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Landslides Triggered by Hurricane Maria:

                        Assessment of an Extreme Event in Puerto Rico






          Erin K. Bessette-Kirton, U.S. Geological Survey, Denver Federal Center, Box 25046, MS 966, Denver, Colorado 80225, USA;
          Corina Cerovski-Darriau, U.S. Geological Survey, 345 Middlefield Road, MS 910, Menlo Park, California 94025, USA;
          William H. Schulz, Jeffrey A. Coe, Jason W. Kean, Jonathan W. Godt, Matthew A. Thomas, U.S. Geological Survey, Denver
          Federal Center, Box 25046, MS 966, Denver, Colorado 80225, USA; and K. Stephen Hughes, Dept. of Geology, University of Puerto
          Rico, Call Box 9000, Mayagüez, Puerto Rico 00681, USA


          ABSTRACT                           triggered widespread landslides through-  triggering storms (TCs and non-TC
            Hurricane Maria hit the island of Puerto   out mountainous areas. Landslides dam-  systems) affect Puerto Rico annually
          Rico on 20 September 2017 and triggered   aged and destroyed structures and roads   (Pando et al., 2005). The frequency of such
          more than 40,000 landslides in at least   (Puerto Rico Highway and Transportation   events constitutes a hazard to ~1 million
          three-fourths of Puerto Rico’s 78 munici-  Authority, 2017, personal commun.; U.S.   U.S. citizens that reside in the predomi-
          palities. The number of landslides that   Geological Survey, 2017a), in some cases   nantly rural interior of the island, much of
          occurred during this event was two orders   isolating communities for days and weeks   which is characterized by rugged moun-
          of magnitude greater than those reported   (e.g., Radebach, 2017; Schmidt and   tainous terrain (Martinuzzi et al., 2007).
          from previous hurricanes. Landslide   Hernández, 2017). Slope failures caused at   Puerto Rico’s vulnerability to extreme
          source areas were commonly limited to   least three fatalities (Hennessy-Fiske, 2017;   rainfall events is sobering given projec-
          surficial soils but also extended into under-  Irizarry Álvarez, 2017), although Kishore   tions of increasingly frequent extreme
          lying saprolite and bedrock. Slope failures   et al. (2018) believe that the death toll from   TCs in the Atlantic Ocean Basin (Knutson
          occurred before, during, and after flood-  Hurricane Maria was underestimated by   et al., 2010).
          ing, and many transitioned into long run-  more than 4000 deaths, some of which   As a step toward reducing landslide risk
          out debris flows. Steep slopes in hilly and   could have been related to landslides.   during extreme, island-wide precipitation
          mountainous regions were particularly   Landslides were also partly responsible    events, we evaluated the extent and charac-
          impacted by landslides due to antecedent   for damage to the communications and   teristics of Maria-induced landslides
          soil moisture levels that were 11%–13%   electrical power transmission infrastruc-  throughout Puerto Rico. Herein, we present
          higher than average and rainfall totals of at   ture that left much of the island without   an assessment of island-wide landslide
          least 250 mm within a 48 h period. High   power for more than six months.  density, which we compare, in conjunction
          landslide densities were especially wide-  Landslides occur frequently in the   with rainfall data, to TCs that have affected
          spread across some geologic formations   mountainous regions of Puerto Rico    Puerto Rico since 1960. Additionally, we
          (e.g., granodiorite of the Utuado batholith);   (e.g., Monroe, 1964, 1979; DeGraff et al.,   discuss the conditions specific to land-
          however, bedrock geology alone did not   1989; Larsen and Simon, 1993; Larsen and   sliding in Puerto Rico and examine the
          determine the location and distribution of   Torres-Sánchez, 1998; Pando et al., 2005;   impact of environmental variables (e.g.,
          landslides caused by Hurricane Maria.   Lepore et al., 2012). Most noteworthy was   rainfall, soil moisture, and geology) on
          While rainfall data collected during   the 7 October 1985 Mameyes disaster,   observed variations in island-wide land-
          Hurricane Maria were inconsistent,    which killed at least 129 people and is rec-  sliding. An improved understanding of
          satellite-based soil moisture data were    ognized as the deadliest landslide in North   causative factors specific to landslides in
          correlated with the distribution of land-  American history (Campbell et al., 1985;   Puerto Rico is important for revised sus-
          slides. In the future, the use of soil mois-  Jibson, 1992). Hurricanes and tropical   ceptibility analyses and risk management
          ture data could enable assessments of   cyclone systems (henceforth referred to   in anticipation of future storms capable of
          regional landslide susceptibility prior to   collectively as TCs) routinely affect Puerto   producing widespread landsliding.
          hurricanes or extreme precipitation events.  Rico (Hernández Ayala and Matyas, 2016)
                                             and are capable of producing landslide-  SETTING
          INTRODUCTION                       triggering rainfall (e.g., Campbell et al.,   Puerto Rico (18° 15′ N, 66° 30′ W) is
            Hurricane Maria struck Puerto Rico on   1985; Jibson, 1989; Larsen and Torres-  the easternmost of the Greater Antilles
          20 September 2017 as the strongest hur-  Sánchez, 1992). Landslides have been   and covers an area of 8750 km  (Fig. 1A).
                                                                                                       2
          ricane to make landfall on the island since   associated with at least 17 major disaster   Two-thirds of the island is mountainous,
          1928 (National Weather Service, 2017a).   declarations in Puerto Rico since 1960   with the east-west–trending Cordillera
          Maria produced heavy rainfall and flood-  (Federal Emergency Management Agency,   Central range spanning most of the island
          ing across most of Puerto Rico and   2018), and, on average, 1.7 major landslide-   and reaching a maximum elevation of



          GSA Today, v. 29, https://doi.org/10.1130/GSATG383A.1. Copyright 2019, The Geological Society of America. CC-BY-NC.

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