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The greater San Francisco Bay Area is recognized by Geologists and Seismologists as one of the most active seismic regions in the United States. Three major fault zones pass through the Bay Area in a northwest direction which have produced approximately 12 earthquakes per century strong enough to cause structural damage. The faults causing such earthquakes are part of the San Andreas Fault System, a major rift in the earth’s crust that extends for at least 700 miles along western California.

The San Andreas Fault System includes the San Andreas, Hayward, and Calaveras Fault Zones. Several smaller potentially-active faults also occur within range-front fault systems associated with these major fault zones. The State of California, Counties, and Cities require Fault Investigations for proposed construction of habitable structures within potentially active fault zones.

I can design a Fault Investigation for your project to satisfy requirements of regulatory agencies. The investigation may include trenching to confirm the presence or absence of faulting. The fault exploration trench will be oriented roughly perpendicular to the mapped fault trend in the area, extending a minimum of 30 feet beyond either end of the proposed building footprint. The walls of the fault trench will be cleaned, closely examined, and logged for evidence of faulting. If fault traces are encountered, appropriate building setbacks may be recommended.

Additional trenching may be required to determine fault trends or to find an appropriate building site, free from faulting. If fault setbacks are not feasible for the proposed project, the age of soils relative to fault traces may be assessed in an effort to determine the age of faulting. If faults are determined to be old and inactive, smaller building setbacks may be feasible.

The scope of the Fault Investigation will be designed to meet the State, County, and City requirements for your project and may include:

  • review of geologic maps, literature, and air photos,
  • site reconnaissance and mapping,
  • consultation with the Reviewing Geologist,
  • consultation with the project Architect and Engineer,
  • excavation and logging of the fault exploration trench,
  • drafting and report preparation.

The Fault Investigation will produce detailed trench logs, a site geologic map, and geologic cross-sections which delineate approximate boundaries of faults and recommended building setbacks, as appropriate.