Abstract Central America is a small and culturally homogeneous region that, since the 1990s, has experienced economic and political integration of its six countries, which share the same threats of volcanic eruptions, disastrous earthquakes and tsunamis. The Pacific coastline of 1700 km is common for Guatemala, El Salvador, Honduras, Nicaragua, Costa Rica and Panama, and the Pacific subduction zone has the potential for creating huge tsunamis that threaten this coast. In addition to the natural hazard, the growing tourist industry is expanding its infrastructure along the Pacific beaches, which again enhances the exposure and tsunami risk. Even though the 1992 tsunami disaster in Nicaragua did not severely hit the tourist beaches, it raised the risk awareness, and special attention is now given to ‘slow’ earthquakes that may be modest in shaking while still having a large tsunami potential. The tsunami hazard mapping is well advanced in Nicaragua, Costa Rica and El Salvador, and initiatives are ongoing to improve the mapping in all countries. National systems for early warning were established in Nicaragua and El Salvador, while the other four countries rely on rapid information from the Pacific Tsunami Warning Center. Mitigation measures and information campaigns are presently conducted on a national basis in all countries, but a regional centre for early tsunami warning and coordinated information campaigns (CATAC) is expected to become operational in the near future.

Disaster risk reduction efforts are lacking in many hazard-prone areas around the globe. Governmental initiatives in El Salvador sought to address challenges to disaster management that became evident following a series of disasters spanning 1998–2005. The region surrounding San Vicente volcano, El Salvador, has a history of disasters but, until recently, has received little attention toward hazard mitigation. The debrisflow disaster in November 2009, triggered by rains from Hurricane Ida, was the first time new systems were tested, and an in-depth review of the evolution of these systems is the focus of this paper. Faculty at the Universidad de El Salvador–Facultad Multi-disciplinaria Paracentral (UES-FMP), in San Vicente experienced the tragedy first-hand and perceived that chaotic project implementation, redundant objectives among various groups, and poor coordination hindered the effectiveness of postevent disaster risk reduction efforts. Poor potential-hazard awareness, no warning or monitoring systems, and unclear crisis-response responsibilities all contributed to >200 deaths in the region. UES-FMP agricultural sciences faculty led a comprehensive effort to identify weaknesses and improve plans for the next catastrophe. Their approach encompassed conceiving and implementing new research, field, and training activities for improving hazard understanding and communication in order to inform decision makers and the public. UES-FMP partnered with research and development groups to gather hydrometeorological data, model hazards, and train local stakeholders. UES-FMP encourages disaster risk reduction practitioners to focus on interdisciplinary methods to help guide project design. Experiences from San Vicente can be applied to improve disaster risk reduction and hazard research efforts in other areas.

The crater lake at Santa Ana Volcano (El Salvador) was monitored during 1992–1993 and 2002–2007. Crater lake chemistry was generally similar until the 2005 eruption. Acidification of the hydrothermal system by condensing magmatic gases yielded fluids that sustained a cool acid sulfate-chloride lake roughly 200 m in diameter (temperature = 16–28 °C, pH = 0.7–2.0, SO 4 2− = 4500–14,000 mg/L, Cl − = 1100–9200 mg/L, total dissolved solids [TDS] = 7000–25,000 mg/L). The phreatomagmatic eruption Volcanic Explosivity Index (VEI) 3 of October 2005 modified the summit crater morphology, leading to physical, thermal, and chemical changes in the lake over the next few years. The lake became hotter and more acidic, with variable chemistry and color (temperature = 24–66 °C, pH = 0.4–1.3, SO 4 2− = 2500–9800 mg/L, Cl − = 3200–22,000 mg/L, TDS = 10,000–36,000 mg/L, turquoise-gray-yellow color). The SO 4 2− /Cl − ratio dropped below 1, indicating an increase in the rate of volcanic gas input and coincident S depletion by abundant precipitation of native sulfur and secondary S-bearing minerals (alunite, gypsum, iron sulfides, and anhydrite). An increase in rare earth element (REE) concentrations in lake waters indicated leaching of the newly intruded magma. The eruption likely enhanced permeability in the edifice, further increasing the amount of available fresh wall rock to react with acidic fluids, and the concentration of rock-forming elements in the lake increased fivefold to a maximum of 90 g rock dissolved per kg water. The magma continued to degas through the lake bottom at the drowned eruptive vent, providing a large, direct gas input into the lake. Direct gas discharge into the lake led to sulfur saturation and formation of hollow sulfur spherules by percolation of gas bubbles through the molten sulfur bottom layer. Increased heat input into the lake (8–830 MW, equivalent SO 2 flux of 16–1600 t/d) led to enhanced evaporation and highly variable lake mass. Consequently, on three occasions during 2006 and 2007, the lake area diminished to 70% of its former size.

