The Trend of Radon Emission Values in Relation to the San Andreas Fault at Point Reyes, California

The main goal of the current study was to determine the trend of radon emission values in relation to an important geologic fault. The study was conducted on the Earthquake Trail and Olema Valley regions of Point Reyes, California. Both sites are located in areas that contain clear evidence of faulting associated with the San Andreas Fault. Forty-nine radon soil-gas detector cups were buried in predetermined grids, with each cup spaced approximately ten meters apart. After collection the detectors were analyzed at Lawrence Hall of Science using an instrument specially made for radon detector analysis. The location of each detector and its radon emission value were mapped, with locations being determined by using a Global Positioning System (GPS) receiver or by its position within the grid, depending on the survey. The maps were then analyzed, and compared with geologic maps of each area. In the Olema Valley study, a northwest-southeast trend of radon outgassing was found, similar to the trend of the San Andreas Fault. In the Earthquake Trail study area, high radon concentration values also followed the fault?s trend. These results indicate the presence of unsealed fractures that provide channels for the transport of radon generated at depth. In the Earthquake Trail area several elongated zones of high outgassing were detected, which indicates the presence of numerous sets of fractures associated with the fault. Overall, the study showed that high radon readings (i.e., those that are greater than three times above background values) occur in fracture zones that have a trend very similar to that of known local features associated with the San Andreas Fault. In addition, results from this study suggest that valuable information regarding fault structure may be obtained through mapping radon outgassing patterns.