Figure 2. Landslide and precipitation time series for 38
selected landslides in California. (a) Cumulative displacement time
series projected onto the downslope direction and separated by water
year (WY). The time series for each landslide are smoothed using a
moving median temporal filter. Solid lines correspond to landslides
occurring within the Franciscan mélange rock unit. Colors correspond to
30-year mean water year precipitation (WY1990-WY2019) for each
landslide. Inset shows the location of the selected landslides on the
30-year mean precipitation map. (b) Cumulative precipitation time series
for each landslide separated by WY and colored by 30-year mean water
year precipitation.
Despite the large variability in rock type throughout California (Figure
S4), we found that 228 of 247 landslides occurred in host rocks broadly
defined as marine and nonmarine sedimentary or metasedimentary rock
units (Figure S4 and Table S3). Of these landslides, 176 (71% of total)
landslides are hosted within the Franciscan complex mélange (Figure 1b),
which indicates a strong lithologic control on the distribution of
slow-moving landslides. Numerous recent studies have made similar
findings indicating that rock type exerts a primary control on
slow-moving landslides in California (Bennett, Miller, et al., 2016;
Handwerger, Fielding, et al., 2019; Scheingross et al., 2013; Xu et al.,
2021) and throughout the world (see refs. in Lacroix, Handwerger, et
al., 2020).
Estimates of precipitation from PRISM data showed that active landslides
are occurring in both wet and dry environments (Figure 1). We found more
than an order of magnitude precipitation variation between the wettest
active landslide in northern California (30-year mean = 2180 mm/yr) and
driest landslide in southern California (30-year mean = 216 mm/yr)
(Figure 1a).
3.2 Seasonal and Annual Landslide Behavior
To assess the behavior of landslides occurring in wet and dry climates,
we selected a subset of 38 landslides spanning more than an order of
magnitude in 30-year mean precipitation (Figure 2 and Table S4). The
subset of landslides consisted of different landslide types, rock types,
and occurred in different environments including coastal and inland
regions, as well as developed and undeveloped areas.
Annual downslope velocities (± bootstrap uncertainty) averaged over the
full study period ranged from 0.85 ± 0.56 to 9.7 ± 1.2 cm/yr (Table S4).
Landslides occurring in both wet and dry regions of California exhibited
seasonal kinematic changes in response to seasonal precipitation each
year (Figure 2). Each landslide accelerated in response to infiltrating
rainfall during the wet season before decelerating back to lower rates,
or completely stopping, during the dry season.
The landslides also responded to changes in seasonal rainfall each year.
We found large changes in seasonal precipitation caused large changes in
displacement (Figures 2 and 3). While there is no clear relationship
between velocity, precipitation, and landslide size (Figures 3 and S5),
the landslides moved faster than average during the wetter WY2017 and
WY2019, and slower than average during the drier WY2016 and WY2018
(Figure 3). Interestingly, we observed the largest displacement at a
single landslide during the drier than average WY2018 (Figure 2). This
landslide is located on the coast and is likely subject to other driving
forces such as debuttressing from wave erosion at its toe (see location
of “landslide 22” in Table S4). However, we cannot constrain
additional driving forces with our dataset. While we did detect several
other coastal landslides, most of their motion appears to be driven by
rainfall (Figure 2).
Landslide sensitivity to precipitation also appeared to change during
the study period. For instance, WY2016, which was the final year of the
historic California drought, had both wetter and drier than average
conditions in certain places, however all of the landslides were moving
slower than average (velocity ratio < 1). WY2018 was drier
than WY2016 across California, but a few landslides exhibited velocity
ratios > 1. Our findings suggest that antecedent rainfall
from the previous wet seasons, particularly the lingering impact of
long-term droughts, likely play an important control on landslide
behavior and sensitivity to rainfall.