State releases new earthquake hazards map | June 5, 2019 | Almanac | Almanac Online |


News - June 5, 2019

State releases new earthquake hazards map

by Rick Radin

The Woodside area has a new reference for liquefaction and earthquake-induced landslide hazards in the form of an online map from the California Geological Survey.

The map of the Woodside quadrangle was released in April and may be used in the future to update or modify a map adopted by the Town Council in 2016 that was formulated over decades by town engineers and geologists, according to Planning Director Jackie Young.

The current geological map is on display on the town website and in the lobby of Town Hall, Young said.

The new map identifies liquefaction areas that are generally around creek beds, as well as areas that are subject to landslides in a quake, said Max Mareschal, a professional geologist with CGS in San Mateo.

The information can be used to put requirements on new construction to make structures more earthquake safe, Mareschal said. Contractors can build thicker and deeper foundations or increase the soil density as mitigation for liquefaction and do grading, reinforce slopes and build retaining structures to protect against landslides.

Most of the areas around Woodside that are prone to landslides are in open space areas in the hills, Mareschal said.

"It's much less expensive and easier to take steps that can help minimize the risk of these seismic hazards before construction takes place than it is to retrofit," said Tim McCrink, a supervising engineering geologist with CGS.

People selling property and real estate agents must tell potential buyers if a parcel is in a seismic hazard zone, as is the case when property is in a designated flood zone.

Mareschal estimated that most structures built in the last 20 years would be able to withstand the shaking of a major earthquake.

Young said the town geologist will be studying the new map, and if there is new information that is relevant it could lead to changes to the existing town map. Proposed changes would be brought before the Town Council.

The CGS mapping efforts were authorized by the Seismic Hazards Mapping Act of 1990, which was passed following the 1989 Loma Prieta earthquake.

The new map, which allows users to find information on individual parcels, can be found at


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