Seventeen years after the Loma Prieta earthquake trashed the Marina District in San Francisco and the Cypress Freeway in Oakland, new regulations are kicking in that require communities to prepare better for earthquakes in such high-hazard zones.
On Oct. 16, the California Geological Survey in Menlo Park released three new Seismic Hazard Zone Maps for the Bay Area, including San Francisco and the Midpeninsula between Redwood City and Palo Alto.
The maps show zones where soft soils are likely to fail when shaken by an earthquake:
• Green indicates where soils are prone to liquefaction. Soft earth, especially close to streams and the Bay, and saturated with water, cam turn to liquid during an earthquake — as happened with the Marina District and Cypress Freeway.
• Blue indicates areas at risk of landslides. Hillsides with loose soils and soft rock can let go in an earthquake, taking homes and roads with them. Damage can be even worse when the ground is wet.
The Palo Alto Quadrangle is one of the new maps. Its green liquefaction zone stretches along the Bayfront and up the creeks. Much of downtown Palo Alto, East Palo Alto, east Menlo Park, and downtown Redwood City are green. Blue landslide zones are scattered around the hills to the west.
Release of these maps on Oct. 18 triggered new rules for jurisdictions with blue or green hazard zones. These are designated “zones of required investigation” in the Seismic Hazards Mapping Act that was passed by the state after the Loma Prieta Earthquake.
From now on, cities must take extra precautions in blue and green zones to make sure that new buildings are safe in a large earthquake. Publication of the maps also triggers disclosure requirements to assure that buyers are notified of earthquake risks.
“Cities and counties are required to start acting,” said Keith Knudsen of the California Geological Survey.
Risks in South County
Locally, Menlo Park and Portola Valley are most affected by the new map. Atherton has very little blue or green, primarily a thin green stripe along the drainage canal. Very little of Woodside is located in the Palo Alto Quadrangle; but survey officials said it will likely be included in the next maps.Menlo Park will be hardest hit by the new requirements. Most of the Bayfront northeast of Bay Road is in the green liquefaction zone; that includes Belle Haven, Tyco Electronics, and most of the city’s industrial area. The green zone extends up San Francisquito Creek on both sides, reflecting the alluvial soil deposited during eons of flooding.
Community Development Director Arlinda Heineck hadn’t seen the map. “We will be looking at it,” she said.
Portola Valley may not have much to do. It has been doing basically what the map requires for 30 years. It mapped the town for faults and landslides long ago; it requires geologic studies and engineering solutions for new building in hazard areas.
Portola Valley also requires disclosure of information when property is sold, said Town Planner George Mader. The real estate agent is required to get a Residential Data Report from the town to share with the buyer.
The new maps show more possible liquefaction areas than town maps, Mr. Mader noted. These lie primarily along Los Trancos Creek and on the Valley floor near the San Andreas Fault zone. While most of the hills are shown in blue, only a small area is included in the Palo Alto Quadrangle. The adjacent Mindego Hill Quadrangle, which has already been mapped, includes a lot of Portola Valley’s western hills and is mostly blue.
“The town needs to look at the map and decide how it wants to implement the state requirements,” Mr. Mader said.
INFORMATION
• On the 17th anniversary of the 6.9 Loma Prieta Earthquake, the U.S. Geological Survey released online comprehensive studies of the earthquake and its effects. The 162 papers totaling 3,000 pages can be viewed at earthquake.usgs.gov/1989.• At the same time, the California Geological Survey released three new Seismic Hazard Zone maps online. They include the Palo Alto Quadrangle, which covers most of Palo Alto, East Palo Alto, Menlo Park, Atherton, Portola Valley, and Redwood City. The maps and associated reports can be downloaded from conservation.ca.gov/cgs/shzp.
Chan Zuckerberg Initiative announces $10M donation for Menlo Park, East Palo Alto familiesApril 13, 2026 12:00 pm