Laserfiche WebLink
<br /> <br /> <br />5.1.2 Surface Fault Rupture <br />A fault is a point of displacement along the fractures of the earth’s crust caused by shifting <br />tectonic plates. When an earthquake occurs, there is a rupture on a fault as built-up energy <br />is suddenly released. Active faults are those that have ruptured in the past 11,000 years.6 <br />Often the rupture occurs deep within the earth, but it is possible for the rupture to extend <br />to the surface and create visible above- ground displacement, called “surface rupture.” The <br />California Geological Survey (CGS) publishes maps of active Bay Area faults that could <br />produce surface rupture, as required by the Alquist-Priolo Earthquake Fault Zoning Act <br />(1972).7 These maps show the most comprehensive depiction of fault traces that can <br />rupture the surface, and the zones directly above and surrounding the fault traces. Cities <br />and counties require special geologic studies within these zones to prevent construction of <br />human-occupied structures. For buildings already in these zones, the surface rupture <br />hazard must be disclosed in real estate transactions. <br />Surface fault rupture varies in size and can change over time. Generally, a large magnitude <br />earthquake can generate a longer rupture and greater displacement, though the surface <br />expression of the displacement can vary widely. The M6.0 2014 South Napa Earthquake <br />resulted in over one foot of displacement in some locations,8 while the M6.9 1989 Loma <br />Prieta Earthquake had no surface fault rupture. In the 1906 Earthquake along the San <br />Andreas Fault, surface rupture displacements were greater than 20 feet in some locations.9 <br />Additionally, though the majority of displacement occurs during the actual earthquake <br />event (called “co-seismic slip”), surface displacement can occur in the days, weeks, and <br />even months after the event (called “post-seismic slip”). This was also observed in Napa <br />and can cause additional damage for up to a year after an earthquake. In a large <br />earthquake on the Hayward Fault the fault rupture displacement could reach 8 feet in some <br />areas. Most of the displacement would occur during the shaking, and in the first day <br />following the earthquake, but as much as 20 percent of the total afterslip could occur in the <br />time between one month and 12 months after the quake the fault continuing to displace a <br />full year after the earthquake.10 <br />5.1.3 Ground Shaking <br />When faults rupture, the slip generates vibrations or waves in the earth that are felt as <br />ground shaking. Larger magnitude earthquakes generally cause a larger area of ground to <br /> <br />6 Bryant, W.A., and Hart, E.W., (2007) <br />7 California Public Resources Code, Division 2, Geology, Mines and Mining, Chapter 7.5, Earthquake Fault Zoning, <br />sections 2621-2630 <br />8 Brocher, T.M., et al, (2015) <br />9 Thatcher W., Marshall, G., Lisowski, M., (1997) <br />10 Aagaard, B., Lienkaemper, J., Schwartz, D. (2012)