Assessment of landslide hazards resulting from the February 13, 2001, El Salvador earthquake; a report to the government of El Salvador and the U. S. Agency for International Development

On February 13, 2001, a
magnitude 6.5 earthquake
occurred about 40 km eastsoutheast
of the capital city of San
Salvador in central El Salvador and
triggered thousands of landslides in
the area east of Lago de Ilopango.
The landslides are concentrated in a
2,500-km2 area and are particularly
abundant in areas underlain by
thick deposits of poorly consolidated,
late Pleistocene and Holocene
Tierra Blanca rhyolitic tephras that
were erupted from Ilopango caldera.
Drainages in the tephra deposits are
deeply incised, and steep valley
walls failed during the strong shaking.
Many drainages are clogged
with landslide debris that locally
buries the adjacent valley floor. The
fine grain-size of the tephra facilitates
its easy mobilization by rainfall
runoff. The potential for remobilizing
the landslide debris as debris flows
and in floods is significant as this
sediment is transported through the
drainage systems during the upcoming
rainy season.
In addition to thousands of shallow
failures, two very large landslides
occurred that blocked the Rio
El Desague and the Rio Jiboa. The
Rio El Desague landslide has an
estimated volume of 1.5 million m3,
and the Rio Jiboa landslide has an
estimated volume of 12 million m3.
Field studies indicate that catastrophic
draining of the Rio El
Desague landslide-dammed lake
would pose a minimal flooding hazard,
whereas catastrophic draining
of the Rio Jiboa lake would pose a
serious hazard and warrants immediate
action. Construction of a spillway
across part of the dam could
moderate the impact of catastrophic
lake draining and the associated
flood.
Two major slope failures on the
northern side of Volcan San Vicente
occurred in the upper reaches of
Quebrada Del Muerto and the
Quebrada El Blanco. The landslide
debris in the Quebrada Del Muerto
consists dominantly of blocks of
well-lithified andesite, whereas the
debris in the Quebrada El Blanco
consists of poorly consolidated pyroclastic
sediment. The large blocks of
lithified rock in Quebrada Del
Muerto are unlikely to be remobilized
during the rainy season;
whereas, the sandy and silty landslide
debris in the channel of
Quebrada El Blanco is susceptible
to remobilization as debris flows
that could extend into populated
areas on the lower slopes of the volcano.
Around the northern and eastern
shore of Lago de Ilopango,
earthquake-induced liquefaction
and lateral-spreading landslides
caused local damage to homes and
other structures; this damage was
most prevalent in the village of San
Agustin. San Agustin is also potentially
threatened by floods because
it is located on the alluvial fan of
the Quebrada El Chaguite drainage
basin, which contains hundreds of
landslides that have choked numerous
small channels with volcanic
tephra. As the easily eroded tephra
is transported down the drainage
system and deposited on the alluvial
fan, it could clog the currently
active channel with sediment, divert
the stream into a new channel, and
possibly direct flow through San
Agustin, causing more damage and
destruction