Seismicity and Seismic Factors


For the San Francisco Bay Area, all sites are considered to be the most seismically active regions of the United States. The Hayward Fault and The San Andreas Fault are the most prominent. The site can be in the Special Studies Zones, also referred to as the Alquist-Priolo for the Hayward Fault, which make the site subject to special geological scrutiny.

The sites can be mapped for landslides and the information is usually contained in USGS maps.

Although research on earthquake prediction has greatly increased in recent years, seismologists have not yet reached the point where they can accurately predict when and where an earthquake will occur.  Nevertheless, on the basis of current technology, and U.S. Geological Survey compiled data, it is reasonable to assume that the proposed constructions will be subjected to at least one moderate or severe earthquake.  The house, building or addition to the existing should be structurally designed to withstand such earthquake.

Five earthquakes with energies exceeding Richter magnitude 7 have occurred in the last 150 years within the San Francisco Bay Area.  The 1906 and 1989 San Andreas Fault earthquakes were the last of these major events.  Moderate earth tremors and a slow creeping movement on some active Bay Area faults evidence present day seismicity.

The maximum probable seismic shock to be expected from the Hayward Fault is a 7.2 magnitude event on the Richter scale.  The site and general area could experience bedrock accelerations of 0.4 to 0.5 g.  The site may be considered to be a firm site with a Characteristic Site Period (Ts) equal to 0.5 seconds and shock duration of 0.5 minutes.

It should be clearly understood that California and especially the greater San Francisco Bay Area is an area of higher seismic risk.  It should also be realized that, in general, it is not economically feasible to build totally earthquake-resistant structures that would be resistant to any and all earthquakes.  Therefore it is possible that if a large or close earthquake to a site occurred, the site and structure could be damaged and there is an irreducible risk associated with living in a seismically active area such as California with many active faults.


Seismic Factors

The seismic lateral forces can be calculated utilizing the California Building Code CBC 2010. Based on the latitude & longitude for site location and the site characteristics, for example, the seismic factors are determined as follows:

Latitude 37.8000 degree North, Longitude -122.000 degree West
Site Classification “D” stiff soil profile.

The soil factors including maximum accelerations and design accelerations are compiled as function of the spectral acceleration and for example they can be:

  • Site Coefficients – Fa=1.0, Fv=1.5
  • Spectral acceleration for 0.2 sec. period (short) – Ss=2.013g
  • Spectral acceleration for 1 sec. period – S1=0.786g
  • Maximum spectral response acceleration (short) – SMs=FaSs=2.013g
  • Maximum spectral response acceleration – SM1=FvS1=1.18g
  • Design spectral response acceleration (short) – SDs=2/3SMs=1.34g
  • Design spectral response acceleration – SD1=2/3SM1=0.79g

The sturctural engineer designing the foundation and framing must follow the guidelines given above by the geotechnical engineer in his calculations.