By Alka Tripathy-Lang, Ph.D. (@DrAlkaTrip)
Urbanization obscures a complex fault zone on which downtown San Diego sits, but decades-old geotechnical studies reveal the faults.
Citation: Alka Tripathy-Lang (2020), Past meets present to help future seismic hazard forecasts in San Diego, Temblor, http://doi.org/10.32858/temblor.065
Fault studies often rely on surface expressions of the ground’s movement. In densely populated urban areas, such as San Diego, this evidence is concealed beneath the cityscape. Now, though, a team has used historical reports to trace faults through downtown San Diego in unprecedented detail, establishing a template that other fault-prone cities can follow to illuminate otherwise hidden hazards.
Urbanization obscures geology
Downtown San Diego, popular for its beaches and parks, also hosts the active Rose Canyon Fault Zone, a complex hazard that underlies the city from northwest of La Jolla through downtown, before curving into San Diego Bay. Like the nearby San Andreas, the Rose Canyon Fault is right-lateral, meaning if you were to stand on one side, the opposite side would appear to move to your right. But it plods along at a rate of 1-2 millimeters per year, unlike its speedy neighbor, which indicates a comparatively lower seismic risk.
“We haven’t had a major rupture” on the Rose Canyon Fault since people have been living atop it, says Jillian Maloney, a geophysicist at San Diego State University and co-author of the new study. So it’s hard to say what kind of damage would be caused, she says. “But, a magnitude-6.9 [of which this fault is capable] is big.”
Because of urbanization, though, “there haven’t been any comprehensive geologic investigations” of the faults underlying downtown San Diego, Maloney says. This presents a problem because detailed knowledge of active and inactive fault locations, especially in a complicated area where the fault zone bends, is key for successful seismic hazard assessments, she says. The state and federal government maintain fault maps and databases, but their accuracy at the small scale was unknown.
A solution to the lack of detailed fault mapping in downtown San Diego resided in decades of old geotechnical reports. These individual studies the size of a city block or smaller are required by the city for any proposed development near active faults, as mapped by the state. Although the data are public once the reports are filed with the city, the reports had not been integrated into a comprehensive or digital resource, and the city does not maintain a list of such reports.
According to Luke Weidman, lead author of this study, which was his master’s project, the first challenge was determining how many reports were even available. Weidman, currently a geologist at geotech firm Geocon, went straight to the source: He asked several of San Diego’s large geotechnical firms for their old publicly available reports in exchange for digitizing them.
Weidman scrutinized more than 400 reports he received, dating from 1979 to 2016. Many were uninterpretable because of faded or illegible pages. He assembled the 268 most legible ones into a fault map and database of downtown San Diego. Because reports lacked geographical coordinates, Weidman resorted to property boundaries, building locations, park benches and even trees to locate the reports on a modern map, says Maloney, one of his master’s advisors. Weidman, Maloney and geologist Tom Rockwell also of San Diego State published the findings from their comprehensive interactive digital map last month in Geosphere, along with an analysis of the Rose Canyon Fault Zone in downtown San Diego.
The team found that downtown San Diego’s active faults—defined in their paper as having ruptured within the past 11,500 years—largely track the state’s active fault maps. However, at the scale of the one-block investigations, they found several faults mapped in the wrong location, and cases of no fault where one was expected. Further, the team uncovered three active faults that were not included in the state or federal maps. At the scale at which geotechnical firms, government, owners and developers need to know active fault locations, the use of this type of data is important, says Diane Murbach, an engineering geologist at Murbach Geotech who was not involved in this study.
Maloney says they also found other faults that haven’t ruptured in the last 11,500 years. This is important, she says, because “you could have a scenario where an active zone ruptures and propagates to [one] that was previously considered inactive.”
This research “is the first of its kind that I know of that takes all these different reports from different scales with no set format, and fits them into one [usable] database,” says Nicolas Barth, a geologist at the University of California, Riverside who was not part of this study. Many cities have been built on active faults, obscuring hints of past seismicity, he notes. “This is a nice template for others to use,” he says, “not just in California, but globally.”
Weidman, L., Maloney, J.M., and Rockwell, T.K. (2019). Geotechnical data synthesis for GIS-based analysis of fault zone geometry and hazard in an urban environment. Geosphere, v.15, 1999-2017. doi:10.1130/GES02098.1