Page 8 - i1052-5173-29-6

A

B

Figure 3. Landslide density distributions and geologic formations. (A) Map showing geologic formations with >10% by
area high landslide density and with >10% by area absent landslides. (B) Chart showing formations with >10% by area
with high landslide density. Ku—granodiorite; Thp—hornblende quartz diorite; Kap—basalt lava and tuffaceous sand-
stone; Kco—basaltic lava and volcanic breccia; TKdi—diorite; Trhp—rhyodacite porphyry; Kav—andesitic lava;
Kmam—andesite lava and volcanic sandstone and siltstone; TKn—crystal tuff and tuffaceous clastics; Kln—basaltic
breccia and basalt lava; Kja—amphibolite; Kper—basalt lava and tuff, volcanic sandstone (Bawiec, 1998).

COMPARISON WITH from 287 to 579 mm, and although these CONCLUSIONS
PAST EVENTS values vary by as much as a factor of two, Although landsliding is frequent on
To place Hurricane Maria in the context even the minimum is nearly twice as large steep hillslopes in Puerto Rico, the abun-
of previous landslide-generating TCs, we as the median MMR for all previous wide- dance and widespread extent of landslides
compiled rainfall data and information on spread landsliding events (152 mm). that occurred during Hurricane Maria
the extent and characteristics of landslides Although TC-triggered widespread land- were unprecedented in comparison to
from 72 TCs that affected Puerto Rico sliding occurs frequently in Puerto Rico, previously recorded hurricane or tropical
between 1960 and 2017 (see [B] in the GSA the number and spatial extent of landslides storm-triggered landsliding events.
Data Repository [see footnote 1]). Based on triggered by Hurricane Maria were unprec- Hurricane Maria provided a unique data
this information, we classified the extent of edented in comparison to previously docu- set to examine the characteristics and con-
landsliding as either none, isolated, fre- mented events. For example, Hurricane tributing factors of landslides with variable
quent, or widespread for each TC (Table S2 Hugo (1989) and the precursor to Tropical failure types and material properties as a
[see footnote 1]; see Table 1 for classifica- Storm Isabel (1985) both produced large means of improving future hazard assess-
tion criteria). In addition to reporting maxi- amounts of rain but only triggered land- ments throughout Puerto Rico.
mum rainfall (Fig. 4A), we calculated the slides that were confined to localized areas While we have shown that the average
average rainfall in mountainous terrain (Jibson, 1989; Larsen and Torres-Sanchez, rainfall from Hurricane Maria in mountain-
(mean mountain rainfall [MMR], Fig. 4B) 1992). Other TC-triggered landslide events ous areas was greater than that of any other
for 56 TCs for which data were available. have not been documented systematically, hurricane or tropical storm in Puerto Rico
We used contoured rainfall maps (National and typically refer to “numerous” landslides since 1960, we also found that inconsisten-
Centers for Environmental Prediction, in one or more municipalities or regions of cies among rainfall data sets and the coarse
2018) to extract the average rainfall in areas Puerto Rico (Table S2 [see footnote 1]). resolution of available data did not allow for
with slopes >20° for each historical TC to Based on our island-wide landslide density a meaningful assessment of the correlation
compare with all three rainfall data sets map (Fig. 1B), we estimate that the number between rainfall amount and landslide dis-
from Hurricane Maria. On average, of landslides triggered by Hurricane Maria tribution. Satellite-based SMAP measure-
Hurricane Maria produced more rain than (at least 40,000) was approximately two ments from both before and after Hurricane
any other TC to affect Puerto Rico in the orders of magnitude greater than the hun- Maria showed larger differences between
58-year record (Fig. 4). Our calculated dreds of landslides reported from previous low and high landslide density areas, indi-
MMR values for Hurricane Maria range detailed inventories. cating that, unsurprisingly, soil moisture

8 GSA Today | June 2019