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may have occurred during Hurricane Irma, north-central and northwestern reaches of ASSESSMENT OF
but an absence of imagery during the the Cordillera Central. Landsliding in the CONTRIBUTING FACTORS
interim precluded us from differentiating municipalities of Utuado and Naranjito was Our evaluation of landslide distribution
between landslides that occurred during particularly severe, with ~38% and 45% of shows that while landslides occurred
each hurricane. We divided the island into each municipality, respectively, classified throughout most of Puerto Rico’s moun-
a 2 km × 2 km grid and classified each grid as having HLD. All five HLD clusters were tainous interior, landsliding was particu-
cell as either having no landslides (NLS), located ~10–20 km north of the Cordillera larly severe in five distinct areas (Fig. 1B).
1–25 landslides/km (low landslide density, Central divide (Fig. 1). Areas along the We examined rainfall, soil moisture, and
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LLD) or more than 25 landslides/km (high southern flank of the Cordillera Central, geology in NLS, LLD, and HLD areas as a
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landslide density, HLD). With the intent of which have some of the highest relief and first-order attempt to understand the dif-
rapidly preparing a product to aid emer- steepest slopes on the island, generally had ferences between severely impacted areas
gency response agencies, we visually lower landslide densities. and neighboring areas in which landslides
examined each grid, but did not map indi- During field work, we observed a vari- were less spatially dense.
vidual landslides. Landslide scars were ety of landslide failure modes and material Elevated pore-water pressure from rain-
readily visible in imagery because of defo- types and properties. Most landslides were fall is the most common trigger for land-
liation from strong winds during Hurricane shallow, translational failures in soil or slides (Terzaghi, 1950). Because subsurface-
Maria, and because of the sharp color con- saprolite, generally measuring decimeters water pressure is not commonly measured,
trast between exposed soil and rock and the to a few meters deep (e.g., Fig. S1A [see rainfall is often used as a proxy. During
remaining vegetation (Fig. S1 [see footnote footnote 1]). We also observed deeper (up Hurricane Maria, rainfall amounts
1]). See Bessette-Kirton et al. (2017) for to ~30 m) complex failures (e.g., Fig. S1B) reported by the NHC and NCEP data sets
further description of the mapping proce- in soil, saprolite, and rock, and rock falls for NLS areas were 7%–12% less than the
dure. We validated and updated our pre- and rock slides. Many landslides transi- average island-wide rainfall (Fig. 2). The
liminary density map (Bessette-Kirton et tioned into debris flows (e.g., Figs. S1C average island-wide rainfall reported by
al., 2017) by helicopter and on the ground, and S1D), and coalescence and subsequent the PERSIANN-CCS data set was about
covering a distance of 1950 km between channelization of debris flows (e.g., Fig. equal to rainfall in NLS areas (Fig. 2).
26 October and 6 November 2017. S1D) was common. Landslides that partly All three data sets showed that rainfall in
From our mapping, we estimated that reactivated preexisting landslides were LLD and HLD areas was greater than
more than 40,000 landslides resulted from also common. Crosscutting relationships average (4%–19% increase; Fig. 2), but
Hurricane Maria. Landslides occurred in at between debris flow and flood deposits differences in the spatial distribution of
least 59 of Puerto Rico’s 78 municipalities indicated that landsliding occurred before, rainfall resulted in inconclusive differ-
(Fig. 1B). Many of the other 19 municipali- during, and after extreme flooding (e.g., ences between LLD and HLD areas.
ties have such low relief that landsliding is Fig. S1C). Landslides continued to occur Variability between rainfall data sets
unlikely. Five HLD clusters, ranging in during storms in the days and weeks calls into question the validity of using
size from 12 to 132 km , occurred in the following Maria. island-wide rainfall estimates for localized
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A B
C

Figure 2. (A) Deviation from average island-wide rainfall and soil moisture in areas with no, low, and high landslide densities. (B)
Soil Moisture Active Passive (SMAP) root zone soil moisture (Reichle et al., 2018) measurements from April 2015–December 2017
(Hurricane Maria shown with stars). (C) Antecedent SMAP root zone soil moisture measurements during the two weeks preceding
Hurricane Maria, including the passage of Hurricane Irma. SMAP data were averaged across the entire island and in areas with
low landslide density (LLD) and high landslide density (HLD). CCS—PERSIANN-Cloud Classification System; NCEP—National
Centers for Environmental Prediction; NHC—National Hurricane Center.

6 GSA Today | June 2019