Living with our Faults

 

We live in an area of earthquake faults and hills with active and potentially active slides. We interviewed local geotechnical engineer William Langbehn about our geologic makeup and how it affects the stability of our homes. 

 

     1.  MG: How would you describe the hazards associated with the geology and soils in the East Bay?

WL:  (GEOLOGY): in the flats, we have strong shaking during a major earthquake combined with the possibility of liquefaction, while in the hills we have faulting, strong shaking and also landsliding, either with or without an earthquake. Liquefaction is a product of strong shaking of saturated sandy soils that lose strength and then essentially flow like water.

     (SOILS): the number one issue is the widespread presence of expansive clay soils. These soils expand and contract with seasonal wetting and drying cycles, frequently making our homes shift slightly depending on the weather patterns. Many of us have experienced a door that will close freely in the summer and stick in the winter. Locally, we benefit from our summer fog’s high humidity as it prevents our soils from drying out too much.

 

     2.  MG: How much do the slide areas move in the greater Berkeley area?

WL:  Active slides move every year, sometimes as little as 1/8” and up to 1” or more in very wet winters. Some slides are so active they have names – the Blakemont, Southampton and Keith Avenue Landslides. Think of them as “mud glaciers” left over from the Pleistocene Ice Ages and typically at least 25 to 60 feet in depth.

 

     3.  MG: What are the clues or red flags to look for to determine that the services of a geotechnical engineer are needed?

WL:  From a structural standpoint, extensive foundation cracking, foundation rotation, excess moisture in the crawl space and/or leaning/cracked retaining walls.  Architecturally, severely sloping floors, widespread wall cracking and wracking of the framing suggest that a geotechnical and/or structural engineering review is prudent.

 

     4. MG: What is the typical role of a geotechnical engineer, especially as it relates to real estate, and what are the approximate costs? 

WL:  Basically, we determine overall geologic hazards, such as shaking, sliding and slope stability issues, then assess the condition and performance of the foundations and retaining walls and look for drainage issues. A verbal report runs just un a written one usually between $700 and $1,000.

     When I do a site reconnaissance, I know that no single map is correct. As a result, I look at 2 or 3 different slide maps, 2 or 3 fault maps and 2 or 3 geologic maps. I have to draw my conclusions using the observations during my site visit combined with the semi-complete data from these various maps as well as considerable past experience in the area. I frequently feel like a doctor who has to make a diagnosis without conducting invasive surgery.

 

     5. MG: What is your informal assessment about how well our houses are built for survival during a major earthquake here?

WL: Newer homes built to modern codes are pretty safe in general, older homes have more risk. In general, single-family homes perform well during earthquakes because they are typically lightweight, relatively flexible and fairly strong for their weight and flexibility. Problems arise in older homes that are not bolted well to the foundation or have either unbraced cripple wall conditions and/or lots of dry rot. Also, unreinforced masonry (like chimneys) is heavy and brittle so tend to have problems in earthquakes.

 

Latest revised version (2008) interactive map, click on the map and you are directly connected to more refined maps, click on the higher resolution pdf for printing. This map is the latest on the Hayward fault and generally supersedes the info on the EFZ or Alquist-Priolo maps on the link above.

 

     6.  MG: Local Realtors work with state-generated geohazard maps as well as the local map developed by Alan Kropp Associates.  Frequently there are differences between them.  Can you give us some insight as to the best use of these maps?

 WL: In a nutshell, take the state hazard maps with a grain of salt!  They are broadly interpreted maps compiled from a variety of data, some of which is pretty general. These maps actually delineate “Zones of Required Investigation” where the possibility of the hazard should be further evaluated but the geohazard may or may not be present. The Kropp map is the best available one for landslides in our area, as it is based on hard data gathered from site-specific investigations, including soil borings.

 

  • State Hazard Maps from CGS:

 

  • Liquefaction and Seismically-induced Landslide Hazards ("Zones of Required Investigation") 

http://gmw.consrv.ca.gov/shmp/html/pdf_maps_no.html

Click on the appropriate quadrangle map and wait for the pdf to load.

 

  • Earthquake Fault Zones for Active Faults ('Alquist-Priolo Special Studies Zones")

http://www.quake.ca.gov/gmaps/ap/ap_maps.htm

This link is new for fault rupture hazard but are the older AP maps. Either search by City or address or click the appropriate quadrangle map wait for the pdf to load. 

 

  • Note: we are in Richmond 7.5' minute Quadrangle, Oakland East is to the south.

 

The Kropp map is the best available one for landslides in our area, as it is based on hard data gathered from site-specific geotechnical investigations, including soil borings. This map also shows the “best-fit” location of the Hayward fault per the USGS. However, it has not been updated since 1995. I spent the early part of my career with Kropp frequently updating the map. Since 1997, I have been further developing a geologic model of our local soils and geology and I input new data onto the Kropp map on a regular basis. I hope to have a more current map available soon.

 

  • Alan Kropp and Associates map:

http://www.akropp.com/uploads/File/resources/downloads/berkeley_hills_slide_map.pdf

 

 

7.  MG: Can you explain the importance of the state designated “Alquist-Priolo Fault Zones”?  We also understand that there appears to be a discrepancy on the determination of where the “Special Studies Zone” is in Kensington with respect to where the actual fault lays.  Can you enlighten us?

WL:  Generated after 1974 following the San Fernando earthquake, the state developed “zones of active faulting” to address fault rupture hazards. The Alquist-Priolo Special Studies Zones (now called “Earthquake Fault Zones”) delineate an area 500 feet on each side of a known active fault. Locally, especially in the hills where there are active slides, the actual location of the fault line can appear on the surface to have moved over time.  Thus, updated, locally generated fault maps, such as the USGS “Best-Fit” map by Lienkaemper are typically more accurate.

 

  • USGS "Best-Fit" Active Traces of the Hayward Fault (J.J. Lienkaemper):

http://pubs.usgs.gov/ds/2006/177/

 

 

8.  MG: We’ve heard conflicting reports that proximity to a known or suspected fault line makes a big difference during an earthquake?

WL:  My response is based on two assumptions: first, the property is not sitting right on the fault rupture line, and second, it is not in an active slide area. The “epicenter” is the point on the surface directly above where the energy is released. However, the “focus” is the actual point where the fault slips, usually at least 3-8 miles below the surface. So being 100 feet or 1 mile from the epicenter doesn’t change the distance much from the focus of the earthquake. Energy waves radiate from the focus of the quake to the surface and can affect structures miles away.  Often, what matters more than proximity to the epicenter or fault line is the nature of the underlining soils and the depth to bedrock.  For example, the Loma Prieta earthquake shook for only 15 seconds at the epicenter, while roughly 65 miles away in the Marina district of San Francisco (which is on soft ground), the length of shaking was about twice that and the intensity of the shaking was about 4 times as strong compared with a bedrock site at similar distance.  This combination created lots of damage even at that distance. 

 

 

Our thanks to William K. Langbehn, Geotechnical Engineer, 1034 Richmond St, El Cerrito, CA 94530 (510) 558-8028. E-mail: wmlangbehn@sbcglobal.net.  

 

 

In order to achieve a successful sale, we can help you navigate around the many obstacles inherent in today’s complicated real estate market.  Call with questions and/or to arrange a courtesy consultation on the value of your property.

 

 

 

Taken from excerpt "Living with our Faults" in Marvin Gardens Real Estate East Bay Essentials Newsletter Vol.26