The Hayward Fault: Is the Bay Area Due for Another 1868 Earthquake?

By Thomas M. Brocher, Jack Boatwright, James J. Lienkaemper, Carol S. Prentice, David P. Schwartz and Howard Bundock, U.S. Geological Survey

The Alameda County Courthouse in San Leandro after the 1868 earthquake: its second story collapsed. Bancroft Library, University of California.

October 21, 1868

Early that morning, a magnitude 6.8 earthquake on the Hayward Fault shook the fog-shrouded San Francisco Bay area.

• Shaking lasted more than 40 seconds; people fled their homes, and even fire-engine horses bolted.
• Several East Bay towns were devastated. Brick buildings, walls and chimneys fell in Oakland, San Francisco, Santa Rosa and San Jose, with serious damage in Napa and Hollister.
• Witnesses saw the ground move in waves. The quake was felt as far away as Nevada, and aftershocks shook the Bay Area for weeks.
• About 30 people were killed. The region was sparsely settled, yet the quake still ranks among the most destructive in California's history.

USGS studies show quakes like it recur, and the fault may be ready for another of magnitude 6.8 to 7.0. Updated building codes, retrofits, planning and preparedness will reduce the damage.

Major faults of the Bay region. The part of the Hayward Fault that broke in 1868 is yellow; dot size shows today's relative city populations. USGS.

A creeping fault

The Hayward Fault is a near-vertical surface along which two huge blocks of bedrock slide past each other in the East Bay. Like several faults in the region, it creeps: the two sides are always moving, slowly.

• Rate: about 1/5 inch (5 millimeters) a year.
• Evidence: roads, curbs and buildings being offset, little by little.
• Depth: creep seems limited to the top 3 miles (5 km). Below that, the fault is locked and building up stress.
• The catch: creep and small quakes account for only about one-third of the fault's long-term movement. The other two-thirds must be released in large earthquakes like 1868's.

A curb in the city of Hayward, offset by creep. USGS.

A lidar image of the fault (red) in Berkeley. It runs through the University of California football stadium (left), and past quakes have offset Hamilton Gulch (center). Arrows show the direction of movement. USGS.

Watching from space: University of California, Berkeley, scientists track creep with InSAR (interferometric satellite aperture radar), repeated radar surveys that show how fast points on the ground move — and so which parts of the fault creep, and which are locked to the surface and may produce even stronger quakes.

InSAR over part of the East Bay. Warm colors: points moving southeast, toward the satellite; cool colors: moving northwest, away from it. The scale is in millimeters per year (1 millimeter = 0.04 inch). USGS.

The earthquake of 1868

• The end of a sequence: it capped a decade of seven increasingly strong Bay Area quakes.
• Rupture: ground cracking was traced for 20 miles, from Warm Springs in Fremont north to San Leandro. Old land-survey data suggest the break reached Berkeley, with an average sideways offset of about 6 feet (2 meters).
• Reach: the strongest shaking covered about 1,000 square miles — the strongest the Bay Area's young towns had ever felt. Until 1906, it was known as the "great San Francisco quake."

Piecing it together

Seismographs had not yet been invented, so there are no recordings of the quake. Much of what is known comes from a chapter of the 1908 report on the 1906 San Francisco earthquake, which reviewed accounts of the faulting, gathered damage and felt reports, and interviewed survivors.

Scientists added newspapers, photographs, pioneer diaries and letters, and weighed how each damaged building was built — adobe, brick or wood — to map the shaking: a ShakeMap, which can also forecast damage to buildings today.

ShakeMaps of 1868 and of the 1989 magnitude 6.9 Loma Prieta earthquake, in Modified Mercalli Intensity — from V (very light damage) to X+ (very heavy). USGS.

• Strongest in Hayward, Fremont and San Leandro; weaker but damaging in Oakland, San Francisco and San Jose.
• Magnitude: estimated at 6.8.
• Compared with 1989: a repeat of 1868 would shake the East Bay much harder, and do much more damage, than Loma Prieta did.

