The Hayward Fault runs for 112km along the western foothills on the east side of San Francisco Bay. Among the most active and dangerous faults in the United States, it runs through a densely urbanised and interconnected region home to seven million people.
Scientists have documented a series of major earthquakes on the Hayward Fault, approximately one every 100 to 220 years during the past 1,900 years; the latest was a M6.8 earthquake in 1868 – 150 years ago.
In 2018, the USGS published the HayWired Earthquake Scenario, which is now considered a template for any government seeking to elevate preparedness for earthquakes. The study anticipates the impacts of a hypothetical M7 earthquake on the Hayward Fault and considers earthquake hazard impacts, mitigation efforts, and resiliency actions around the San Francisco Bay Area. It assumes sudden fault offset – the movement of opposite sides of the fault relative to each other – of more than two metres. Aftershocks, or slow offset (afterslip) along the fault continue for several months.
Under this scenario, an area from Richmond in the north to Fremont in the south – including Berkeley, Oakland, San Leandro, and Hayward – are hardest hit by violent ground shaking, while strong shaking is felt throughout the San Francisco Bay region.
Shaking causes liquefaction in sandy, water-saturated soils along the margin of San Francisco Bay and along streams. It also causes landslides in the hills and mountains surrounding the bay. Many aftershocks occur in the minutes to years following the main shock.
The impact could affect two million buildings in the San Francisco Bay region. Damage could displace about 77,000 households and render older steel-frame high-rise office buildings and newer reinforced-concrete residential buildings unusable for as long as ten months.
Even if all buildings in the bay region met current building code standards, 0.4 per cent could collapse and five per cent could be unsafe to occupy. Crucially, the report examines the benefits of stronger building code and concludes that for only a small cost increase, more resilient buildings could allow 95 per cent of the bay region’s population to remain in their homes and workplaces following such an earthquake.
The report estimates up to 800 deaths and 18,000 casualties in the San Francisco Bay region, mainly from building and structural damage. More than 2,500 people in the region could require rescue from collapsed buildings, and more than 22,000 people could require rescue from stalled elevators. East-bay residents could lose water service for up to six months; the ability to suppress fires would be reduced.
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