We test the suitability of short-occupation differential and absolute positioning methods using data from a 13 station global positioning system (GPS) network that spans the Santa Ana Volcano and Coatepeque caldera of western El Salvador for monitoring intereruptive activity and tectonic movements near these potentially hazardous features. Data spanning a 1 yr period from 12 GPS benchmarks located 1.9 km to 9.7 km from a continuous reference GPS station were processed with Trimble differential processing software to determine the repeatabilities and hence precisions of the differential station coordinates. For observation sessions spanning 20 min, the coordinates of the benchmark closest to our reference station are repeatable to within 4–6 mm in the horizontal component of the baseline between the two sites and 12–13 mm for the vertical component. In contrast, the horizontal and vertical repeatabilities at the site farthest from the reference station are 7–11 mm and 30–34 mm, respectively. These results suggest that any horizontal movement of the benchmarks larger than ~10 mm relative to the reference site, or vertical movement larger than 10–30 mm (depending on the baseline distance) should be detectable. Five sites adjacent to and within Coatepeque caldera moved upward at rates of 180–470 mm/yr relative to the reference site from February to July 2009. In contrast, no uplift pattern during this period was observed for the other network sites, suggesting an uplift source beneath the caldera. No increase in microseismicity coincided with the transient inflation event, and no other possibly corroborating observations from the caldera are available. The cause of the uplift is thus unknown. Our results suggest that differential GPS with occupation times of 20 min or more is a useful monitoring tool at subtropical volcanoes and calderas for networks with baselines that are shorter than ~10 km. Absolute positioning as an alternative GPS processing method gives precisions of ~10 mm (95%) in the absolute station coordinate estimates for occupations as short as 3 h, i.e., sufficiently precise for monitoring volcano deformation. This new monitoring strategy, whereby one dual-frequency receiver is used for short benchmark occupations without any need for a reference station, is simpler than that required for differential monitoring and minimizes tropospheric water vapor as a source of noise in station position estimates.

Boquerón Volcano, formed on the old San Salvador Volcano, is the youngest and active central edifice of the San Salvador volcanic complex, which also includes 25 secondary vents. The San Salvador volcanic complex is located in the vicinity of the San Salvador metropolitan area and is considered one of the most hazardous volcanic centers in El Salvador and Central America. Boquerón Volcano has a long record of effusive and explosive eruptions spanning 36,000 yr; the most recent eruption was in 1917. We reviewed and updated its eruptive history through detailed fieldwork, allowing the recognition of up to 25 different eruptions. Lava flows, ash-fall, and ballistic projectile deposits produced by Strombolian or violent-Strombolian eruptions are the most recurrent events preserved in the stratigraphic record of Boquerón Volcano. Pyroclastic-flow, and especially pyroclastic-surge, deposits are also present, indicative of explosive subplinian and Plinian eruptions, some of which had significant phreatomagmatic components. We define three hazard scenarios regarding ash fall, ballistic projectiles, and pyroclastic density currents for Boquerón Volcano and constrain them using its documented explosive eruptive history, fieldwork, and computer simulations. Each scenario is characterized by a likelihood of occurrence (high, medium, low), assigned to eruptive events of small, intermediate, or large magnitude, which are mainly characterized for the areal distribution of the related volcanic products. Resulting hazard maps show areas likely to be affected by future eruptions, enabling decision makers and the general public to consider volcanic hazards in land development and risk mitigation planning.

Edifice-collapse phenomena have, to date, received relatively little attention in Central America, although ∼40 major collapse events (≥0.1 km 3 ) from about two dozen volcanoes are known or inferred in this volcanic arc. Volcanoes subjected to gravitational failure are concentrated at the arc's western and eastern ends. Failures correlate positively with volcano elevation, substrate elevation, edifice height, volcano volume, and crustal thickness and inversely with slab descent angle. Collapse orientations are strongly influenced by the direction of slope of the underlying basement, and hence are predominately perpendicular to the arc (preferentially to the south) at its extremities and display more variable failure directions in the center of the arc. The frequency of collapse events in Central America is poorly constrained because of the lack of precise dating of deposits, but a collapse interval of ∼1000–2000 yr has been estimated during the Holocene. These high-impact events fortunately occur at low frequency, but the proximity of many Central American volcanoes to highly populated regions, including some of the region's largest cities, requires evaluation of their hazards. The primary risks are from extremely mobile debris avalanches and associated lahars, which in Central America have impacted now-populated areas up to ∼50 km from a source volcano. Lower probability risks associated with volcanic edifice collapse derive from laterally directed explosions and tsunamis. The principal hazards of the latter here result from potential impact of debris avalanches into natural or man-made lakes. Much work remains on identifying and describing debris-avalanche deposits in Central America. The identification of potential collapse sites and assessing and monitoring the stability of intact volcanoes is a major challenge for the next decade.