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areas (Fig. S2 [see footnote 1]). Collecting Additionally, the antecedent soil moisture Lorenzo Formation granodiorite (Ksl) had
accurate rainfall data during hurricanes is in LLD and HLD areas was 13% and 11% no HLD areas and lacked landslides in
intrinsically difficult due to high winds and above average, respectively. SMAP data 53% of its area, yet the formation is essen-
the inability to measure sideways rainfall. showed that prior to the passage of tially equivalent to the Ku granodiorite
During Hurricane Maria, these complica- Hurricane Irma, the average island-wide (Bawiec, 1998), which displayed the
tions were compounded by damage to rain soil moisture was already higher than greatest areal percentage of HLD (63%).
gages (at least 14 of the U.S. Geological normal, and rainfall from Irma only To the extent of the resolution of our
Survey’s 24 gaging stations; U.S. Geo- caused a slight increase in island-wide soil data, these variations in landslide density
logical Survey, 2017b) and the failure of moisture (Fig. 2C). The increase in soil cannot be attributed to differences in rain-
two Federal Aviation Administration moisture caused by heavy rainfall during fall or slope. Instead, it appears that rela-
Doppler radars (Buchanan, 2017; National Hurricane Maria was unprecedented tions between geologic formation and
Weather Service, 2017b). Additionally, (41%–48% above average) during the time landslide density were largely due to soil
localized effects, such as orographically period for which SMAP data are available. moisture variability. For example, Ksl
enhanced rainfall, may have produced However, increased antecedent soil mois- occurs in the eastern quarter of the island
pockets of heavy rain that were not accu- ture prior to Hurricane Maria evidently where soil moisture was relatively low
rately represented by any of the existing influenced the susceptibility to land- (0.25–0.26 m /m ), whereas Ku occurs
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data sets. sliding in LLD and HLD areas. This where soil moisture was relatively high
Soil moisture is a better proxy for pore- finding, along with the physical relation (Fig. S5 [see footnote 1]). Individual for-
water pressure than rainfall because it is a between increased pore-water pressure mations with large HLD areas (Fig. 3) also
subsurface hydrologic response variable. and decreased effective material strength reveal trends between landsliding and soil
We found that soil moisture measurements (Terzaghi, 1950), may make soil moisture moisture (Fig. S5) when considering for-
from NASA’s Soil Moisture Active a useful tool for estimating differential mations that cover large areas of Puerto
Passive (SMAP) mission (see [A] in the susceptibility to landsliding prior to future Rico (necessitated by the gross resolution
GSA Data Repository [see footnote 1]) storms with predicted heavy and wide- of SMAP data). The three formations that
differentiated between LLD and HLD spread rainfall. Although the resolution of are most widespread from east to west
areas more effectively than did rainfall soil moisture data is coarse (9 km × 9 km), (basaltic breccia and basalt lava [Kln],
data. We compared landslide density to and the data for Puerto Rico are primarily hornblende quartz diorite porphyry [Thp],
SMAP’s 9 km × 9 km gridded global esti- derived from a land-surface model with- and diorite [TKdi]; spanning east-west
mates of root zone (0–100 cm) volumetric out local calibration, our preliminary distances of ~60, 70, and 160 km, respec-
water content before (9:30 a.m. AST on 18 assessments indicate that SMAP data tively) display strong positive correlations
September 2017) and after (9:30 p.m. AST could be a useful component of landslide between soil moisture and landslide den-
on 21 September 2017) Hurricane Maria forecasting across widespread areas prior sity (Fig. S5). We conclude that landslide
(Reichle et al., 2018; Fig S3 [see footnote to future hurricanes. densities were not strongly controlled by
1]). These estimates rely on remotely An examination of mapped rock forma- variable susceptibility to landsliding
sensed measurements of brightness tem- tions in mountainous areas indicated that among the geologic formations, but by
peratures and the solution of a water and geologic material did not consistently cor- variable soil moisture.
energy balance (see [A] in the GSA Data relate with landslide density. Twelve of the Although we have identified spatially
Repository for details [see footnote 1]). island’s 145 rock formations (Bawiec, averaged correlations between landslide
Both antecedent and post-event root zone 1998) had >10% HLD by area. Landslides density and soil moisture, the resolution
SMAP measurements showed that soil were observed nearly everywhere in half of our island-wide data sets and our
moisture was 10%–23% greater than the of these 12 formations and had HLD clas- method of grid analysis do not allow
island-wide average in areas where land- sification in ≥11% of their areas. In con- for differentiation of contributing factors
slides occurred (LLD and HLD areas) and trast, the other six formations had HLD at a scale sufficient for detailed analyses.
5%–7% less than the island-wide average in ≥14% of their areas but also had no For example, our analysis of island-wide
in NLS areas (Fig. 2A). Additionally, the observed landslides in ≥11% of their areas. geology (1:100,000 scale) does not
difference in deviation from the mean Similar variability is also apparent when account for localized variations in geo-
between LLD and HLD areas for anteced- grouping geologic formations by terrane logic units that may affect rock weather-
ent and post-event root zone soil moisture (Fig. S4 [see footnote 1]). For example, the ing rates and soil formation, and therefore
measurements (11%) was nearly twice as intrusive igneous rock terrane had the two landslide susceptibility. The resolution of
large as the spread between LLD and formations with the highest proportion of our 2 km × 2 km grid framework is com-
HLD areas for rainfall (3%–6% increase HLD area, both of which also had land- patible with island-wide rainfall and soil
and 7% decrease). slides across nearly their entire areas moisture data sets but is likely less useful
SMAP measurements from the two (granodiorite-quartz diorite of the Utuado for comparison with higher resolution
years prior to Hurricane Maria (Fig. 2B) batholith [Ku] and hornblende quartz dio- data sets (<4 km ). While our rapidly pro-
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show that soil moisture across Puerto Rico rite porphyry [Thp]; Fig. 3). However, two duced landslide density map allowed for
immediately before Maria (18 September similar formations both lacked landslides the analysis of contributing factors on a
2017) was 10% above the average island- and had large areas of high density land- regional scale, a detailed inventory would
wide soil moisture during the period of slides (diorite [TKdi] and rhyodacite por- be invaluable for more localized assess-
record (April 2015–December 2017). phyry [Trhp]; Fig. 3). Finally, the San ments of landslide susceptibility.

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