanding of ground motion.

Council and public comments

Council member Nottoli, who represents the Delta on the Sacramento County Board of Supervisors, said, “The levees held in 1906. They have resiliency.”

Council member Johnston commented that the first warnings of levee risk were introduced to CALFED (Delta Stewardship Council’s predecessor) as recently as 2000-01.

In response to a question by Council member Nordhoff about the relative safety of an underground tunnel, Schwartz said, “Tunnels tend to do pretty well.”

During the public comment period, people who live and work in the Delta shared their experiences and knowledge.

Gilbert Cosio, a levee engineer who works with several reclamation districts in the Delta, said he has been on levees when the ground swells up and down during an earthquake. He explained different levee designs that provide added protection and the process of confining weak foundations. “They have a lot of resiliency,” he said, echoing Nottoli.

Tom Zuckerman, Central Delta Water Agency, described his experience in the 1995 Kobe earthquake, magnitude 6.9. “The bullet train failed, but there was little damage to the levees,” he said.

Connie Ford, Sacramento County Water Agency, said that her understanding is that the most damage to buildings occurs when the harmonic levels of the earthquake are the same as the motion of the building.

Grindstaff said there are no earthquake standards for levees in California, although the Department of Water Resources is working on them. Urban areas are the priority.

Mitigating the effects of an earthquake on Delta levees so they do not fail, or last longer in an event, is part of a complex set of policy decisions. Which levees to strengthen to prepare for a disaster will depend on a balance of considerations and the money available.

Infrastructure risk

The Hetch Hetchy Reservoir, on the Tuolumne River, provides 85 percent of the drinking water in San Francisco and all San Francisco municipal power uses, including Muni trolleys and the International Airport. The water pipes and power lines cross the Delta. PG&E has power lines crossing the Delta. Burlington Northern Santa Fe Railroad runs across the Delta. East Bay Municipal Utility District has three large water pipes crossing the Delta to serve Bay area communities. A network of roads, highways and bridges provides transportation around and through the Delta.

Risk to infrastructure that would affect population centers in and outside the Delta was mentioned during the levees discussion, but is a topic for another day.
For more information, visit www.deltascouncil.ca.gov.

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