Figure 1. Maps of precipitation, rock type, and landslide locations. (a) 30-year mean water year precipitation (m/yr) with period WY1990-WY2019 calculated from PRISM data. (b) Simplified geologic map showing the areal extent of the Franciscan mélange rock unit. (c) Location of active landslides identified with our InSAR analyses. Well-studied landslide groups labeled Eel = Eel River, BH = Berkeley Hills, CSAF = Central San Andreas Fault, PBL = Portuguese Bend landslide. (d-i) Precipitation Ratio (total WY precipitation / 30-year mean precipitation) for WY2015-WY2020. Red colors correspond to drier than average years and blue colors correspond to wetter than average years. Yellow circles in (d) show landslides selected for detailed time series analyses.
We focus our study between the 2015 and 2020 water years (WY), during which California experienced extreme changes in rainfall (Figure 1d-i and Figure S1). WY2015 and WY2016 were the last two years of a historic drought, which was one of the most severe for hundreds of years (Robeson, 2015; Swain et al., 2014). The drought officially ended in WY2017, which was an extremely wet year across most of California, and was the second wettest year on record in places (Swain et al., 2018; Wang et al., 2017). There were many landslides that were triggered or accelerated and reactivated in WY2017 (Finnegan et al., 2021; Handwerger, Fielding, et al., 2019), including the catastrophic Mud Creek landslide that destroyed State Highway 1 (Handwerger, Huang, et al., 2019; Jacquemart & Tiampo, 2021; Warrick et al., 2019). Dry conditions returned in WY2018 due to below average rainfall, followed by wet conditions in WY2019 due to above average rainfall, and finally a return to dry conditions in WY2020 due to below average rainfall. These back and forth changes from dry to wet conditions are consistent with long-term climate predictions and therefore may be representative of climate patterns in California over the next century (Persad et al., 2020; Polade et al., 2017; Swain et al., 2018).