Volcano hazards in the San Salvador region, El Salvador

San Salvador volcano is one of many volcanoes along the volcanic arc in El Salvador
(figure 1). This volcano, having a volume of about
110 cubic kilometers, towers above San Salvador,
the country’s capital and largest city. The city has a
population of approximately 2 million, and a
population density of about 2100 people per square
kilometer. The city of San Salvador and other
communities have gradually encroached onto the
lower flanks of the volcano, increasing the risk that
even small events may have serious societal
consequences. San Salvador volcano has not
erupted for more than 80 years, but it has a long
history of repeated, and sometimes violent, eruptions. The volcano is composed of remnants of
multiple eruptive centers, and these remnants are
commonly referred to by several names. The
central part of the volcano, which contains a large
circular crater, is known as El Boquerón, and it
rises to an altitude of about 1890 meters. El
Picacho, the prominent peak of highest elevation
(1960 meters altitude) to the northeast of the crater,
and El Jabali, the peak to the northwest of the
crater, represent remnants of an older, larger
edifice. The volcano has erupted several times
during the past 70,000 years from vents central to
the volcano as well as from smaller vents and
fissures on its flanks [1] (numerals in brackets refer
to end notes in the report). In addition, several
small cinder cones and explosion craters are
located within 10 kilometers of the volcano. Since
about 1200 A.D., eruptions have occurred almost
exclusively along, or a few kilometers beyond, the
northwest flank of the volcano, and have consisted
primarily of small explosions and emplacement of
lava flows. However, San Salvador volcano has
erupted violently and explosively in the past, even
as recently as 800 years ago. When such eruptions
occur again, substantial population and infrastructure will be at risk.

Volcanic eruptions are not the only events that
present a risk to local communities. Another
concern is a landslide and an associated debris flow
(a watery flow of mud, rock, and debris--also
known as a lahar) that could occur during periods
of no volcanic activity. An event of this type
occurred in 1998 at Casita volcano in Nicaragua
when extremely heavy rainfall from Hurricane
Mitch triggered a landslide that moved down slope
and transformed into a rapidly moving debris flow
that destroyed two villages and killed more than
2000 people. Historical landslides up to a few
hundred thousand cubic meters in volume have
been triggered on San Salvador volcano by torrential rainstorms and earthquakes, and some have
transformed into debris flows that have inundated
populated areas down stream. Destructive rainfall- and earthquake-triggered landslides and debris
flows on or near San Salvador volcano in September 1982 and January 2001 demonstrate that such
mass movements in El Salvador have also been
lethal.

This report describes the kinds of hazardous
events that occur at volcanoes in general and the
kinds of hazardous geologic events that have
occurred at San Salvador volcano in the past. The
accompanying volcano-hazards-zonation maps
show areas that are likely to be at risk when
hazardous events occur again.