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Volcanism and earthquakes are the most dire geological hazards that threaten San Salvador, El Salvador's largest city (population, 1.7 million), with a rapidly growing population, and few plans or resources for hazard mitigation. Financial, transportation, and governmental centers of El Salvador all reside within San Salvador. The city lies on a plain within severe hazard zones of two active volcanoes (San Salvador to the W and Ilopango to the E) and also lies in a zone of major subduction earthquakes.
In the past 60 Ka, a caldera 12 km E of the city now occupied by Lake Ilopango erupted at least four significant pyroclastic deposits, each of which blanketed the valley that San Salvador now occupies. The last of these eruptions devastated all El Salvador in AD 260 and drove pre-Classic Mayan civilization northward into the jungles. A dome eruption in 1880 and small seismic swarms beneath the lake indicate continuing unrest. Neither San Salvador volcano nor its numerous flank vents has erupted catastrophically since European colonization, however numerous explosive eruptions occurred prior to that. San Salvador volcano erupted ash-rich tephra and pyroclastic flows 800 years ago and caused mudslides that would likely kill many thousands today. Because of the frequency of earthquakes, the people of San Salvador are much more aware of seismic risk than volcanic risk. The city has had to rebuild 7 times after earthquakes since 1712. The most recent devastating earthquake in 1986, killed 1500, injured 10,000 and left 100,000 homeless. Thick unconsolidated pyroclastic and tephra deposits amplify the effects of shallow-focus, moderate magnitude earthquakes beneath the city.
Timely international involvement could provide the impetus that San Salvador needs to begin effective hazard-assessment, monitoring, and educational programs. A significant part of this work will be to make the population of a volcanic country more aware of its vulnerability to volcanic hazards. The recent occurrence of hurricane Mitch offers an opportunity to advance all hazard work in Central America.
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