Lessons learned — and forgotten

After the 1868 quake, and one in 1865 that damaged San Francisco, engineers strengthened Bay Area buildings: iron tie-rods and anchors in masonry, stronger brick-laying with iron binders, interior iron framing and — most important — steel frames, from 1885. San Francisco's buildings of 1868–1906 came through the 1906 shaking, except City Hall.

But two lessons, though widely discussed at the time, were not heeded: the danger of building on landfill in the bay ("made land"), and the advice to "build no more cornices." By 1906 they were largely forgotten.

A lesson from Kobe

The next big Hayward quake may resemble the magnitude 6.9 earthquake that struck Kobe, Japan, in 1995. Kobe, a port on Osaka Bay along the Nojima Fault, has geography strikingly like the East Bay's, and the two faults are similar in length and type of movement.

The quake and its fires killed more than 5,000 people. Shaking and liquefaction — shaken sandy soil acting like a liquid — devastated homes and infrastructure; the damaged port lost shipping lines for good; many lower-income areas went unrebuilt for more than a decade. Japan's codes were less strict in 1995, and Kobe less prepared than the Bay Area is now — but a repeat of 1868 is still likely to bring significant loss of life and heavy damage.

The nation's most dangerous fault?

First, where it is. The Hayward is the most urbanized earthquake fault in the United States. Alameda County had 24,000 residents in 1868; it now has more than 2.4 million.

Hundreds of homes sit on the fault trace, and transit lines, freeways, roads, gas and water pipelines and power lines cross it. Slip and aftershocks could go on for months after a large quake. The USGS HayWired scenario (2018) forecast housing lost to shaking and fire and long-term drinking-water outages.

Second, when it last broke. Its last damaging quake was more than 150 years ago. USGS scientists have found evidence of 12 quakes on the southern Hayward Fault in the past 1,900 years. The last six came at intervals of 95 to 183 years — about 150 years on average.

Hayward Fault earthquakes from trenches dug across the fault — all estimated at magnitude 6.3 or greater; those before 1868 dated by radiocarbon. The gaps: 183, 158, 153, 97 and 142 years — and now 150 years and counting. USGS.

The next one

The 150th anniversary of 1868 passed in 2018. Scientists are convinced the fault could produce a powerful, damaging quake at any time.

• Damage: extensive, to homes, businesses, transportation and utilities, according to a 1996 Earthquake Engineering Research Institute report.
• Homeless: likely several hundred thousand people.
• Cost: a 2013 study estimated economic losses — buildings and contents, business interruption, living expenses — above $100 billion, with 85 percent or more of home and business losses uninsured. Damage to transportation and utilities, and other long-term effects, could add much more.
• Recovery: years, as after Hurricanes Katrina, Sandy and Maria.

The population at greatest risk is 100 times that of 1868, and the region's infrastructure has been tested only by the distant 1989 Loma Prieta quake and the smaller 2014 magnitude 6.0 South Napa quake. Preparing now for the next magnitude 7 can greatly reduce the loss of life and property.

More:

• The 1868 Hayward Fault earthquake (USGS)
• The HayWired Earthquake Scenario — We Can Outsmart Disaster (USGS)
• Putting Down Roots in Earthquake Country
• Earthquake Country Alliance, Bay Area

Sources

Based on The Hayward Fault—Is It Due for a Repeat of the Powerful 1868 Earthquake?, by Thomas M. Brocher, Jack Boatwright, James J. Lienkaemper, Carol S. Prentice, David P. Schwartz and Howard Bundock, USGS Fact Sheet 2018–3052 (superseding Fact Sheet 2008–3019), U.S. Geological Survey; a work of the United States government in the public domain. The maps, images, timeline and photographs, with their captions, are taken from the fact sheet's PDF, which the import had left out; the 1868 photograph of the courthouse is in the public domain. The satellite images of Kobe, from Google Earth, are not reproduced.