But no one has any illusions, it is not a question of if, but when a major earthquake in southern California will occur. As readers of the LA Times may already have noted, the Hollywood fault, and its westerly extension is known as the Santa Monica fault, earlier this year have been zoned as active faults by the California Geologic Survey, and as such, properties within these delineated fault zones need an earthquake fault study to be performed by an engineering geologist to ensure that the fault, or a strand or branch thereof, does not cross through the property before building permits can be issued.
At Arroyo Geo and Applied Earth Sciences, we have performed several major fault studies just this year alone. One of our more recent projects involved a fault study very near the San Fernando fault zone. This fault ruptured in the 1971 San Fernando/Sylmar earthquake and was the singular reason for the passing of the Alquist-Priolo Act in California in 1972, which required fault studies on properties within active fault zones.
Here are some pictures from this recent study. What made this investigation particularly challenging was that it was formerly a gasoline station, and as such we needed to stay clear of the locations of former underground storage tanks and dispenser plumbing/piping. There was also the risk of encountering contaminated soils or digging up previously advanced borings or monitoring wells. All in all, a challenging job with both geologic and environmental components. Here are some pictures:
Prior to mobilizing any field crew to advance any excavations, we had a geophysical survey conducted to verify the locations of previously disturbed soils. Then we advanced a 200’ long trench up to 13 feet deep and 10 feet wide along the perimeter of the property.
We brought in peer review geologists and invited the city geologist to observe the trench as well. After the trench was logged in detail, it was backfilled, and then we advanced three 24” wide, 40’ deep boreholes, which were downhole logged by the engineering geologist (yours truly!).