Tuesday September 29, 2015

Dr. Kenneth Hudnut, Geophysicist with the Earthquake Science Center in Pasadena, recently contacted the City of Napa to bring attention to a potentially hazardous after-effect of the South Napa Quake as the rainy season approaches. Dr. Hudnut reports that surface fault ruptures may be prone to erosion along fractures. As water from heavy rains enters cracks formed by the earthquake, these cracks can enlarge and lead to formation of visible fissures.

The articles linked below show a number of examples of this after-effect that is known as “ground fissuring” or “piping.” This phenomenon has been seen along fault breaks during the years after a surface rupture in the arid regions that have been studied. With different soil types in the Napa area, it is not known to what degree the potential for soil erosion along ruptures may exist after heavy rains, which are expected in this upcoming winter. However, in the abundance of caution, property owners should keep an eye out for soil subsidence along the quake’s surface ruptures and should contact the City’s Building Division at (707) 257-9540 in the event soil erosion is witnessed on their property. The City can provide owners with a list of geotechnical engineers and engineering geologists known to have worked in the area after the South Napa Earthquake and who can be contacted for further analysis regarding soils-related issues.

https://profile.usgs.gov/myscience/upload_folder/ci2011Jul2016445842954Holzer%20IAHS%20339%202010.pdf

http://faculty.fortlewis.edu/KENNY_R/images/Fissures.pdf

http://pubs.usgs.gov/circ/circ1182/pdf/09Arizona.pdf

http://files.geology.utah.gov/online/ss/ss-115.pdf